Simple full micromagnetic model of exchange bias behavior in ferro/antiferromagnetic layered structures (abstract)

NASA Astrophysics Data System (ADS)

Koon, Norman C.

1997-04-01

It is shown using full micromagnetic relaxation calculations that exchange bias behavior is predicted for single-crystal ferro/antiferromagnetic layers with a fully compensated interface. The particular example most fully studied has a bcc/bct lattice structure with a fully compensated (110) interface plane. Only bilinear Heisenberg exchange was assumed, with anisotropy only in the antiferromagnet. In spite of the intuitive notion that exchange coupling between a ferromagnet and an antiferromagnet across a fully compensated plane of the antiferromagnet should be zero, we find strong coupling, comparable to the bilinear exchange, with a 90° angle between the ferromagnetic and antiferromagnetic axes of layers far from the interface in absence of an applied field. Even though the 90° coupling has characteristics resembling "biquadratic" exchange, it originates entirely from frustrated bilinear exchange. The development of exchange bias is found to originate from the formation of a domain wall in the antiferromagnet via the strong 90° exchange coupling and pinning of the wall by the magnetocrystalline anisotropy in the antiferromagnet. Because the large demagnetizing factor of the ferromagnet tends to confine its magnetization to the plane, the exchange bias is found to depend mainly on the strength and the symmetry of the in-plane component of anisotropy. Although little effort was made to analyze specific systems, the model reproduces many of the qualitative features observed in real exchange bias systems and gives reasonable semiquantitative estimates for the bias field when exchange and anisotropy values consistent with real systems are used.

A new reversal mode in exchange coupled antiferromagnetic/ferromagnetic disks: distorted viscous vortex

DOE PAGES

Gilbert, Dustin A.; Ye, Li; Varea, Aïda; ...

2015-04-28

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir 20Mn 80/Fe 20Ni 80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet,more » which leads to unexpected asymmetries in the annihilation and nucleation fields. Lastly, these results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.« less

Ultrafast optical modification of exchange interactions in iron oxides

PubMed Central

Mikhaylovskiy, R.V.; Hendry, E.; Secchi, A.; Mentink, J.H.; Eckstein, M.; Wu, A.; Pisarev, R.V.; Kruglyak, V.V.; Katsnelson, M.I.; Rasing, Th.; Kimel, A.V.

2015-01-01

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 103 Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm−2 acts as a pulsed effective magnetic field of 0.01 Tesla. PMID:26373688

Ultrafast optical modification of exchange interactions in iron oxides.

PubMed

Mikhaylovskiy, R V; Hendry, E; Secchi, A; Mentink, J H; Eckstein, M; Wu, A; Pisarev, R V; Kruglyak, V V; Katsnelson, M I; Rasing, Th; Kimel, A V

2015-09-16

Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

A β-Ta system for current induced magnetic switching in the absence of external magnetic field

NASA Astrophysics Data System (ADS)

Chen, Wenzhe; Qian, Lijuan; Xiao, Gang

2018-05-01

Magnetic switching via Giant Spin Hall Effect (GSHE) has received great interest for its role in developing future spintronics logic or memory devices. In this work, a new material system (i.e. a transition metal sandwiched between two ferromagnetic layers) with interlayer exchange coupling is introduced to realize the deterministic field-free perpendicular magnetic switching. This system uses β-Ta, as the GSHE agent to generate a spin current and as the interlayer exchange coupling medium to generate an internal field. The critical switching current density at zero field is on the order of 106 A/cm2 due to the large spin Hall angle of β-Ta. The internal field, along with switching efficiency, depends strongly on the orthogonal magnetization states of two ferromagnetic coupling layers in this system.

Voltage control of ferromagnetic resonance

NASA Astrophysics Data System (ADS)

Zhou, Ziyao; Peng, Bin; Zhu, Mingmin; Liu, Ming

2016-05-01

Voltage control of magnetism in multiferroics, where the ferromagnetism and ferroelectricity are simultaneously exhibiting, is of great importance to achieve compact, fast and energy efficient voltage controllable magnetic/microwave devices. Particularly, these devices are widely used in radar, aircraft, cell phones and satellites, where volume, response time and energy consumption is critical. Researchers realized electric field tuning of magnetic properties like magnetization, magnetic anisotropy and permeability in varied multiferroic heterostructures such as bulk, thin films and nanostructure by different magnetoelectric (ME) coupling mechanism: strain/stress, interfacial charge, spin-electromagnetic (EM) coupling and exchange coupling, etc. In this review, we focus on voltage control of ferromagnetic resonance (FMR) in multiferroics. ME coupling-induced FMR change is critical in microwave devices, where the electric field tuning of magnetic effective anisotropic field determines the tunability of the performance of microwave devices. Experimentally, FMR measurement technique is also an important method to determine the small effective magnetic field change in small amount of magnetic material precisely due to its high sensitivity and to reveal the deep science of multiferroics, especially, voltage control of magnetism in novel mechanisms like interfacial charge, spin-EM coupling and exchange coupling.

Exchange coupling and magnetic anisotropy of exchanged-biased quantum tunnelling single-molecule magnet Ni3Mn2 complexes using theoretical methods based on Density Functional Theory.

PubMed

Gómez-Coca, Silvia; Ruiz, Eliseo

2012-03-07

The magnetic properties of a new family of single-molecule magnet Ni(3)Mn(2) complexes were studied using theoretical methods based on Density Functional Theory (DFT). The first part of this study is devoted to analysing the exchange coupling constants, focusing on the intramolecular as well as the intermolecular interactions. The calculated intramolecular J values were in excellent agreement with the experimental data, which show that all the couplings are ferromagnetic, leading to an S = 7 ground state. The intermolecular interactions were investigated because the two complexes studied do not show tunnelling at zero magnetic field. Usually, this exchange-biased quantum tunnelling is attributed to the presence of intermolecular interactions calculated with the help of theoretical methods. The results indicate the presence of weak intermolecular antiferromagnetic couplings that cannot explain the ferromagnetic value found experimentally for one of the systems. In the second part, the goal is to analyse magnetic anisotropy through the calculation of the zero-field splitting parameters (D and E), using DFT methods including the spin-orbit effect.

Interlayer Exchange Coupling: A General Scheme Turning Chiral Magnets into Magnetic Multilayers Carrying Atomic-Scale Skyrmions.

PubMed

Nandy, Ashis Kumar; Kiselev, Nikolai S; Blügel, Stefan

2016-04-29

We report on a general principle using interlayer exchange coupling to extend the regime of chiral magnetic films in which stable or metastable magnetic Skyrmions can appear at a zero magnetic field. We verify this concept on the basis of a first-principles model for a Mn monolayer on a W(001) substrate, a prototype chiral magnet for which the atomic-scale magnetic texture is determined by the frustration of exchange interactions, impossible to unwind by laboratory magnetic fields. By means of ab initio calculations for the Mn/W_{m}/Co_{n}/Pt/W(001) multilayer system we show that for certain thicknesses m of the W spacer and n of the Co reference layer, the effective field of the reference layer fully substitutes the required magnetic field for Skyrmion formation.

Brillouin light scattering study of spin waves in NiFe/Co exchange spring bilayer films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Haldar, Arabinda; Banerjee, Chandrima; Laha, Pinaki

2014-04-07

Spin waves are investigated in Permalloy(Ni{sub 80}Fe{sub 20})/Cobalt(Co) exchange spring bilayer thin films using Brillouin light scattering (BLS) experiment. The magnetic hysteresis loops measured by magneto-optical Kerr effect show a monotonic decrease in coercivity of the bilayer films with increasing Py thickness. BLS study shows two distinct modes, which are modelled as Damon-Eshbach and perpendicular standing wave modes. Linewidths of the frequency peaks are found to increase significantly with decreasing Py layer thickness. Interfacial roughness causes to fluctuate exchange coupling at the nanoscale regimes and the effect is stronger for thinner Py films. A quantitative analysis of the magnon linewidthsmore » shows the presence of strong local exchange coupling field which is much larger compared to macroscopic exchange field.« less

Calculation of exchange interaction for modified Gaussian coupled quantum dots

NASA Astrophysics Data System (ADS)

Khordad, R.

2017-08-01

A system of two laterally coupled quantum dots with modified Gaussian potential has been considered. Each quantum dot has an electron under electric and magnetic field. The quantum dots have been considered as hydrogen-like atoms. The physical picture has translated into the Heisenberg spin Hamiltonian. The Schrödinger equation using finite element method has been numerically solved. The exchange energy factor has been calculated as a functions of electric field, magnetic field, and the separation distance between the centers of the dots ( d). According to the results, it is found that there is the transition from anti-ferromagnetic to ferromagnetic for constant electric field. Also, the transition occurs from ferromagnetic to anti-ferromagnetic for constant magnetic field (B>1 T). With decreasing the distance between the centers of the dots and increasing magnetic field, the transition occurs from anti-ferromagnetic to ferromagnetic. It is found that a switching of exchange energy factor is presented without canceling the interactions of the electric and magnetic fields on the system.

Exchange field and Hc dependence on the ferromagnetic material in exchange couples with CoO (abstract)

NASA Astrophysics Data System (ADS)

Takano, Kentaro; Berkowitz, A. E.

1997-04-01

As recording density increases, magnetoresistive (MR) sensors are becoming increasingly important in read heads. NixCo(1-x)O is receiving technological attention for biasing magnetoresistive sensors as a robust alternative to FeMn. The interfacial exchange coupling between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) is observed as an exchange field and an enhanced coercive field of the FM layer. The AFM/FM coupling is sensitive to the interfacial structure and the AFM and FM magnetic parameters. In this work, we deposited various FM layers on similar 300 Å CoO base layers to study the dependence of the FM exchange integral parameter J on the exchange HE and coercive HC fields. CoO was selected as the AFM material because (i) its simple spin and crystal structures facilitate the structural characterization and modeling of its magnetic properties, and (ii) it's modest Néel temperature of 300 K facilitates the use of a superconducting quantum interference device for the magnetic measurements at temperatures ranging from 5 to 400 K. The 300 Å CoO films were reactively sputtered on silicon substrates and capped with various 300 Å FM films, Ni, Co, Fe, and permalloy (Ni81Fe19). The 300 Å CoO base layer films were polycrystalline with columnar grains. The CoO deposition conditions were reproduced to ensure similar structural and magnetic interfacial AF environments. The observed HE temperature dependence cannot be explained by current theoretical models. The temperature dependence of the exchange fields have the common features (i) a blocking temperature Tb=300 K, which corresponds to the bulk Néel temperature of CoO, (ii) a rise in the exchange field with decreasing temperature, (iii) an intermediate temperature region of constant HE (plateau value), and (iv) a second region of linearly increasing HE with decreasing temperatures down to 0 K. The plateau value of the HE decreased inversely with increasing FM magnetization as predicted by theory. The low-temperature increase of HE is more significant in the FM with higher exchange integral J values. The crossover temperature from the plateau to the low-temperature rise in HE appears to be dependent on FM's J value. The increase in the interfacial coupling strength could suggest the magnetic ordering of a secondary phase localized at the interfacial atoms. The temperature dependence of HC enhancement does not share the nonlinear temperature behavior of HE. For T<300 K, HC increases linearly with decreasing temperatures down to 10 K. Although the HC enhancement may have magnetoelastic contributions, the disappearance of the linear enhancement at 300 K, the Néel temperature of CoO, indicates that the dominant mechanism is the interfacial magnetic coupling.

Monte Carlo study of magnetization reversal in the model of a hard/soft magnetic bilayer

NASA Astrophysics Data System (ADS)

Taaev, T. A.; Khizriev, K. Sh.; Murtazaev, A. K.

2017-06-01

Magnetization reversal in the model of a hard/soft magnetic bilayer under the action of an external magnetic field has been investigated by the Monte Carlo method. Calculations have been performed for three systems: (i) the model without a soft-magnetic layer (hard-magnetic layer), (ii) the model with a soft-magnetic layer of thickness 25 atomic layers (predominantly exchange-coupled system), and (iii) with 50 (weak exchange coupling) atomic layers. The effect of a soft-magnetic phase on the magnetization reversal of the magnetic bilayer and on the formation of a 1D spin spring in the magnetic bilayer has been demonstrated. An inf lection that has been detected on the arch of the hysteresis loop only for the system with weak exchange coupling is completely determined by the behavior of the soft layer in the external magnetic field. The critical fields of magnetization reversal decrease with increasing thickness of the soft phase.

Inelastic Neutron Scattering and Magnetisation Investigation of an Exchange-Coupled Dy2 SMM

NASA Astrophysics Data System (ADS)

Baker, Michael L.; Zhang, Qing; Sarachik, Myriam P.; Kent, Andrew D.; Chen, Yizhang; Butch, Nicholas; Pineda, Eufemio M.; McInnes, Eric

The strong spin orbit coupling and weak crystal field energies of simple exchange-coupled rare earth SMMs makes the precise evaluation of their magnetic properties nontrivial. Here we report a detailed investigation of the single molecule magnet hqH2Dy2(hq)4(NO3)3MeOH. Inelastic neutron scattering is used to obtain direct access to several low energy crystal field excitations. The INS results display several features that are not found in earlier FIR absorption experiments, while other features found in the latter are absent. Based on the effective point charge model, numerical calculations are currently underway to resolve these apparent discrepancies using complementary magnetisation measurements to resolve the exchange between Dy ions. Work supported by ARO W911NF-13-1-1025 (CCNY) and NSF-DMR-1309202 (NYU).

Orthogonal interfacial exchange coupling in GaMnAsP/GaMnAs bilayers

NASA Astrophysics Data System (ADS)

Li, Xiang; Bac, Seul-Ki; Dong, Sining; Liu, Xinyu; Lee, Sanghoon; Rouvimov, Sergei; Dobrowolska, Margaret; Furdyna, Jacek K.

2018-05-01

We carried out a systematic study of magnetic ordering and magnetic interlayer coupling in Ga1-xMnxAs1-yPy/Ga1-xMnxAs bilayers using superconducting quantum interference device magnetometry and ferromagnetic resonance. Such bilayers are interesting, because the easy axis of the constituent materials are orthogonal. Our results show that the bilayers are strongly exchange-coupled at the interface, that manifests itself in the form of horizontal exchange-bias-like shifts of the hysteresis loops of the Ga1-xMnxAs layer, as observed in field-cooled magnetic measurements.

Magnetosphere-ionosphere interactions: Near Earth manifestations of the plasma universe

NASA Technical Reports Server (NTRS)

Faelthammar, Carl-Gunne

1986-01-01

As the universe consists almost entirely of plasma, the understanding of astrophysical phenomena must depend critically on the understanding of how matter behaves in the plasma state. In situ observations in the near Earth cosmical plasma offer an excellent opportunity of gaining such understanding. The near Earth cosmical plasma not only covers vast ranges of density and temperature, but is the site of a rich variety of complex plasma physical processes which are activated as a results of the interactions between the magnetosphere and the ionosphere. The geomagnetic field connects the ionosphere, tied by friction to the Earth, and the magnetosphere, dynamically coupled to the solar wind. This causes an exchange of energy an momentum between the two regions. The exchange is executed by magnetic-field-aligned electric currents, the so-called Birkeland currents. Both directly and indirectly (through instabilities and particle acceleration) these also lead to an exchange of plasma, which is selective and therefore causes chemical separation. Another essential aspect of the coupling is the role of electric fields, especially magnetic field aligned (parallel) electric fields, which have important consequences both for the dynamics of the coupling and, especially, for energization of charged particles.

Interfacial exchange coupling and magnetization reversal in perpendicular [Co/Ni]N/TbCo composite structures.

PubMed

Tang, M H; Zhang, Zongzhi; Tian, S Y; Wang, J; Ma, B; Jin, Q Y

2015-06-15

Interfacial exchange coupling and magnetization reversal characteristics in the perpendicular heterostructures consisting of an amorphous ferrimagnetic (FI) TbxCo(100-x) alloy layer exchange-coupled with a ferromagnetic (FM) [Co/Ni]N multilayer have been investigated. As compared with pure TbxCo(100-x) alloy, the magnetization compensation composition of the heterostructures shift to a higher Tb content, implying Co/Ni also serves to compensate the Tb moment in TbCo layer. The net magnetization switching field Hc⊥ and interlayer interfacial coupling field Hex, are not only sensitive to the magnetization and thickness of the switched TbxCo(100-x) or [Co/Ni]N layer, but also to the perpendicular magnetic anisotropy strength of the pinning layer. By tuning the layer structure we achieve simultaneously both large Hc⊥ = 1.31 T and Hex = 2.19 T. These results, in addition to the fundamental interest, are important to understanding of the interfacial coupling interaction in the FM/FI heterostructures, which could offer the guiding of potential applications in heat-assisted magnetic recording or all-optical switching recording technique.

Calculations of Exchange Bias in Thin Films with Ferromagnetic/Antiferromagnetic Interfaces

NASA Astrophysics Data System (ADS)

Koon, N. C.

1997-06-01

A microscopic explanation of exchange bias in thin films with compensated ferro/antiferromagnetic interfaces is presented. Full micromagnetic calculations show the interfacial exchange coupling to be relatively strong with a perpendicular orientation between the ferro/antiferromagnetic axis directions, similar to the classic ``spin-flop'' state in bulk antiferromagnets. With reasonable parameters the calculations predict bias fields comparable to those observed and provide a possible explanation for both anomalous high field rotational hysteresis and recently discovered ``positive'' exchange bias.

Electric field induced reversible 180° magnetization switching through tuning of interfacial exchange bias along magnetic easy-axis in multiferroic laminates

PubMed Central

Xue, Xu; Zhou, Ziyao; Peng, Bin; Zhu, Mingmin; Zhang, Yijun; Ren, Wei; Ren, Tao; Yang, Xi; Nan, Tianxiang; Sun, Nian X.; Liu, Ming

2015-01-01

E-field control of interfacial exchange coupling and deterministic switching of magnetization have been demonstrated in two sets of ferromagnetic(FM)/antiferromagnetic(AFM)/ferroelectric(FE) multiferroic heterostructures, including NiFe/NiCoO/glass/PZN-PT (011) and NiFe/FeMn/glass/PZN-PT (011). We designed this experiment to achieve exchange bias tuning along the magnetic easy axis, which is critical for realizing reversible 180° magnetization deterministic switching at zero or small magnetic bias. Strong exchange coupling were established across AFM-FM interfaces, which plays an important role in voltage control of magnetization switching. Through the competition between the E-field induced uniaxial anisotropy in ferromagnetic layer and unidirectional anisotropy in antiferromagnetic layer, the exchange bias was significantly shifted by up to |∆Hex|/Hex = 8% in NiFe/FeMn/glass/PZN-PT (011) and 13% in NiFe/NiCoO/glass/PZN-PT (011). In addition, the square shape of the hysteresis loop, as well as a strong shape tunability of |∆Hex|/Hc = 67.5 ~ 125% in NiFe/FeMn/glass/PZN-PT and 30 ~ 38% in NiFe/NiCoO/glass/PZN-PT were achieved, which lead to a near 180° magnetization switching. Electrical tuning of interfacial exchange coupling in FM/AFM/FE systems paves a new way for realizing magnetoelectric random access memories and other memory technologies. PMID:26576658

Electric field induced reversible 180° magnetization switching through tuning of interfacial exchange bias along magnetic easy-axis in multiferroic laminates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xue, Xu; Zhou, Ziyao; Peng, Bin

2015-11-18

E-field control of interfacial exchange coupling and deterministic switching of magnetization have been demonstrated in two sets of ferromagnetic(FM)/antiferromagnetic(AFM)/ferroelectric(FE) multiferroic heterostructures, including NiFe/NiCoO/glass/PZN-PT (011) and NiFe/FeMn/glass/PZN-PT (011). We designed this experiment to achieve exchange bias tuning along the magnetic easy axis, which is critical for realizing reversible 180° magnetization deterministic switching at zero or small magnetic bias. Strong exchange coupling were established across AFM-FM interfaces, which plays an important role in voltage control of magnetization switching. Through the competition between the E-field induced uniaxial anisotropy in ferromagnetic layer and unidirectional anisotropy in antiferromagnetic layer, the exchange bias was significantly shiftedmore » by up to |ΔH ex|/H ex=8% in NiFe/FeMn/glass/PZN-PT (011) and 13% in NiFe/NiCoO/glass/PZN-PT (011). In addition, the square shape of the hysteresis loop, as well as a strong shape tunability of |ΔH ex|/H c=67.5~125% in NiFe/FeMn/glass/PZN-PT and 30~38% in NiFe/NiCoO/glass/PZN-PT were achieved, which lead to a near 180° magnetization switching. Lastly, electrical tuning of interfacial exchange coupling in FM/AFM/FE systems paves a new way for realizing magnetoelectric random access memories and other memory technologies.« less

Electric field induced reversible 180° magnetization switching through tuning of interfacial exchange bias along magnetic easy-axis in multiferroic laminates

NASA Astrophysics Data System (ADS)

Xue, Xu; Zhou, Ziyao; Peng, Bin; Zhu, Mingmin; Zhang, Yijun; Ren, Wei; Ren, Tao; Yang, Xi; Nan, Tianxiang; Sun, Nian X.; Liu, Ming

2015-11-01

E-field control of interfacial exchange coupling and deterministic switching of magnetization have been demonstrated in two sets of ferromagnetic(FM)/antiferromagnetic(AFM)/ferroelectric(FE) multiferroic heterostructures, including NiFe/NiCoO/glass/PZN-PT (011) and NiFe/FeMn/glass/PZN-PT (011). We designed this experiment to achieve exchange bias tuning along the magnetic easy axis, which is critical for realizing reversible 180° magnetization deterministic switching at zero or small magnetic bias. Strong exchange coupling were established across AFM-FM interfaces, which plays an important role in voltage control of magnetization switching. Through the competition between the E-field induced uniaxial anisotropy in ferromagnetic layer and unidirectional anisotropy in antiferromagnetic layer, the exchange bias was significantly shifted by up to |ΔHex|/Hex = 8% in NiFe/FeMn/glass/PZN-PT (011) and 13% in NiFe/NiCoO/glass/PZN-PT (011). In addition, the square shape of the hysteresis loop, as well as a strong shape tunability of |ΔHex|/Hc = 67.5 ~ 125% in NiFe/FeMn/glass/PZN-PT and 30 ~ 38% in NiFe/NiCoO/glass/PZN-PT were achieved, which lead to a near 180° magnetization switching. Electrical tuning of interfacial exchange coupling in FM/AFM/FE systems paves a new way for realizing magnetoelectric random access memories and other memory technologies.

Electric field induced reversible 180° magnetization switching through tuning of interfacial exchange bias along magnetic easy-axis in multiferroic laminates.

PubMed

Xue, Xu; Zhou, Ziyao; Peng, Bin; Zhu, Mingmin; Zhang, Yijun; Ren, Wei; Ren, Tao; Yang, Xi; Nan, Tianxiang; Sun, Nian X; Liu, Ming

2015-11-18

E-field control of interfacial exchange coupling and deterministic switching of magnetization have been demonstrated in two sets of ferromagnetic(FM)/antiferromagnetic(AFM)/ferroelectric(FE) multiferroic heterostructures, including NiFe/NiCoO/glass/PZN-PT (011) and NiFe/FeMn/glass/PZN-PT (011). We designed this experiment to achieve exchange bias tuning along the magnetic easy axis, which is critical for realizing reversible 180° magnetization deterministic switching at zero or small magnetic bias. Strong exchange coupling were established across AFM-FM interfaces, which plays an important role in voltage control of magnetization switching. Through the competition between the E-field induced uniaxial anisotropy in ferromagnetic layer and unidirectional anisotropy in antiferromagnetic layer, the exchange bias was significantly shifted by up to |∆Hex|/Hex = 8% in NiFe/FeMn/glass/PZN-PT (011) and 13% in NiFe/NiCoO/glass/PZN-PT (011). In addition, the square shape of the hysteresis loop, as well as a strong shape tunability of |∆Hex|/Hc = 67.5 ~ 125% in NiFe/FeMn/glass/PZN-PT and 30 ~ 38% in NiFe/NiCoO/glass/PZN-PT were achieved, which lead to a near 180° magnetization switching. Electrical tuning of interfacial exchange coupling in FM/AFM/FE systems paves a new way for realizing magnetoelectric random access memories and other memory technologies.

Spin- and valley-dependent electronic band structure and electronic heat capacity of ferromagnetic silicene in the presence of strain, exchange field and Rashba spin-orbit coupling

NASA Astrophysics Data System (ADS)

Hoi, Bui Dinh; Yarmohammadi, Mohsen; Kazzaz, Houshang Araghi

2017-10-01

We studied how the strain, induced exchange field and extrinsic Rashba spin-orbit coupling (RSOC) enhance the electronic band structure (EBS) and electronic heat capacity (EHC) of ferromagnetic silicene in presence of external electric field (EF) by using the Kane-Mele Hamiltonian, Dirac cone approximation and the Green's function approach. Particular attention is paid to investigate the EHC of spin-up and spin-down bands at Dirac K and K‧ points. We have varied the EF, strain, exchange field and RSOC to tune the energy of inter-band transitions and consequently EHC, leading to very promising features for future applications. Evaluation of EF exhibits three phases: Topological insulator (TI), valley-spin polarized metal (VSPM) and band insulator (BI) at given aforementioned parameters. As a new finding, we have found a quantum anomalous Hall phase in BI regime at strong RSOCs. Interestingly, the effective mass of carriers changes with strain, resulting in EHC behaviors. Here, exchange field has the same behavior with EF. Finally, we have confirmed the reported and expected symmetry results for both Dirac points and spins with the study of valley-dependent EHC.

Role of the antiferromagnetic pinning layer on spin wave properties in IrMn/NiFe based spin-valves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gubbiotti, G., E-mail: gubbiotti@fisica.unipg.it; Tacchi, S.; Del Bianco, L.

2015-05-07

Brillouin light scattering (BLS) was exploited to study the spin wave properties of spin-valve (SV) type samples basically consisting of two 5 nm-thick NiFe layers (separated by a Cu spacer of 5 nm), differently biased through the interface exchange coupling with an antiferromagnetic IrMn layer. Three samples were investigated: a reference SV sample, without IrMn (reference); one sample with an IrMn underlayer (10 nm thick) coupled to the bottom NiFe film; one sample with IrMn underlayer and overlayer of different thickness (10 nm and 6 nm), coupled to the bottom and top NiFe film, respectively. The exchange coupling with the IrMn, causing the insurgence ofmore » the exchange bias effect, allowed the relative orientation of the NiFe magnetization vectors to be controlled by an external magnetic field, as assessed through hysteresis loop measurements by magneto-optic magnetometry. Thus, BLS spectra were acquired by sweeping the magnetic field so as to encompass both the parallel and antiparallel alignment of the NiFe layers. The BLS results, well reproduced by the presented theoretical model, clearly revealed the combined effects on the spin dynamic properties of the dipolar interaction between the two NiFe films and of the interface IrMn/NiFe exchange coupling.« less

Nuclear magnetic relaxation by the dipolar EMOR mechanism: General theory with applications to two-spin systems.

PubMed

Chang, Zhiwei; Halle, Bertil

2016-02-28

In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. We have embarked on a systematic program to develop, from the stochastic Liouville equation, a general and rigorous theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole coupling strengths, and Larmor frequencies. Here, we present a general theoretical framework applicable to spin systems of arbitrary size with symmetric or asymmetric exchange. So far, the dipolar EMOR theory is only available for a two-spin system with symmetric exchange. Asymmetric exchange, when the spin system is fragmented by the exchange, introduces new and unexpected phenomena. Notably, the anisotropic dipole couplings of non-exchanging spins break the axial symmetry in spin Liouville space, thereby opening up new relaxation channels in the locally anisotropic sites, including longitudinal-transverse cross relaxation. Such cross-mode relaxation operates only at low fields; at higher fields it becomes nonsecular, leading to an unusual inverted relaxation dispersion that splits the extreme-narrowing regime into two sub-regimes. The general dipolar EMOR theory is illustrated here by a detailed analysis of the asymmetric two-spin case, for which we present relaxation dispersion profiles over a wide range of conditions as well as analytical results for integral relaxation rates and time-dependent spin modes in the zero-field and motional-narrowing regimes. The general theoretical framework presented here will enable a quantitative analysis of frequency-dependent water-proton longitudinal relaxation in model systems with immobilized macromolecules and, ultimately, will provide a rigorous link between relaxation-based magnetic resonance image contrast and molecular parameters.

Nuclear magnetic relaxation by the dipolar EMOR mechanism: General theory with applications to two-spin systems

NASA Astrophysics Data System (ADS)

Chang, Zhiwei; Halle, Bertil

2016-02-01

In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. We have embarked on a systematic program to develop, from the stochastic Liouville equation, a general and rigorous theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole coupling strengths, and Larmor frequencies. Here, we present a general theoretical framework applicable to spin systems of arbitrary size with symmetric or asymmetric exchange. So far, the dipolar EMOR theory is only available for a two-spin system with symmetric exchange. Asymmetric exchange, when the spin system is fragmented by the exchange, introduces new and unexpected phenomena. Notably, the anisotropic dipole couplings of non-exchanging spins break the axial symmetry in spin Liouville space, thereby opening up new relaxation channels in the locally anisotropic sites, including longitudinal-transverse cross relaxation. Such cross-mode relaxation operates only at low fields; at higher fields it becomes nonsecular, leading to an unusual inverted relaxation dispersion that splits the extreme-narrowing regime into two sub-regimes. The general dipolar EMOR theory is illustrated here by a detailed analysis of the asymmetric two-spin case, for which we present relaxation dispersion profiles over a wide range of conditions as well as analytical results for integral relaxation rates and time-dependent spin modes in the zero-field and motional-narrowing regimes. The general theoretical framework presented here will enable a quantitative analysis of frequency-dependent water-proton longitudinal relaxation in model systems with immobilized macromolecules and, ultimately, will provide a rigorous link between relaxation-based magnetic resonance image contrast and molecular parameters.

The role of the (111) texture on the exchange bias and interlayer coupling effects observed in sputtered NiFe/IrMn/Co trilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Castro, I. L.; Nascimento, V. P.; Passamani, E. C.

2013-05-28

Magnetic properties of sputtered NiFe/IrMn/Co trilayers grown on different seed layers (Cu or Ta) deposited on Si (100) substrates were investigated by magnetometry and ferromagnetic resonance measurements. Exchange bias effect and magnetic spring behavior have been studied by changing the IrMn thickness. As shown by X-ray diffraction, Ta and Cu seed layers provoke different degrees of (111) fcc-texture that directly affect the exchange bias and indirectly modify the exchange spring coupling behavior. Increasing the IrMn thickness, it was observed that the coupling angle between the Co and NiFe ferromagnetic layers increases for the Cu seed system, but it reduces formore » the Ta case. The results were explained considering (i) different anisotropies of the Co and IrMn layers induced by the different degree of the (111) texture and (ii) the distinct exchange bias set at the NiFe/IrMn and IrMn/Co interfaces in both systems. The NiFe and Co interlayer coupling angle is strongly correlated with both exchange bias and exchange magnetic spring phenomena. It was also shown that the highest exchange bias field occurs when an unstressed L1{sub 2} IrMn structure is stabilized.« less

Room temperature exchange bias in multiferroic BiFeO3 nano- and microcrystals with antiferromagnetic core and two-dimensional diluted antiferromagnetic shell

NASA Astrophysics Data System (ADS)

Zhang, Chuang; Wang, Shou Yu; Liu, Wei Fang; Xu, Xun Ling; Li, Xiu; Zhang, Hong; Gao, Ju; Li, De Jun

2017-05-01

Exchange bias (EB) of multiferroics presents many potential opportunities for magnetic devices. However, instead of using low-temperature field cooling in the hysteresis loop measurement, which usually shows an effective approach to obtain obvious EB phenomenon, there are few room temperature EB. In this article, extensive studies on room temperature EB without field cooling were observed in BiFeO3 nano- and microcrystals. Moreover, with increasing size the hysteresis loops shift from horizontal negative exchange bias (NEB) to positive exchange bias (PEB). In order to explain the tunable EB behaviors with size dependence, a phenomenological qualitative model based on the framework of antiferromagnetic (AFM) core-two-dimensional diluted antiferromagnet in a field (2D-DAFF) shell structure was proposed. The training effect (TE) ascertained the validity of model and the presence of unstable magnetic structure using Binek's model. Experimental results show that the tunable EB effect can be explained by the competition of ferromagnetic (FM) exchange coupling and AFM exchange coupling interaction between AFM core and 2D-DAFF shell. Additionally, the local distortion of lattice fringes was observed in hexagonal-shaped BiFeO3 nanocrystals with well-dispersed behavior. The electrical conduction properties agreed well with the space charge-limited conduction mechanism.

Electric field effect on exchange interaction in ultrathin Co films with ionic liquids

NASA Astrophysics Data System (ADS)

Ishibashi, Mio; Yamada, Kihiro T.; Shiota, Yoichi; Ando, Fuyuki; Koyama, Tomohiro; Kakizakai, Haruka; Mizuno, Hayato; Miwa, Kazumoto; Ono, Shimpei; Moriyama, Takahiro; Chiba, Daichi; Ono, Teruo

2018-06-01

Electric-field modulations of magnetic properties have been extensively studied not only for practical applications but also for fundamental interest. In this study, we investigated the electric field effect on the exchange interaction in ultrathin Co films with ionic liquids. The exchange coupling J was characterized from the direct magnetization measurement as a function of temperature using Pt/ultrathin Co/MgO structures. The trend of the electric field effect on J is in good agreement with that of the theoretical prediction, and a large change in J by applying a gate voltage was observed by forming an electric double layer using ionic liquids.

Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores.

PubMed

Malkin, B Z; Lummen, T T A; van Loosdrecht, P H M; Dhalenne, G; Zakirov, A R

2010-07-14

The experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R(2)Ti(2)O(7) (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.

Perspectives of voltage control for magnetic exchange bias in multiferroic heterostructures

NASA Astrophysics Data System (ADS)

Yang, Q.; Zhou, Z.; Sun, N. X.; Liu, M.

2017-04-01

Exchange bias, as an internal magnetic bias induced by a ferromagnetic-antiferromagnetic exchange coupling, is extremely important in many magnetic applications such as memories, sensors and other devices. Voltage control of exchange bias in multiferroics provides an energy-efficient way to achieve a rapidly 180° deterministic switching of magnetization, which has been considered as a key challenge in realizing next generation of fast, compact and ultra-low power magnetoelectric memories and sensors. Additionally, exchange bias can enhance dynamic magnetoelectric coupling strength in an external-field-free manner. In this paper, we provide a perspective on voltage control of exchange bias in different multiferroic heterostructures. Brief mechanization and related experiments are discussed as well as future trend and challenges that can be overcome by electrically tuning of exchange bias in state-of-the-art magnetoelectric devices.

Monte Carlo Study of Magnetic Properties of Mixed Spins in a Fullerene X 30 Y 30-Like Structure

NASA Astrophysics Data System (ADS)

Mhirech, A.; Aouini, S.; Alaoui-Ismaili, A.; Bahmad, L.

2018-03-01

In this work, inspiring form of the fullerene-C60 structures, we study the mixed X_{30} Y_{30} fullerene-like structure and investigate its magnetic properties. In a such a structure, the carbons are assumed to be replaced by magnetic atoms having spin moments σ = 1/2 and S = 1. Firstly, we elaborate the ground-state phase diagrams in different physical parameter planes. In a second stage, we investigate the exchange coupling interaction effects in the absence or presence of both external magnetic and crystal fields. Using the Monte Carlo method, we carried out a study of the system magnetic properties and the thermal behavior of such a system for the ferromagnetic case. It is found that the critical temperature increases when increasing the coupling exchange interactions. On the other hand, the coercive magnetic field increases also when increasing the coupling exchange interactions. However, this physical parameter decreases when increasing the reduced temperature.

Landau-Lifshitz-Bloch equation for exchange-coupled grains

NASA Astrophysics Data System (ADS)

Vogler, Christoph; Abert, Claas; Bruckner, Florian; Suess, Dieter

2014-12-01

Heat-assisted recording is a promising technique to further increase the storage density in hard disks. Multilayer recording grains with graded Curie temperature is discussed to further assist the write process. Describing the correct magnetization dynamics of these grains, from room temperature to far above the Curie point, during a write process is required for the calculation of bit error rates. We present a coarse-grained approach based on the Landau-Lifshitz-Bloch (LLB) equation to model exchange-coupled grains with low computational effort. The required temperature-dependent material properties such as the zero-field equilibrium magnetization as well as the parallel and normal susceptibilities are obtained by atomistic Landau-Lifshitz-Gilbert simulations. Each grain is described with one magnetization vector. In order to mimic the atomistic exchange interaction between the grains a special treatment of the exchange field in the coarse-grained approach is presented. With the coarse-grained LLB model the switching probability of a recording grain consisting of two layers with graded Curie temperature is investigated in detail by calculating phase diagrams for different applied heat pulses and external magnetic fields.

Exchange bias for core/shell magnetic nanoparticles

NASA Astrophysics Data System (ADS)

Lemos, C. G. O.; Figueiredo, W.; Santos, M.

2015-09-01

We study the properties of a finite magnetic system to model a magnetic nanoparticle, which is formed by a reduced number of magnetic dipole moments due to the spin of the atoms. The nanoparticle is of the type core/shell where the shell is formed by spins interacting through an antiferromagnetic exchange coupling while for the spins belonging to the core the coupling is ferromagnetic. The interaction between the spins at the interface core/shell can be either ferro or antiferromagnetic. To describe the states of the spins we used the XY model in which the spins are considered as continuous variables, free to point in any direction of the xy plane. We also consider a magnetocrystalline anisotropy, exchange anisotropy and the Zeeman effect. Our model is studied in a lattice with square symmetry, using the Monte Carlo method along with the Metropolis prescription. The results show that in the absence of an external magnetic field and exchange anisotropy, the system continuously goes to a disordered state from an ordered state at a well defined temperature. In the presence of external magnetic fields the system displays the exchange bias phenomenon, that is, the displacement of the hysteresis loops, due to the introduction of the exchange anisotropy term. However, this displacement depends on the core and shell sizes, as well as on the magnitude of the coupling between the shell and the core moments.

Effect of nano oxide layer on exchange bias and GMR in Mn-Ir-Pt based spin valve

NASA Astrophysics Data System (ADS)

Jeon, D. M.; Lee, J. P.; Lee, D. H.; Yoon, S. Y.; Kim, Y. S.; Suh, S. J.

2004-05-01

We have investigated the effect of nano oxide layers (NOLs), which were fabricated by a plasma oxidation of CoFe layer on the magnetic properties and magneto-resistance (MR) in a Mn-Ir-Pt based spin valve. The adjusted NOL could result in the high MR and the strong exchange coupling field ( Hex). From a high resolution electron microscopy analysis the oxide was about 1 nm. The strong reflectivity at the interface of a free and oxide capping layer should lead to the decrease of an interlayer coupling field, which could possibly improve the Hex.

Co nanoparticles inserted into a porous carbon amorphous matrix: the role of cooling field and temperature on the exchange bias effect.

PubMed

Fernández-García, María Paz; Gorria, Pedro; Sevilla, Marta; Fuertes, Antonio B; Boada, Roberto; Chaboy, Jesús; Aquilanti, Giuliana; Blanco, Jesús A

2011-01-21

We report unusual cooling field dependence of the exchange bias in oxide-coated cobalt nanoparticles embedded within the nanopores of a carbon matrix. The size-distribution of the nanoparticles and the exchange bias coupling observed up to about 200 K between the Co-oxide shell (∼3-4 nm) and the ferromagnetic Co-cores (∼4-6 nm) are the key to understand the magnetic properties of this system. The estimated values of the effective anisotropy constant and saturation magnetization obtained from the fit of the zero-field cooling and field cooling magnetization vs. temperature curves agree quite well with those of the bulk fcc-Co.

First-order exchange coefficient coupling for simulating surface water-groundwater interactions: Parameter sensitivity and consistency with a physics-based approach

USGS Publications Warehouse

Ebel, B.A.; Mirus, B.B.; Heppner, C.S.; VanderKwaak, J.E.; Loague, K.

2009-01-01

Distributed hydrologic models capable of simulating fully-coupled surface water and groundwater flow are increasingly used to examine problems in the hydrologic sciences. Several techniques are currently available to couple the surface and subsurface; the two most frequently employed approaches are first-order exchange coefficients (a.k.a., the surface conductance method) and enforced continuity of pressure and flux at the surface-subsurface boundary condition. The effort reported here examines the parameter sensitivity of simulated hydrologic response for the first-order exchange coefficients at a well-characterized field site using the fully coupled Integrated Hydrology Model (InHM). This investigation demonstrates that the first-order exchange coefficients can be selected such that the simulated hydrologic response is insensitive to the parameter choice, while simulation time is considerably reduced. Alternatively, the ability to choose a first-order exchange coefficient that intentionally decouples the surface and subsurface facilitates concept-development simulations to examine real-world situations where the surface-subsurface exchange is impaired. While the parameters comprising the first-order exchange coefficient cannot be directly estimated or measured, the insensitivity of the simulated flow system to these parameters (when chosen appropriately) combined with the ability to mimic actual physical processes suggests that the first-order exchange coefficient approach can be consistent with a physics-based framework. Copyright ?? 2009 John Wiley & Sons, Ltd.

Dipole-induced exchange bias.

PubMed

Torres, Felipe; Morales, Rafael; Schuller, Ivan K; Kiwi, Miguel

2017-11-09

The discovery of dipole-induced exchange bias (EB), switching from negative to positive sign, is reported in systems where the antiferromagnet and the ferromagnet are separated by a paramagnetic spacer (AFM-PM-FM). The magnitude and sign of the EB is determined by the cooling field strength and the PM thickness. The same cooling field yields negative EB for thin spacers, and positive EB for thicker ones. The EB decay profile as a function of the spacer thickness, and the change of sign, are attributed to long-ranged dipole coupling. Our model, which accounts quantitatively for the experimental results, ignores the short range interfacial exchange interactions of the usual EB theories. Instead, it retains solely the long range dipole field that allows for the coupling of the FM and AFM across the PM spacer. The experiments allow for novel switching capabilities of long range EB systems, while the theory allows description of the structures where the FM and AFM are not in atomic contact. The results provide a new approach to design novel interacting heterostructures.

Symmetric and asymmetric exchange stiffnesses of transition-metal thin film interfaces in external electric field

NASA Astrophysics Data System (ADS)

Nakamura, K.; Pradipto, A.-M.; Akiyama, T.; Ito, T.; Oguchi, T.; Weinert, M.

2018-07-01

The electric-field induced modifications of the symmetric and asymmetric exchange stiffness constants for the prototypical transition-metal system of a Co monolayer on Pt(111) are determined from first-principles calculated total energy differences of spin-spiral states with oppositely rotating magnetizations in the presence of both the external field and spin-orbit coupling. The trend underlying the modifications is shown to be linked to orbital magnetism. The results demonstrate that an electric field may be a promising approach to manipulate macroscopically magnetic textures.

Induced magnetic structure in exchange-coupled ferro-/antiferromagnet thin films

NASA Astrophysics Data System (ADS)

Morales, Rafael

2007-03-01

The most prominent feature observed in exchange-coupled ferromagnetic/ antiferromagnetic (FM/AF) bilayers is the so-called exchange bias field (HEB), i.e. the shift of the hysteresis loop along the magnetic field axis. However the exchange bias phenomenon can induce other interesting effects on the FM. In this talk we show two methods to establish a bi-domain state in the FM, due to the coexistence of domains with opposite sign of HEB [1-3]. Magneto-optical, polarized neutron and soft X-ray measurements show that this lateral structure becomes more complex for low magnetocrystalline anisotropy materials where a spin depth profile is created in the FM due to the exchange coupling with the AF [4-6]. The internal magnetic structure in the AF and its role on exchange bias has also been investigated using FM/AF/FM trilayers. These studies demonstrate that the bulk spin configuration in the AF plays a crucial role in the pinning of uncompensated spins at the interface thus determining the HEB . Supported by the US-DOE, European Marie-Curie-OIF and the Alfred P. Sloan Foundation. [1] O. Petracic et al. Appl. Phys. Lett. 87, 222509 (2005) [2] I. V. Roshchin et al. Europhys. Lett. 71, 297 (2005) [3] J. Olamit et al. Phys. Rev. B 72, 012408 (2005) [4] R. Morales et al. Appl. Phys. Lett. 89, 072504 (2006) [5] S. Roy et al. Phys. Rev. Lett. 95, 047201 (2005) [6] Z-P. Li et al. Phys. Rev. Lett. 96, 217205 (2006)

Interfacial coupling in multiferroic BiFeO3 and ferromagnetic La2/3Sr1/3MnO3 thin films

NASA Astrophysics Data System (ADS)

Dominguez, C.; E Ordoñez, J.; E Gomez, M.

2017-12-01

Antiferromagnetic/Ferromagnetic coupling mechanics have been studying by growing successfully BiFeO3/La2/3Sr1/3MnO3 bilayers on SrTiO3 single crystals by using rf and dc sputtering technique at pure oxygen pressures. We have investigated the magnetic behaviour of this samples, field cooling loops evidence interfacial coupling effect when antiferromagnetic ferroelectric BiFeO3 is placed in contact with ferromagnetic La2/3Sr1/3MnO3 indicate by the shift of the magnetization loop to negative values of the applied magnetic field. Our samples exhibited an exchange bias of 76Oe at 5K after field cooling the sample under 5000Oe. Temperature dependence of the exchange bias field showed exponential decay. The BFO/LSMO bilayer exhibits excellent ferroelectric behaviour (Ps=65μC/cm2 at 4V and 100Hz). Coexistence of ferroelectric and ferromagnetic properties in the BFO/LSMO bilayer make it a promising candidate system for applications where the magnetoelectric behaviour is required.

Exchange bias in zinc ferrite-FeNiMoB based metallic glass composite thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

R, Lisha; P, Geetha; B, Aravind P.

2015-06-24

The Exchange bias phenomenon and methods to manipulate the bias field in a controlled manner are thrust areas in magnetism due to its sophisticated theoretical concepts as well as advanced technological utility in the field of spintronics. The Exchange bias effect is observed as a result of ferromagnetic-antiferromagnetic (FM-AFM) exchange interaction, usually observed as a loop shift on field cooling below the Neel temperature of AFM. In the present study, we have chosen zinc ferrite which is a well known antiferromagnet, and FeNiMoB based metallic glass as the ferromagnet. The films were prepared by RF sputtering technique. The thickness andmore » composition was obtained by RBS. The magnetic studies using SQUID VSM indicate exchange bias effect in the system. The effect of thermal annealing on exchange bias effect was studied. The observed exchange bias in the zinc ferrite-FeNiMoB system is not due to FM-AFM coupling but due to spin glass-ferromagnetic interaction.« less

Exchange bias in zinc ferrite-FeNiMoB based metallic glass composite thin films

NASA Astrophysics Data System (ADS)

R, Lisha; T, Hysen; P, Geetha; B, Aravind P.; Ojha, S.; Avasthi, D. K.; Ramanujan, R. V.; Anantharaman, M. R.

2015-06-01

The Exchange bias phenomenon and methods to manipulate the bias field in a controlled manner are thrust areas in magnetism due to its sophisticated theoretical concepts as well as advanced technological utility in the field of spintronics. The Exchange bias effect is observed as a result of ferromagnetic-antiferromagnetic (FM-AFM) exchange interaction, usually observed as a loop shift on field cooling below the Neel temperature of AFM. In the present study, we have chosen zinc ferrite which is a well known antiferromagnet, and FeNiMoB based metallic glass as the ferromagnet. The films were prepared by RF sputtering technique. The thickness and composition was obtained by RBS. The magnetic studies using SQUID VSM indicate exchange bias effect in the system. The effect of thermal annealing on exchange bias effect was studied. The observed exchange bias in the zinc ferrite-FeNiMoB system is not due to FM-AFM coupling but due to spin glass-ferromagnetic interaction.

Large exchange-dominated domain wall velocities in antiferromagnetically coupled nanowires

NASA Astrophysics Data System (ADS)

Kuteifan, Majd; Lubarda, M. V.; Fu, S.; Chang, R.; Escobar, M. A.; Mangin, S.; Fullerton, E. E.; Lomakin, V.

2016-04-01

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for methods of information storage and processing. A major obstacle for their practical use is the domain-wall velocity, which is traditionally limited for low fields and currents due to the Walker breakdown occurring when the driving component reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for large domain-wall velocities driven by field and/or current as compared to conventional nanowires.

Finite-size effects and magnetic exchange coupling in thin CoO layers

NASA Astrophysics Data System (ADS)

Ambrose, Thomas Francis

Finite size effects in CoO have been observed in CoO/SiOsb2 multilayers. The Neel temperatures of the CoO layers, as determined by dc susceptibility measurements, follow a finite-size scaling relation with a shift exponent lambda = 1.55 ± 0.05. This determined exponent is close to the theoretical value for finite size scaling in an Ising system. The value of the zero temperature correlation length has also been determined to be 18A, while antiferromagnetic ordering persists down to a CoO layer thickness of 10A. The properties of exchange biasing have been extensively studied in NiFe/CoO bilayers. The effects of the cooling field (Hsb{FC}), up to 50 kOe, on the resultant exchange field (Hsb{E}) and coercivity (Hsb{C}) have been examined. The value of Hsb{E} increases rapidly at low cooling fields (Hsb{FC} < 1kOe) and levels off for Hsb{FC} larger than 4 kOe. The value of Hsb{C} also depends upon Hsb{FC}, but less sensitively. The bilayer thickness also influences exchange biasing. We find that Hsb{E} varies inversely proprotional to both tsb{FM} and tsb{AF} where tsb{FM} and tsb{AF} are the ferromagnetic and antiferromagnetic layer thickness respectively. Because of the 1/tsb{AF}, the simple picture of interfacial coupling between ferromagnet and antiferromagnet spins appears to be inadequate. The assertion of long range coupling between ferromagnetic and antiferromagnetic layers has been verified by the observation of antiferromagnetic exchange coupling across spacer layers in NiFe/NM/CoO trilayers, where NM is a non-magnetic material. Exchange biasing has been observed in trilayers with metallic spacer layers up to 50A thick using Ag, Cu and Au, while no exchange field was observed for insulating spacer layers of any thickness using Alsb2Osb3, SiOsb2 and MgO. The temperature dependence of Hsb{E} and Hsb{C} and the effect of the deposition order have been studied in a series of bilayer (NiFe/CoO and CoO/NiFe) and trilayer (NiFe/CoO/NiFe) films. A profound difference in Hsb{E} was observed in samples with NiFe deposited on top of CoO compared to samples with CoO deposited on top of NiFe. When CoO is on top of NiFe Hsb{E} varies linearly with temperature, while for samples with NiFe on top of CoO Hsb{E} has a plateau followed by a rapid decrease. These distinct temperature dependences have been reproduced in NiFe/CoO/NiFe trilayers which contain both geometries. Structural analysis using Transmission Electron Microscopy indicate no apparent differences in the top and bottom interfaces. The angular dependence of the exchange coupling in a NiFe/CoO bilayer has been measured. Both Hsb{E} and Hsb{C} with unidirectional and uniaxial characteristics, respectively, are integral parts of the exchange coupling. The values of Hsb{E} can be expressed by a series of odd angle cosine terms, while the values of Hsb{C} can be expressed by a series of even angle cosine terms. Finally, exchange biasing has been used to "spin engineer" ferromagnetic layers in NiFe/CoO/NiFe trilayers. Four different spin structures have been observed. A phase diagram, for the four spin structures and the conditions with which each spin structure is obtained, has been determined. (Abstract shortened by UMI.)

Effect of exchange coupling on magnetic property in Sm-Co/α-Fe layered system

NASA Astrophysics Data System (ADS)

C, X. Sang; G, P. Zhao; W, X. Xia; X, L. Wan; F, J. Morvan; X, C. Zhang; L, H. Xie; J, Zhang; J, Du; A, R. Yan; P, Liu

2016-03-01

The hysteresis loops as well as the spin distributions of Sm-Co/α-Fe bilayers have been investigated by both three-dimensional (3D) and one-dimensional (1D) micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses ts. The exchange coupling coefficient Ahs between the hard and soft layers varies from 1.8 × 10-6 erg/cm to 0.45 × 10-6 erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm-Co/α-Fe multilayers. Project supported by the National Natural Science Foundation of China (Grant Nos. 11074179 and 10747007), the National Basic Research Program of China (Grant No. 2014CB643702), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14E010006), the Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan Province, China (Grant No. 12TD008), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of the Education Ministry, China, and the Program for Key Science and Technology Innovation Team of Zhejiang Province, China (Grant No. 2013TD08).

General formalism of local thermodynamics with an example: Quantum Otto engine with a spin-1/2 coupled to an arbitrary spin.

PubMed

Altintas, Ferdi; Müstecaplıoğlu, Özgür E

2015-08-01

We investigate a quantum heat engine with a working substance of two particles, one with a spin-1/2 and the other with an arbitrary spin (spin s), coupled by Heisenberg exchange interaction, and subject to an external magnetic field. The engine operates in a quantum Otto cycle. Work harvested in the cycle and its efficiency are calculated using quantum thermodynamical definitions. It is found that the engine has higher efficiencies at higher spins and can harvest work at higher exchange interaction strengths. The role of exchange coupling and spin s on the work output and the thermal efficiency is studied in detail. In addition, the engine operation is analyzed from the perspective of local work and efficiency. We develop a general formalism to explore local thermodynamics applicable to any coupled bipartite system. Our general framework allows for examination of local thermodynamics even when global parameters of the system are varied in thermodynamic cycles. The generalized definitions of local and cooperative work are introduced by using mean field Hamiltonians. The general conditions for which the global work is not equal to the sum of the local works are given in terms of the covariance of the subsystems. Our coupled spin quantum Otto engine is used as an example of the general formalism.

General formalism of local thermodynamics with an example: Quantum Otto engine with a spin-1 /2 coupled to an arbitrary spin

NASA Astrophysics Data System (ADS)

Altintas, Ferdi; Müstecaplıoǧlu, Ã.-zgür E.

2015-08-01

We investigate a quantum heat engine with a working substance of two particles, one with a spin-1 /2 and the other with an arbitrary spin (spin s ), coupled by Heisenberg exchange interaction, and subject to an external magnetic field. The engine operates in a quantum Otto cycle. Work harvested in the cycle and its efficiency are calculated using quantum thermodynamical definitions. It is found that the engine has higher efficiencies at higher spins and can harvest work at higher exchange interaction strengths. The role of exchange coupling and spin s on the work output and the thermal efficiency is studied in detail. In addition, the engine operation is analyzed from the perspective of local work and efficiency. We develop a general formalism to explore local thermodynamics applicable to any coupled bipartite system. Our general framework allows for examination of local thermodynamics even when global parameters of the system are varied in thermodynamic cycles. The generalized definitions of local and cooperative work are introduced by using mean field Hamiltonians. The general conditions for which the global work is not equal to the sum of the local works are given in terms of the covariance of the subsystems. Our coupled spin quantum Otto engine is used as an example of the general formalism.

A Monte Carlo study of the spin-1 Blume-Emery-Griffiths phase diagrams within biquadratic exchange anisotropy

NASA Astrophysics Data System (ADS)

Dani, Ibtissam; Tahiri, Najim; Ez-Zahraouy, Hamid; Benyoussef, Abdelilah

2014-08-01

The effect of the bi-quadratic exchange coupling anisotropy on the phase diagram of the spin-1 Blume-Emery-Griffiths model on simple-cubic lattice is investigated using mean field theory (MFT) and Monte Carlo simulation (MC). It is found that the anisotropy of the biquadratic coupling favors the stability of the ferromagnetic phase. By decreasing the parallel and/or perpendicular bi-quadratic coupling, the ferrimagnetic and the antiquadrupolar phases broaden in contrast, the ferromagnetic and the disordered phases become narrow. The behavior of magnetization and quadrupolar moment as a function of temperature is also computed, especially in the ferrimagnetic phase.

Exchange bias effect in L10-ordered FePt and FeCo-based bilayer structure: effect of increasing applied field

NASA Astrophysics Data System (ADS)

Singh, Sadhana; Kumar, Dileep; Bhagat, Babli; Choudhary, R. J.; Reddy, V. R.; Gupta, Ajay

2018-02-01

The applied magnetic field (H APP) dependence of the exchange bias (EB) is studied in an exchange-coupled thin-film bilayer composed of a hard ferromagnetic FePt layer in the proximity of a soft ferromagnetic FeCo layer. FePt/FeCo structure is deposited in an ultra-high vacuum chamber, where the FePt layer was first annealed at 823 K for 30 min and subsequently cooled to room temperature in the presence of an in-plane magnetic field, H MAX ~ 1.5 kOe to promote L10-ordered hard magnetic phase with magnetic moments aligned in one of the in-plane directions in the FePt layer. In-situ magneto-optical Kerr effect measurements during different stages of bilayer growth and detailed ex-situ superconducting quantum interference device-vibrating sample magnetometer measurements jointly revealed that due to the interplay between exchange coupling at the interface and dipolar energies of the saturated hard FePt layer, a hysteresis loop of FeCo layer shifts along the magnetic field axis. A clear dependence of EB field (H EB) on increasing maximum value of the H APP during the hysteresis loop measurement is understood in terms of the magnetic state of soft and hard magnetic layers, where EB increases with increasing H APP until the hard layer moment remains undisturbed in its remanence state. As soon as the field was sufficient to rotate the spins of the FePt layer, the loop became symmetric with respect to the field axis.

Magnonic waveguide based on exchange-spring magnetic structure

NASA Astrophysics Data System (ADS)

Wang, Lixiang; Gao, Leisen; Jin, Lichuan; Liao, Yulong; Wen, Tianlong; Tang, Xiaoli; Zhang, Huaiwu; Zhong, Zhiyong

2018-05-01

A soft/hard exchange-spring coupled bilayer magnetic structure is proposed to obtain a narrow channel for spin-wave propagation. Micromagnetic simulations show that broad-band Damon-Eshbach geometry spin waves are strongly constrained within the channel and propagate effectively with a high group velocity. The beam width of the bound spin waves is almost independent from the frequency and is smaller than 24nm. Two side spin beams appearing at the low-frequency excitation are demonstrated to be coupled with the channel spins by dipole-dipole interaction. In contrast to a domain wall, the channel formed by exchange-spring coupling is easier to be realized in experimental scenarios and holds stronger immunity to surroundings. This work is expected to open new possibilities for energy-efficient spin-wave guiding as well as to help shape the field of beam magnonics.

The thermal stability of magnetically exchange coupled MnBi/FeCo composites at electric motor working temperature

NASA Astrophysics Data System (ADS)

Cheng, Ye; Wang, Hongying; Li, Zhigang; Liu, Wanhui; Bao, Ilian

2018-04-01

The magnetically exchange coupled MnBi/FeCo composites were synthesized through a magnetic self-assembly process. The MnBi/FeCo composites were then hot pressed in a magnetic field to form magnets. The thermal stability of the magnets were tested by annealing at electric motor working temperature of 200 °C for 20, 40 and 60 h, respectively. It was found that after heating for 20 h, there was negligible change in its hysteresis loop. However, when the heating time was increased 40 and 60 h, the magnetic hysteresis loops presented two-phase magnetic behaviors, and the maximum energy products of the magnet were decreased. This research showed that the magnetically exchange coupled MnBi/FeCo composites had low thermal stability at electric motor working temperature.

Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling.

PubMed

Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E

2016-06-01

The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.

Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

DOE PAGES

Chesnel, Karine; Safsten, Alex; Rytting, Matthew; ...

2016-06-01

The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer.more » The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. Lastly, if the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling.« less

Shaping nanoscale magnetic domain memory in exchange-coupled ferromagnets by field cooling

PubMed Central

Chesnel, Karine; Safsten, Alex; Rytting, Matthew; Fullerton, Eric E.

2016-01-01

The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling. PMID:27248368

Coexisting exchange bias effect and ferroelectricity in geometrically frustrated ZnCr2O4

NASA Astrophysics Data System (ADS)

Dey, J. K.; Majumdar, S.; Giri, S.

2018-06-01

Concomitant occurrence of exchange bias effect and ferroelectric order is revealed in antiferromagnetic spinel ZnCr2O4. The exchange bias effect is observed below antiferromagnetic Neél temperature (T N) with a reasonable value of exchange bias field ( Oe at 2 K). Intriguingly, the ratio is found unusually high as  ∼2.2, where H C is the coercivity. This indicates that large H C is not always primary for obtaining large exchange bias effect. Ferroelectric order is observed at T N, where non-centrosymmetric magnetic structure with space group associated with the magnetoelectric coupling correlates the ferroelectric order, proposing that, ZnCr2O4 is an improper multiferroic material. Rare occurrence of exchange bias effect and ferroelectric order in ZnCr2O4 attracts the community for fundamental interest and draws special attention in designing new materials for possible electric field control of exchange bias effect.

Superior bit error rate and jitter due to improved switching field distribution in exchange spring magnetic recording media

PubMed Central

Suess, D.; Fuger, M.; Abert, C.; Bruckner, F.; Vogler, C.

2016-01-01

We report two effects that lead to a significant reduction of the switching field distribution in exchange spring media. The first effect relies on a subtle mechanism of the interplay between exchange coupling between soft and hard layers and anisotropy that allows significant reduction of the switching field distribution in exchange spring media. This effect reduces the switching field distribution by about 30% compared to single-phase media. A second effect is that due to the improved thermal stability of exchange spring media over single-phase media, the jitter due to thermal fluctuation is significantly smaller for exchange spring media than for single-phase media. The influence of this overall improved switching field distribution on the transition jitter in granular recording and the bit error rate in bit-patterned magnetic recording is discussed. The transition jitter in granular recording for a distribution of Khard values of 3% in the hard layer, taking into account thermal fluctuations during recording, is estimated to be a = 0.78 nm, which is similar to the best reported calculated jitter in optimized heat-assisted recording media. PMID:27245287

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zohar, S.; Choi, Y.; Love, D. M.

We use X-ray Excited Luminescence Microscopy to investigate the elemental and layer resolved magnetic reversal in an interlayer exchange coupled (IEC) epitaxial Fe/Cr wedge/Co heterostructure. The transition from strongly coupled parallel Co-Fe reversal for Cr thickness t(Cr) < 0.34 nm to weakly coupled layer independent reversal for t(Cr) > 1.5 nm is punctuated at 0.34 < t(Cr) < 1.5 nm by a combination of IEC guided domain wall motion and stationary zig zag domain walls. Domain walls nucleated at switching field minima are guided by IEC spatial gradients and collapse at switching field maxima.

Spatial distribution characteristics of magnetization in exchange-coupled bilayers with mutually orthogonal anisotropies

NASA Astrophysics Data System (ADS)

Xiang, Y.; Chen, C. W.

2017-05-01

The magnetization distribution of a bilayer exchange spring system with mutually orthogonal anisotropies was investigated by micromagnetic simulation. Results showed that the spatial change rate of the magnetization direction could be engineered by varying the material parameters, layer thicknesses, and magnetic field. When no magnetic field is applied, this angular change rate is determined by three parameter ratios: a ratio of the exchange energy and anisotropy constants of both layers and two thickness ratios of both layers. If these three ratios are kept invariant, the ratio of the angular change of the soft layer over the hard layer will remain the same. When a magnetic field is applied, two more ratios concerning the magnetic field should be added to determine the spatial angular change of the magnetization direction.

Antiferromagnetic exchange and magnetoresistance enhancement in ultrathin Co-Re sandwiches

NASA Astrophysics Data System (ADS)

Freitas, P. P.; Melo, L. V.; Trindade, I.; From, M.

1992-10-01

Co-Re ultrathin sandwiches were prepared that show antiferromagnetic coupling and enhanced saturation magnetoresistance for Re spacer thicknesses below 9 Å. A field of 2.5 kOe is needed to saturate the antiferromagnetically coupled Co layers. These results are similar to those found in Co-Re superlattices.

Temperature dependence of the exchange coupling in the Fe(001) whisker/11 ML Cr/20 ML Fe structure

NASA Astrophysics Data System (ADS)

From, M.; Liao, L. X.; Cochran, J. F.; Heinrich, B.

1994-05-01

The exchange coupling between iron layers separated by 11 monolayers (ML) of Cr(001) has been investigated using a structure in which the Cr(001) was grown on a bulk iron whisker Fe(001) surface at a temperature of approximately 300 °C. This temperature was selected to produce near optimum smoothness of the Cr layer. The Cr(001) deposition was followed by the deposition of 20 ML of Fe(001) at room temperature, and by the deposition of a 20 ML Au(001) protective layer. The frequencies corresponding to the magnetic excitations in this structure were measured by means of Brillouin light scattering (BLS). One of the observed frequencies corresponds to a surface mode in the bulk iron whisker. Another observed frequency corresponds to the lowest lying precessional mode of the magnetization in the 20 ML thick Fe(001) thin film. Typically, the thin film frequency exhibits a dependence on applied magnetic field that displays two cusps. The positions of the cusps are dependent on the exchange coupling between the 20 ML Fe film and the bulk iron substrate. The surface mode frequency increases monotonically with increasing field over most of the field range investigated. However, at the field corresponding to the low field cusp in the thin film frequency, the surface mode frequency undergoes an abrupt jump in magnitude. We have used the position of the cusps in the thin film data to deduce values for the bilinear, J1, and biquadratic, J2, coupling terms, where the coupling energy is written in the form EAB=-J1 cos(Δφ)+J2 cos2(Δφ); Δφ is the angle between the thin film and bulk iron magnetizations. Measurements of J1 and J2 have been carried out at six temperatures that span the range 100-350 K. Both J1 and J2 are found to depend strongly on temperature. The data are well described by the quadratic expression J2=-0.54+1.46 ×‖J1‖-0.52×J12, where J1 and J2 are expressed in erg/cm2. The large nonzero intercept and the linear term probably imply a significant intrinsic contribution to the biquadratic exchange.

Control of magnetic direction in multi-layer ferromagnetic devices by bias voltage

DOEpatents

You, Chun-Yeol; Bader, Samuel D.

2001-01-01

A system for controlling the direction of magnetization of materials comprising a ferromagnetic device with first and second ferromagnetic layers. The ferromagnetic layers are disposed such that they combine to form an interlayer with exchange coupling. An insulating layer and a spacer layer are located between the first and second ferromagnetic layers. A direct bias voltage is applied to the interlayer exchange coupling, causing the direction of magnetization of the second ferromagnetic layer to change. This change of magnetization direction occurs in the absence of any applied external magnetic field.

Spontaneous exchange bias formation driven by a structural phase transition in the antiferromagnetic material.

PubMed

Migliorini, A; Kuerbanjiang, B; Huminiuc, T; Kepaptsoglou, D; Muñoz, M; Cuñado, J L F; Camarero, J; Aroca, C; Vallejo-Fernández, G; Lazarov, V K; Prieto, J L

2018-01-01

Most of the magnetic devices in advanced electronics rely on the exchange bias effect, a magnetic interaction that couples a ferromagnetic and an antiferromagnetic material, resulting in a unidirectional displacement of the ferromagnetic hysteresis loop by an amount called the 'exchange bias field'. Setting and optimizing exchange bias involves cooling through the Néel temperature of the antiferromagnetic material in the presence of a magnetic field. Here we demonstrate an alternative process for the generation of exchange bias. In IrMn/FeCo bilayers, a structural phase transition in the IrMn layer develops at room temperature, exchange biasing the FeCo layer as it propagates. Once the process is completed, the IrMn layer contains very large single-crystal grains, with a large density of structural defects within each grain, which are promoted by the FeCo layer. The magnetic characterization indicates that these structural defects in the antiferromagnetic layer are behind the resulting large value of the exchange bias field and its good thermal stability. This mechanism for establishing the exchange bias in such a system can contribute towards the clarification of fundamental aspects of this exchange interaction.

New Trends in Magnetic Exchange Bias

NASA Astrophysics Data System (ADS)

Mougin, Alexandra; Mangin, Stéphane; Bobo, Jean-Francois; Loidl, Alois

2005-05-01

The study of layered magnetic structures is one of the hottest topics in magnetism due to the growing attraction of applications in magnetic sensors and magnetic storage media, such as random access memory. For almost half a century, new discoveries have driven researchers to re-investigate magnetism in thin film structures. Phenomena such as giant magnetoresistance, tunneling magnetoresistance, exchange bias and interlayer exchange coupling led to new ideas to construct devices, based not only on semiconductors but on a variety of magnetic materials Upon cooling fine cobalt particles in a magnetic field through the Néel temperature of their outer antiferromagnetic oxide layer, Meiklejohn and Bean discovered exchange bias in 1956. The exchange bias effect through which an antiferromagnetic AF layer can cause an adjacent ferromagnetic F layer to develop a preferred direction of magnetization, is widely used in magnetoelectronics technology to pin the magnetization of a device reference layer in a desired direction. However, the origin and effects due to exchange interaction across the interface between antiferromagneic and ferromagnetic layers are still debated after about fifty years of research, due to the extreme difficulty associated with the determination of the magnetic interfacial structure in F/AF bilayers. Indeed, in an AF/F bilayer system, the AF layer acts as “the invisible man” during conventional magnetic measurements and the presence of the exchange coupling is evidenced indirectly through the unusual behavior of the adjacent F layer. Basically, the coercive field of the F layer increases in contact with the AF and, in some cases, its hysteresis loop is shifted by an amount called exchange bias field. Thus, AF/F exchange coupling generates a new source of anisotropy in the F layer. This induced anisotropy strongly depends on basic features such as the magnetocrystalline anisotropy, crystallographic and spin structures, defects, domain patterns etc of the constituant layers. The spirit of this topical issue is, for the first time, to gather and survey recent and original developments, both experimental and theoretical, which bring new insights into the physics of exchange bias. It has been planned in relation with an international workshop exclusively devoted to exchange bias, namely IWEBMN’04 (International Workshop on Exchange Bias in Magnetic Nanostructures) that took place in Anglet, in the south west of France, from 16th to 18th September 2004. The conference gathered worldwide researchers in the area, both experimentalists and theoreticians. Several research paths are particularly active in the field of magnetic exchange coupling. The conference, as well as this topical issue, which was also open to contributions from scientists not participating in the conference, has been organized according to the following principles: 1. Epitaxial systems: Since the essential behavior of exchange bias critically depends on the atomic-level chemical and spin structure at the interface between the ferromagnetic and antiferromagnetic components, epitaxial AF/F systems in which the quality of the interface and the crystalline coherence are optimized and well known are ideal candidates for a better understanding of the underlying physics of exchange bias. The dependence of exchange bias on the spin configurations at the interfaces can be accomplished by selecting different crystallographic orientations. The role of interface roughness can also be understood from thin-film systems by changing the growth parameters, and correlations between the interface structure and exchange bias can be made, as reported in this issue. 2. Out-of-plane magnetized systems: While much important work has been devoted to the study of structures with in-plane magnetization, little has been done on the study of exchange bias and exchange coupling in samples with out-of-plane magnetization. Some systems can exhibit either in-plane or out-of-plane exchange bias, depending on the field cooling direction. This is of particular interest since it allows probing of the three-dimensional spin structure of the AF layer. The interface magnetic configuration is extremely important in the perpendicular geometry, as the short-range exchange coupling competes with a long-range dipolar interaction; the induced uniaxial anisotropy must overcome the demagnetization energy to establish perpendicular anisotropy films. Those new studies are of primary importance for the magnetic media industry as perpendicular recording exhibits potential for strongly increased storage densities. 3. Parameters tuning exchange bias in polycrystalline samples and magnetic configurations: Different parameters can be used to tune the exchange bias coupling in polycrystalline samples similar to those used in devices. Particularly fascinating aspects are the questions of the appearance of exchange bias or coercivity in ferromagnet/antiferromagnet heterostructures, and its relation to magnetic configurations formed on either side of the interface. Several papers report on either growth choices or post preparation treatments that enable tuning of the exchange bias in bilayers. The additional complexity and novel features of the exchange coupled interface make the problem particularly rich. 4. Dynamics and magnetization reversal: Linear response experiments, such as ferromagnetic resonance, have been used with great success to identify interface, surface anisotropies and interlayer exchange in multilayer systems. The exchange bias structure is particularly well suited to study because interface driven changes in the spin wave frequencies in the ferromagnet can be readily related to interlayer exchange and anisotropy parameters associated with the antiferromagnet. Because the exchange bias is intimately connected with details of the magnetization process during reversal and the subsequent formation of hysteresis, considerations of time dependence and irreversible processes are also relevant. Thermal processes like the training effect manifesting itself in changes in the hysteretic characteristics depending on magnetic history can lead to changes in the magnetic configurations. This section contains an increasing number of investigations of dynamics in exchange bias coupled bilayers, and in particular those of the intriguing asymmetric magnetization reversal in both branches of a hysteresis loop. The Editors of the topical issue: Alexandra Mougin Laboratoire de Physique des Solides, UMR CNRS 8502, Université Paris Sud, F-91405 Orsay, France Stéphane Mangin Laboratoire de Physique des Matériaux, UMR CNRS 7556, Université Henri Poincaré, F-54506 Nancy, France Jean-Francois Bobo Laboratoire de Physique de la Matière Condensée - NMH, FRE 2686 CNRS ONERA, 2 avenue Edouard Belin, F-31400 Toulouse, France Alois Loidl Experimentalphysik V, EKM, Institut für Physik, Universität Augsburg, Universitätsstrasse 1, D-86135, Augsburg, Germany

Tunneling magnetoresistance sensors with different coupled free layers

NASA Astrophysics Data System (ADS)

Liu, Yen-Fu; Yin, Xiaolu; Yang, Yi; Ewing, Dan; De Rego, Paul J.; Liou, Sy-Hwang

2017-05-01

Large differences of magnetic coercivity (HC), exchange coupling field (HE), and tunneling magnetoresistance ratio (TMR) in magnetic tunnel junctions with different coupled free layers are discussed. We demonstrate that the magnetization behavior of the free layer is not only dominated by the interfacial barrier layer but also affected largely by the magnetic or non-magnetic coupled free layers. All these parameters are sensitively controlled by the magnetic nanostructure, which can be tuned also by the magnetic annealing process. The optimized sensors exhibit a large field sensitivity of up to 261%/mT in the region of the reversal synthetic ferrimagnet at the pinned layers.

Storage and retrieval of light pulse in coupled quantum wells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Jibing, E-mail: liu0328@foxmail.com; Liu, Na; Shan, Chuanjia

In this paper, we propose an effective scheme to create a frequency entangled states based on bound-to-bound inter-subband transitions in an asymmetric three-coupled quantum well structure. A four-subband cascade configuration quantum well structure is illuminated with a pulsed probe field and two continuous wave control laser fields to generate a mixing field. By properly adjusting the frequency detunings and the intensity of coupling fields, the conversion efficiency can reach 100%. A maximum entangled state can be achieved by selecting a proper length of the sample. We also numerically investigate the propagation dynamics of the probe pulse and mixing pulse, themore » results show that two frequency components are able to exchange energy through a four-wave mixing process. Moreover, by considering special coupling fields, the storage and retrieval of the probe pulse is also numerically simulated.« less

Anisotropic exchange interaction induced by a single photon in semiconductor microcavities

NASA Astrophysics Data System (ADS)

Chiappe, G.; Fernández-Rossier, J.; Louis, E.; Anda, E. V.

2005-12-01

We investigate coupling of localized spins in a semiconductor quantum dot embedded in a microcavity. The lowest cavity mode and the quantum dot exciton are coupled and close in energy, forming a polariton. The fermions forming the exciton interact with localized spins via exchange. Exact diagonalization of a Hamiltonian in which photons, spins, and excitons are treated quantum mechanically shows that a single polariton induces a sizable indirect anisotropic exchange interaction between spins. At sufficiently low temperatures strong ferromagnetic correlations show up without an appreciable increase in exciton population. In the case of a (Cd,Mn)Te quantum dot, Mn-Mn ferromagnetic coupling is still significant at 1 K : spin-spin correlation around 3 for exciton occupation smaller than 0.3. We find that the interaction mediated by photon-polaritons is 10 times stronger than the one induced by a classical field for equal Rabi splitting.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

DOE PAGES

Chen, Aiping; Wang, Qiang; Fitzsimmons, Michael R.; ...

2017-05-02

In a variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. But, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed inmore » terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. These results shed new light on using oxide interfaces to design functional spintronic devices.« less

Exchange interaction and the tunneling induced transparency in coupled quantum dots

NASA Astrophysics Data System (ADS)

Borges, Halyne; Alcalde, Augusto; Ulloa, Sergio

2014-03-01

Stacked semiconductor quantum dots coupled by tunneling are unique ``quantum molecule'' where it is possible to create a multilevel structure of excitonic states. This structure allows the investigation of quantum interference processes and their control via electric external fields. In this work, we investigate the optical response of a quantum molecule coherently driven by a polarized laser, considering the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. In our model we consider interdot transitions mediated by the the hole tunneling between states with the same total spin and, between bright and dark exciton states. Using realistic experimental parameters, we demonstrate that the excitonic states coupled by tunneling exhibit an enriched and controllable optical response. Our results show that through the appropriate control of the external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra, whenever states of appropriate symmetry are mixed by the hole tunneling. We explore the relevant parameters space that would allows with the experiments. CAPES, INCT-IQ and MWN/CIAM-NSF.

Kondo peak splitting and Kondo dip in single molecular magnet junctions

NASA Astrophysics Data System (ADS)

Niu, Pengbin; Shi, Yunlong; Sun, Zhu; Nie, Yi-Hang; Luo, Hong-Gang

2016-01-01

Many factors containing bias, spin-orbit coupling, magnetic fields applied, and so on can strongly influence the Kondo effect, and one of the consequences is Kondo peak splitting (KPS). It is natural that KPS should also appear when another spin degree of freedom is involved. In this work we study the KPS effects of single molecular magnets (SMM) coupled with two metallic leads in low-temperature regime. It is found that the Kondo transport properties are strongly influenced by the exchange coupling and anisotropy of the magnetic core. By employing Green's function method in Hubbard operator representation, we give an analytical expression for local retarded Green's function of SMM and discussed its low-temperature transport properties. We find that the anisotropy term behaves as a magnetic field and the splitting behavior of exchange coupling is quite similar to the spin-orbit coupling. These splitting behaviors are explained by introducing inter-level or intra-level transitions, which account for the seven-peak splitting structure. Moreover, we find a Kondo dip at Fermi level under proper parameters. These Kondo peak splitting behaviors in SMM deepen our understanding to Kondo physics and should be observed in the future experiments.

Selective Transient Cooling by Impulse Perturbations in a Simple Toy Model

NASA Astrophysics Data System (ADS)

Fabrizio, Michele

2018-06-01

We show in a simple exactly solvable toy model that a properly designed impulse perturbation can transiently cool down low-energy degrees of freedom at the expense of high-energy ones that heat up. The model consists of two infinite-range quantum Ising models: one, the high-energy sector, with a transverse field much bigger than the other, the low-energy sector. The finite-duration perturbation is a spin exchange that couples the two Ising models with an oscillating coupling strength. We find a cooling of the low-energy sector that is optimized by the oscillation frequency in resonance with the spin exchange excitation. After the perturbation is turned off, the Ising model with a low transverse field can even develop a spontaneous symmetry breaking despite being initially above the critical temperature.

Field coupling-induced pattern formation in two-layer neuronal network

NASA Astrophysics Data System (ADS)

Qin, Huixin; Wang, Chunni; Cai, Ning; An, Xinlei; Alzahrani, Faris

2018-07-01

The exchange of charged ions across membrane can generate fluctuation of membrane potential and also complex effect of electromagnetic induction. Diversity in excitability of neurons induces different modes selection and dynamical responses to external stimuli. Based on a neuron model with electromagnetic induction, which is described by magnetic flux and memristor, a two-layer network is proposed to discuss the pattern control and wave propagation in the network. In each layer, gap junction coupling is applied to connect the neurons, while field coupling is considered between two layers of the network. The field coupling is approached by using coupling of magnetic flux, which is associated with distribution of electromagnetic field. It is found that appropriate intensity of field coupling can enhance wave propagation from one layer to another one, and beautiful spatial patterns are formed. The developed target wave in the second layer shows some difference from target wave triggered in the first layer of the network when two layers are considered by different excitabilities. The potential mechanism could be pacemaker-like driving from the first layer will be encoded by the second layer.

Enhancement of exchange bias in ferromagnetic/antiferromagnetic core-shell nanoparticles through ferromagnetic domain wall formation

NASA Astrophysics Data System (ADS)

Wu, Rui; Ding, Shilei; Lai, Youfang; Tian, Guang; Yang, Jinbo

2018-01-01

The spin configuration in the ferromagnetic part during the magnetization reversal plays a crucial role in the exchange bias effect. Through Monte Carlo simulation, the exchange bias effect in ferromagnetic-antiferromagnetic core-shell nanoparticles is investigated. Magnetization reversals in the ferromagnetic core were controlled between the coherent rotation and the domain wall motion by modulating the ferromagnetic domain wall width with parameters of uniaxial anisotropy constant and exchange coupling strength. An anomalous monotonic dependence of exchange bias on the uniaxial anisotropy constant is found in systems with small exchange coupling, showing an obvious violation of classic Meiklejohn-Bean model, while domain walls are found to form close to the interface and propagate in the ferromagnetic core with larger uniaxial anisotropy in both branches of the hysteresis. The asymmetric magnetization reversal with the formation of a spherical domain wall dramatically reduces the coercive field in the ascending branch, leading to the enhancement of the exchange bias. The results provide another degree of freedom to optimize the magnetic properties of magnetic nanoparticles for applications.

Temperature dependence of the effective interdimer exchange interaction in a weakly coupled antiferromagnetic dimer copper compound

NASA Astrophysics Data System (ADS)

Calvo, Rafael; Santana, Vinicius T.; Nascimento, Otaciro R.

2017-08-01

We report a variation with temperature T of the effective interdimeric interaction Jeff' in the antiferromagnetic (AFM) copper dimeric organic compound Cu2[TzTs] 4 (N -thiazol-2-yl-toluenesulfonamidate CuII). This T dependence was obtained from measurements of the effects in the electron paramagnetic resonance (EPR) spectra of the proposed quantum phase transition associated with the exchange-narrowing processes. Cu2[TzTs] 4 contains exchange-coupled pairs of CuII spins SA and SB (S =1 /2 ), with intradimeric AFM exchange coupling J0=(-115 ±1 ) cm-1 (Hex=-J0SA.SB ). The variation of the EPR linewidth of single crystals with field orientation around a "magic angle" where the transitions intersect and the integrated signal intensity of the so-called U peak of the powder spectrum were measured as a function of T . Modeling these data using arguments of exchange narrowing in the adiabatic regime considering the angular variation of the single-crystal spectra and a geometric description, we find that the effective interdimeric coupling | Jeff'| associated with the exchange frequency ωex is negligible for T ≪| J0/kB| when the units are uncoupled and | Jeff'|=(0.080 ±0.005 ) cm-1 (| Jeff'/J0|=7.0 × 10-4 ) at 298 K. Within this T interval, two ranges of | Jeff'| with linear temperature variation but different slopes, with a kink at ˜80 K, are observed and discussed. This T dependence arises from the growing population of the triplet state, and its relevance to the properties of various arrays of dimeric units is discussed. Our experimental procedures and results are compared with those of previous works in ion radical salts and dimeric metal compounds. The relation between the effective coupling | Jeff'| and the real interdimeric exchange coupling | J'| related to the chemical paths connecting neighbor units is discussed.

Interface adjustment and exchange coupling in the IrMn/NiFe system

NASA Astrophysics Data System (ADS)

Spizzo, F.; Tamisari, M.; Chinni, F.; Bonfiglioli, E.; Del Bianco, L.

2017-01-01

The exchange bias effect was investigated, in the 5-300 K temperature range, in samples of IrMn [100 Å]/NiFe [50 Å] (set A) and in samples with inverted layer-stacking sequence (set B), produced at room temperature by DC magnetron sputtering in a static magnetic field of 400 Oe. The samples of each set differ for the nominal thickness (tCu) of a Cu spacer, grown at the interface between the antiferromagnetic and ferromagnetic layers, which was varied between 0 and 2 Å. It has been found out that the Cu insertion reduces the values of the exchange field and of the coercivity and can also affect their thermal evolution, depending on the stack configuration. Indeed, the latter also determines a peculiar variation of the exchange bias properties with time, shown and discussed with reference to the samples without Cu of the two sets. The results have been explained considering that, in this system, the exchange coupling mechanism is ruled by the glassy magnetic behavior of the IrMn spins located at the interface with the NiFe layer. Varying the stack configuration and tCu results in a modulation of the structural and magnetic features of the interface, which ultimately affects the spins dynamics of the glassy IrMn interfacial component.

Thermosphere Dynamics Workshop, volume 2

NASA Technical Reports Server (NTRS)

Mayr, H. G. (Editor); Miller, N. J. (Editor)

1986-01-01

Atmospheric observations reported on include recent measurements of thermospherical composition, gas temperatures, auroral emissions, ion-neutral collisional coupling, electric fields, and plasma convection. Theoretical studies reported on include model calculations of thermospherical general circulation, thermospheric tides, thermospheric tidal coupling to the lower atmosphere, interactions between thermospheic chemistry and dynamics and thermosphere-ionosphere coupling processes. The abstracts provide details given in each talk but the figures represent the fundamental information exchanged within the workshop

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in the dimerized quantum magnet Ba 2 CuSi 2 O 6 Cl 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Okada, Makiko; Tanaka, Hidekazu; Kurita, Nobuyuki

We synthesized single crystals of composition Ba 2CuSi 2O 6Cl 2 and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi 2O 6 also known as Han purple. Ba 2CuSi 2O 6Cl 2 has a singlet ground state with an excitation gap of Δ/k B = 20.8 K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the g factor except for a small jump anomaly for a magnetic field perpendicular to the c axis. The magnetization curve with a nonlinear slope above themore » critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. Lastly, the magnetic-field-induced spin ordering in Ba 2CuSi 2O 6Cl 2 is described as the quasi-2D Bose-Einstein condensation of triplets.« less

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in the dimerized quantum magnet Ba 2 CuSi 2 O 6 Cl 2

DOE PAGES

Okada, Makiko; Tanaka, Hidekazu; Kurita, Nobuyuki; ...

2016-09-20

We synthesized single crystals of composition Ba 2CuSi 2O 6Cl 2 and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi 2O 6 also known as Han purple. Ba 2CuSi 2O 6Cl 2 has a singlet ground state with an excitation gap of Δ/k B = 20.8 K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the g factor except for a small jump anomaly for a magnetic field perpendicular to the c axis. The magnetization curve with a nonlinear slope above themore » critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. Lastly, the magnetic-field-induced spin ordering in Ba 2CuSi 2O 6Cl 2 is described as the quasi-2D Bose-Einstein condensation of triplets.« less

Engineering the quantum anomalous Hall effect in graphene with uniaxial strains

DOE Office of Scientific and Technical Information (OSTI.GOV)

Diniz, G. S., E-mail: ginetom@gmail.com; Guassi, M. R.; Qu, F.

2013-12-28

We theoretically investigate the manipulation of the quantum anomalous Hall effect (QAHE) in graphene by means of the uniaxial strain. The values of Chern number and Hall conductance demonstrate that the strained graphene in presence of Rashba spin-orbit coupling and exchange field, for vanishing intrinsic spin-orbit coupling, possesses non-trivial topological phase, which is robust against the direction and modulus of the strain. Besides, we also find that the interplay between Rashba and intrinsic spin-orbit couplings results in a topological phase transition in the strained graphene. Remarkably, as the strain strength is increased beyond approximately 7%, the critical parameters of themore » exchange field for triggering the quantum anomalous Hall phase transition show distinct behaviors—decrease (increase) for strains along zigzag (armchair) direction. Our findings open up a new platform for manipulation of the QAHE by an experimentally accessible strain deformation of the graphene structure, with promising application on novel quantum electronic devices with high efficiency.« less

Ultrafast Photoinduced Multimode Antiferromagnetic Spin Dynamics in Exchange-Coupled Fe/RFeO3 (R = Er or Dy) Heterostructures.

PubMed

Tang, Jin; Ke, Yajiao; He, Wei; Zhang, Xiangqun; Zhang, Wei; Li, Na; Zhang, Yongsheng; Li, Yan; Cheng, Zhaohua

2018-05-25

Antiferromagnetic spin dynamics is important for both fundamental and applied antiferromagnetic spintronic devices; however, it is rarely explored by external fields because of the strong exchange interaction in antiferromagnetic materials. Here, the photoinduced excitation of ultrafast antiferromagnetic spin dynamics is achieved by capping antiferromagnetic RFeO 3 (R = Er or Dy) with an exchange-coupled ferromagnetic Fe film. Compared with antiferromagnetic spin dynamics of bare RFeO 3 orthoferrite single crystals, which can be triggered effectively by ultrafast laser heating just below the phase transition temperature, the ultrafast photoinduced multimode antiferromagnetic spin dynamic modes, for exchange-coupled Fe/RFeO 3 heterostructures, including quasiferromagnetic resonance, impurity, coherent phonon, and quasiantiferromagnetic modes, are observed in a temperature range of 10-300 K. These experimental results not only offer an effective means to trigger ultrafast antiferromagnetic spin dynamics of rare-earth orthoferrites, but also shed light on the ultrafast manipulation of antiferromagnetic magnetization in Fe/RFeO 3 heterostructures. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Antiferromagnetic exchange bias of a ferromagnetic semiconductor by a ferromagnetic metal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Olejnik, K.; Wadley, P.; Haigh, J.

2009-11-05

We demonstrate an exchange bias in (Ga,Mn)As induced by antiferromagnetic coupling to a thin overlayer of Fe. Bias fields of up to 240 Oe are observed. Using element-specific x-ray magnetic circular dichroism measurements, we distinguish an interface layer that is strongly pinned antiferromagnetically to the Fe. The interface layer remains polarized at room temperature.

Exchange bias and perpendicular anisotropy study of ultrathin Pt-Co-Pt-IrMn multilayers sputtered on float glass

NASA Astrophysics Data System (ADS)

Laval, M.; Lüders, U.; Bobo, J. F.

2007-09-01

We have prepared ultrathin Pt-Co-Pt-IrMn polycrystalline multilayers on float-glass substrates by DC magnetron sputtering. We have determined the optimal set of thickness for both Pt layers, the Co layer and the IrMn biasing layer so that these samples exhibit at the same time out-of-plane magnetic anisotropy and exchange bias. Kerr microscopy domain structure imaging evidences an increase of nucleation rate accompanied with inhomogeneous magnetic behavior in the case of exchange-biased films compared to Pt-Co-Pt trilayers. Polar hysteresis loops are measured in obliquely applied magnetic field conditions, allowing us to determine both perpendicular anisotropy effective constant Keff and exchange-bias coupling JE, which are significantly different from the ones determined by standard switching field measurements.

Direct measurement of the long-range p -d exchange coupling in a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure

NASA Astrophysics Data System (ADS)

Akimov, I. A.; Salewski, M.; Kalitukha, I. V.; Poltavtsev, S. V.; Debus, J.; Kudlacik, D.; Sapega, V. F.; Kopteva, N. E.; Kirstein, E.; Zhukov, E. A.; Yakovlev, D. R.; Karczewski, G.; Wiater, M.; Wojtowicz, T.; Korenev, V. L.; Kusrayev, Yu. G.; Bayer, M.

2017-11-01

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered to be a prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range p -d exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10-nm-thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10 nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift ΔS on the external magnetic field B . We show that in a strong magnetic field, ΔS is a linear function of B with an offset of Δp d=50 -100 μ eV at zero field from the FM induced effective exchange field. On the other hand, the s -d exchange interaction between conduction band electrons and FM, as well as the p -d contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure.

Excitations of interface pinned domain walls in constrained geometries

NASA Astrophysics Data System (ADS)

Martins, S. M. S. B.; Oliveira, L. L.; Rebouças, G. O. G.; Dantas, Ana L.; Carriço, A. S.

2018-05-01

We report a theoretical investigation of the equilibrium pattern and the spectra of head-to-head and Neel domain walls of flat Fe and Py stripes, exchange coupled with a vicinal antiferromagnetic substrate. We show that the domain wall excitation spectrum is tunable by the strength of the interface field. Furthermore, strong interface coupling favors localized wall excitations.

Cascading and local-field effects in non-linear optics revisited: a quantum-field picture based on exchange of photons.

PubMed

Bennett, Kochise; Mukamel, Shaul

2014-01-28

The semi-classical theory of radiation-matter coupling misses local-field effects that may alter the pulse time-ordering and cascading that leads to the generation of new signals. These are then introduced macroscopically by solving Maxwell's equations. This procedure is convenient and intuitive but ad hoc. We show that both effects emerge naturally by including coupling to quantum modes of the radiation field that are initially in the vacuum state to second order. This approach is systematic and suggests a more general class of corrections that only arise in a QED framework. In the semi-classical theory, which only includes classical field modes, the susceptibility of a collection of N non-interacting molecules is additive and scales as N. Second-order coupling to a vacuum mode generates an effective retarded interaction that leads to cascading and local field effects both of which scale as N(2).

Field theory of hyperfluid

NASA Astrophysics Data System (ADS)

Ariki, Taketo

2018-02-01

A hyperfluid model is constructed on the basis of its action entirely free from external constraints, regarding the hyperfluid as a self-consistent classical field. Intrinsic hypermomentum is no longer a supplemental variable given by external constraints, but arises purely from the diffeomorphism covariance of dynamical field. The field-theoretic approach allows natural classification of a hyperfluid on the basis of its symmetry group and corresponding homogeneous space; scalar, spinor, vector, and tensor fluids are introduced as simple examples. Apart from phenomenological constraints, the theory predicts the hypermomentum exchange of fluid via field-theoretic interactions of various classes; fluid–fluid interactions, minimal and non-minimal SU(n) -gauge couplings, and coupling with metric-affine gravity are all successfully formulated within the classical regime.

Bit patterned media with composite structure for microwave assisted magnetic recording

NASA Astrophysics Data System (ADS)

Eibagi, Nasim

Patterned magnetic nano-structures are under extensive research due to their interesting emergent physics and promising applications in high-density magnetic data storage, through magnetic logic to bio-magnetic functionality. Bit-patterned media is an example of such structures which is a leading candidate to reach magnetic densities which cannot be achieved by conventional magnetic media. Patterned arrays of complex heterostructures such as exchange-coupled composites are studied in this thesis as a potential for next generation of magnetic recording media. Exchange-coupled composites have shown new functionality and performance advantages in magnetic recording and bit patterned media provide unique capability to implement such architectures. Due to unique resonant properties of such structures, their possible application in spin transfer torque memory and microwave assisted switching is also studied. This dissertation is divided into seven chapters. The first chapter covers the history of magnetic recording, the need to increase magnetic storage density, and the challenges in the field. The second chapter introduces basic concepts of magnetism. The third chapter explains the fabrication methods for thin films and various lithographic techniques that were used to pattern the devices under study for this thesis. The fourth chapter introduces the exchanged coupled system with the structure of [Co/Pd] / Fe / [Co/Pd], where the thickness of Fe is varied, and presents the magnetic properties of such structures using conventional magnetometers. The fifth chapter goes beyond what is learned in the fourth chapter and utilizes polarized neutron reflectometry to study the vertical exchange coupling and reversal mechanism in patterned structures with such structure. The sixth chapter explores the dynamic properties of the patterned samples, and their reversal mechanism under microwave field. The final chapter summarizes the results and describes the prospects for future applications of these structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nozaki, T., E-mail: nozaki@ecei.tohoku.ac.jp; Oida, M.; Ashida, T.

We investigated the effect of Pt insertion on a Cr{sub 2}O{sub 3}/Co exchange coupling system. The perpendicular exchange bias μ{sub 0}H{sub ex} decreased with increasing Pt insertion layer thickness, and we observed positive μ{sub 0}H{sub ex} for samples with relatively thick Pt insertion layers. We also examined the cooling field μ{sub 0}H{sub fc} dependence of μ{sub 0}H{sub ex} for the samples. At small μ{sub 0}H{sub fc}, all samples exhibited negative μ{sub 0}H{sub ex}. With increasing μ{sub 0}H{sub fc}, a shift of μ{sub 0}H{sub ex} from negative to positive was observed. In the past, similar behaviors were observed for FeF{sub 2}/Femore » systems exhibiting positive μ{sub 0}H{sub ex}. In addition, the μ{sub 0}H{sub fc} dependence of μ{sub 0}H{sub ex} was well fitted by an equation taking into account the Zeeman energy at the surface of an antiferromagnet as well as an antiferromagnetic exchange coupling. The results strongly suggest that (1) Cr{sub 2}O{sub 3} surface spin is affected by the external magnetic field and (2) the coupling at the Cr{sub 2}O{sub 3}/Pt/Co interface is antiferromagnetic.« less

Thermally activated switching at long time scales in exchange-coupled magnetic grains

NASA Astrophysics Data System (ADS)

Almudallal, Ahmad M.; Mercer, J. I.; Whitehead, J. P.; Plumer, M. L.; van Ek, J.; Fal, T. J.

2015-10-01

Rate coefficients of the Arrhenius-Néel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of M H hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modeled as two exchange-coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of noninteracting ECC grains are then integrated numerically in an applied field with constant sweep rate R =-d H /d t and the magnetization calculated as a function of the applied field H . M H hysteresis loops are presented for a range of values I for sweep rates 105Oe /s ≤R ≤1010Oe /s and a figure of merit that quantifies the advantages of ECC media is proposed. M H hysteresis loops are also calculated based on the stochastic Landau-Lifshitz-Gilbert equations for 108Oe /s ≤R ≤1010Oe /s and are shown to be in good agreement with those obtained from the direct integration of rate equations. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer-based formalism and which provide some useful insight into the reversal process and its dependence on the coupling strength and sweep rate. Of particular interest is the clustering of minimum energy states that are separated by relatively low-energy barriers into "metastates." It is shown that while approximating the reversal process in terms of "metastates" results in little loss of accuracy, it can reduce the run time of a kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by 2 -3 orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC-based simulation studies on systems of interacting dual layer ECC media.

Interlayer-coupled spin vortex pairs and their response to external magnetic fields

NASA Astrophysics Data System (ADS)

Wintz, Sebastian; Bunce, Christopher; Banholzer, Anja; Körner, Michael; Strache, Thomas; Mattheis, Roland; McCord, Jeffrey; Raabe, Jörg; Quitmann, Christoph; Erbe, Artur; Fassbender, Jürgen

2012-06-01

We report on the response of multilayer spin textures to static magnetic fields. Coupled magnetic vortex pairs in trilayer elements (ferromagnetic/nonmagnetic/ferromagnetic) are imaged directly by means of layer-selective magnetic x-ray microscopy. We observe two different circulation configurations with parallel and opposing senses of magnetization rotation at remanence. Upon application of a field, all of the vortex pairs investigated react with a displacement of their cores. For purely dipolar coupled pairs, the individual core displacements are similar to those of an isolated single-layer vortex, but also a noticeable effect of the mutual stray fields is detected. Vortex pairs that are linked by an additional interlayer exchange coupling (IEC), which is either ferromagnetic or antiferromagnetic, mainly exhibit a layer-congruent response. We find that, apart from a possible decoupling at higher fields, these strict IEC vortex pairs can be described by a single-layer model with effective material parameters. This result implies the possibility to design multilayer spin structures with arbitrary effective magnetization.

Compressible Analysis of Bénard Convection of Magneto Rotatory Couple-Stress Fluid

NASA Astrophysics Data System (ADS)

Mehta, C. B.; Singh, M.

2018-02-01

Thermal Instability (Benard's Convection) in the presence of uniform rotation and uniform magnetic field (separately) is studied. Using the linearized stability theory and normal mode analyses the dispersion relation is obtained in each case. In the case of rotatory Benard's stationary convection compressibility and rotation postpone the onset of convection whereas the couple-stress have duel character onset of convection depending on rotation parameter. While in the absence of rotation couple-stress always postpones the onset of convection. On the other hand, magnetic field on thermal instability problem on couple-stress fluid for stationary convection couple-stress parameter and magnetic field postpones the onset of convection. The effect of compressibility also postpones the onset of convection in both cases as rotation and magnetic field. Graphs have been plotted by giving numerical values to the parameters to depict the stationary characteristics. Further, the magnetic field and rotation are found to introduce oscillatory modes which were non-existent in their absence and then the principle of exchange of stability is valid. The sufficient conditions for non-existence of overstability are also obtained.

Effect of capping layer on interlayer coupling in synthetic spin valves

NASA Astrophysics Data System (ADS)

Li, Kebin; Qiu, Jinjun; Han, Guchang; Guo, Zaibing; Zheng, Yuankai; Wu, Yihong; Li, Jinshan

2005-01-01

The magnetic and transport properties of high quality synthetic spin-valves with the structure of Ta/NiFe/IrMn/CoFe/Ru/CoFe/NOL/CoFe/Cu/CoFe/CL were studied by using magnetoresistance measurements. Here Ti, Hf, and Al are used as the capping layer. It is found that both the thickness and materials properties of the capping layers can affect the interlayer coupling field. The interlayer coupling field oscillates weakly with respect to the thickness of the Ti and Hf capping layers. Extremely strong ferromagnetic coupling has been observed when the thickness of the Al capping layer is in a certain range where resonant exchange coupling takes place. The strength of the interlayer coupling is inversely proportional to the square of the thickness of the spacer. It is a typical characteristic of quantum size effect.

Orbital-ordering-driven multiferroicity and magnetoelectric coupling in GeV₄S₈.

PubMed

Singh, Kiran; Simon, Charles; Cannuccia, Elena; Lepetit, Marie-Bernadette; Corraze, Benoit; Janod, Etienne; Cario, Laurent

2014-09-26

We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system GeV₄S₈. Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon.

Combined DFT and BS study on the exchange coupling of dinuclear sandwich-type POM: comparison of different functionals and reliability of structure modeling.

PubMed

Yin, Bing; Xue, GangLin; Li, JianLi; Bai, Lu; Huang, YuanHe; Wen, ZhenYi; Jiang, ZhenYi

2012-05-01

The exchange coupling of a group of three dinuclear sandwich-type polyoxomolybdates [MM'(AsMo7O27)2](12-) with MM' = CrCr, FeFe, FeCr are theoretically predicted from combined DFT and broken-symmetry (BS) approach. Eight different XC functionals are utilized to calculate the exchange-coupling constant J from both the full crystalline structures and model structures of smaller size. The comparison between theoretical values and accurate experimental results supports the applicability of DFT-BS method in this new type of sandwich-type dinuclear polyoxomolybdates. However, a careful choice of functionals is necessary to achieve the desired accuracy. The encouraging results obtained from calculations on model structures highlight the great potential of application of structure modeling in theoretical study of POM. Structural modeling may not only reduce the computational cost of large POM species but also be able to take into account the external field effect arising from solvent molecules in solution or counterions in crystal.

Single Domain SmCo5@Co Exchange-coupled Magnets Prepared from Core/shell Sm[Co(CN)6]·4H2O@GO Particles: A Novel Chemical Approach

PubMed Central

Yang, Ce; Jia, Lihui; Wang, Shouguo; Gao, Chen; Shi, Dawei; Hou, Yanglong; Gao, Song

2013-01-01

SmCo5 based magnets with smaller size and larger maximum energy product have been long desired in various fields such as renewable energy technology, electronic industry and aerospace science. However, conventional relatively rough synthetic strategies will lead to either diminished magnetic properties or irregular morphology, which hindered their wide applications. In this article, we present a facile chemical approach to prepare 200 nm single domain SmCo5@Co core/shell magnets with coercivity of 20.7 kOe and saturation magnetization of 82 emu/g. We found that the incorporation of GO sheets is responsible for the generation of the unique structure. The single domain SmCo5 core contributes to the large coercivity of the magnets and the exchange-coupled Co shell enhances the magnetization. This method can be further utilized in the synthesis other Sm-Co based exchange-coupled magnets. PMID:24356309

Induced dipole-dipole coupling between two atoms at a migration resonance

NASA Astrophysics Data System (ADS)

Kaur, Maninder; Mian, Mahmood

2018-05-01

Results of numerical simulations for the resonant energy exchange phenomenon called Migration reaction between two cold Rydberg atoms are presented. The effect of spatial interatomic distance on the onset of peculiar coherent mechanism is investigated. Observation of Rabi-like population inversion oscillation at the resonance provides a clear signature of dipole induced exchange of electronic excitations between the atoms. Further we present the results for the dependence of expectation value of the interaction hamiltonian on the interatomic distance, which is responsible for energy exchange process. The results of this observation endorse the range of inter atomic distance within which the excitation exchange process occurs completely or partially. Migration process enhance the Rydberg-Rydberg interaction in the absence of an external field, under the condition of the zero permanent dipole moments. Our next observation sheds light on the fundamental mechanism of induced electric fields initiated by the oscillating dipoles in such energy exchange processes. We explore the dependence of induced electric field on the interatomic distance and angle between the dipoles highlighting the inverse power law dependence and anisotropic property of the field. We put forward an idea to utilise the coherent energy exchange process to build efficient and fast energy transfer channels by incorporating more atoms organised at successive distances with decreasing distance gradient.

Partially massless fields during inflation

NASA Astrophysics Data System (ADS)

Baumann, Daniel; Goon, Garrett; Lee, Hayden; Pimentel, Guilherme L.

2018-04-01

The representation theory of de Sitter space allows for a category of partially massless particles which have no flat space analog, but could have existed during inflation. We study the couplings of these exotic particles to inflationary perturbations and determine the resulting signatures in cosmological correlators. When inflationary perturbations interact through the exchange of these fields, their correlation functions inherit scalings that cannot be mimicked by extra massive fields. We discuss in detail the squeezed limit of the tensor-scalar-scalar bispectrum, and show that certain partially massless fields can violate the tensor consistency relation of single-field inflation. We also consider the collapsed limit of the scalar trispectrum, and find that the exchange of partially massless fields enhances its magnitude, while giving no contribution to the scalar bispectrum. These characteristic signatures provide clean detection channels for partially massless fields during inflation.

Normal-Mode Splitting in a Weakly Coupled Optomechanical System

NASA Astrophysics Data System (ADS)

Rossi, Massimiliano; Kralj, Nenad; Zippilli, Stefano; Natali, Riccardo; Borrielli, Antonio; Pandraud, Gregory; Serra, Enrico; Di Giuseppe, Giovanni; Vitali, David

2018-02-01

Normal-mode splitting is the most evident signature of strong coupling between two interacting subsystems. It occurs when two subsystems exchange energy between themselves faster than they dissipate it to the environment. Here we experimentally show that a weakly coupled optomechanical system at room temperature can manifest normal-mode splitting when the pump field fluctuations are antisquashed by a phase-sensitive feedback loop operating close to its instability threshold. Under these conditions the optical cavity exhibits an effectively reduced decay rate, so that the system is effectively promoted to the strong coupling regime.

Search for exotic spin-dependent interactions with a spin-exchange relaxation-free magnetometer

DOE PAGES

Chu, Pinghan; Kim, Young Jin; Savukov, Igor Mykhaylovich

2016-08-15

We propose a novel experimental approach to explore exotic spin-dependent interactions using a spin-exchange relaxation-free (SERF) magnetometer, the most sensitive noncryogenic magnetic-field sensor. This approach studies the interactions between optically polarized electron spins located inside a vapor cell of the SERF magnetometer and unpolarized or polarized particles of external solid-state objects. The coupling of spin-dependent interactions to the polarized electron spins of the magnetometer induces the tilt of the electron spins, which can be detected with high sensitivity by a probe laser beam similarly as an external magnetic field. Lastly, we estimate that by moving unpolarized or polarized objects nextmore » to the SERF Rb vapor cell, the experimental limit to the spin-dependent interactions can be significantly improved over existing experiments, and new limits on the coupling strengths can be set in the interaction range below 10 –2 m.« less

A nonequilibrium model for a moderate pressure hydrogen microwave discharge plasma

NASA Technical Reports Server (NTRS)

Scott, Carl D.

1993-01-01

This document describes a simple nonequilibrium energy exchange and chemical reaction model to be used in a computational fluid dynamics calculation for a hydrogen plasma excited by microwaves. The model takes into account the exchange between the electrons and excited states of molecular and atomic hydrogen. Specifically, electron-translation, electron-vibration, translation-vibration, ionization, and dissociation are included. The model assumes three temperatures, translational/rotational, vibrational, and electron, each describing a Boltzmann distribution for its respective energy mode. The energy from the microwave source is coupled to the energy equation via a source term that depends on an effective electric field which must be calculated outside the present model. This electric field must be found by coupling the results of the fluid dynamics and kinetics solution with a solution to Maxwell's equations that includes the effects of the plasma permittivity. The solution to Maxwell's equations is not within the scope of this present paper.

Tunnel magnetoresistance and linear conductance of double quantum dots strongly coupled to ferromagnetic leads

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weymann, Ireneusz, E-mail: weymann@amu.edu.pl

2015-05-07

We analyze the spin-dependent linear-response transport properties of double quantum dots strongly coupled to external ferromagnetic leads. By using the numerical renormalization group method, we determine the dependence of the linear conductance and tunnel magnetoresistance on the degree of spin polarization of the leads and the position of the double dot levels. We focus on the transport regime where the system exhibits the SU(4) Kondo effect. It is shown that the presence of ferromagnets generally leads the suppression of the linear conductance due to the presence of an exchange field. Moreover, the exchange field gives rise to a transition frommore » the SU(4) to the orbital SU(2) Kondo effect. We also analyze the dependence of the tunnel magnetoresistance on the double dot levels' positions and show that it exhibits a very nontrivial behavior.« less

Exchange coupled CoPt/FePtC media for heat assisted magnetic recording

NASA Astrophysics Data System (ADS)

Dutta, Tanmay; Piramanayagam, S. N.; Ru, Tan Hui; Saifullah, M. S. M.; Bhatia, C. S.; Yang, Hyunsoo

2018-04-01

L10 FePtC granular media are being studied as potential future magnetic recording media and are set to be used in conjunction with heat assisted magnetic recording (HAMR) to enable recording at write fields within the range of current day recording heads. Media structures based on a FePtC storage layer and a capping layer can alleviate the switching field distribution (SFD) requirements of HAMR and reduce the noise originating from the writing process. However, the current designs suffer from SFD issues due to high temperature writing. To overcome this problem, we study a CoPt/FePtC exchange coupled composite structure, where FePtC serves as the storage layer and CoPt (with higher Curie temperature, Tc) as the capping layer. CoPt remains ferromagnetic at near Tc of FePtC. Consequently, the counter exchange energy from CoPt would reduce the noise resulting from the adjacent grain interactions during the writing process. CoPt/FePtC bilayer samples with different thicknesses of CoPt were investigated. Our studies found that CoPt forms a continuous layer at a thickness of 6 nm and leads to considerable reduction in the saturation field and its distribution.

Using the Model Coupling Toolkit to couple earth system models

USGS Publications Warehouse

Warner, J.C.; Perlin, N.; Skyllingstad, E.D.

2008-01-01

Continued advances in computational resources are providing the opportunity to operate more sophisticated numerical models. Additionally, there is an increasing demand for multidisciplinary studies that include interactions between different physical processes. Therefore there is a strong desire to develop coupled modeling systems that utilize existing models and allow efficient data exchange and model control. The basic system would entail model "1" running on "M" processors and model "2" running on "N" processors, with efficient exchange of model fields at predetermined synchronization intervals. Here we demonstrate two coupled systems: the coupling of the ocean circulation model Regional Ocean Modeling System (ROMS) to the surface wave model Simulating WAves Nearshore (SWAN), and the coupling of ROMS to the atmospheric model Coupled Ocean Atmosphere Prediction System (COAMPS). Both coupled systems use the Model Coupling Toolkit (MCT) as a mechanism for operation control and inter-model distributed memory transfer of model variables. In this paper we describe requirements and other options for model coupling, explain the MCT library, ROMS, SWAN and COAMPS models, methods for grid decomposition and sparse matrix interpolation, and provide an example from each coupled system. Methods presented in this paper are clearly applicable for coupling of other types of models. ?? 2008 Elsevier Ltd. All rights reserved.

Topological Hall Effect from Strong to Weak Coupling

NASA Astrophysics Data System (ADS)

Nakazawa, Kazuki; Bibes, Manuel; Kohno, Hiroshi

2018-03-01

The topological Hall effect (THE) of electrons coupled to a noncoplanar spin texture has been studied so far for the strong- and weak-coupling regimes separately; the former in terms of the Berry phase and the latter by perturbation theory. In this letter, we present a unified treatment in terms of spin gauge field by considering not only the adiabatic (Berry phase) component of the gauge field but also the nonadiabatic component. While only the adiabatic contribution is important in the strong-coupling regime, it is completely canceled by a part of the nonadiabatic contribution in the weak-coupling regime, where the THE is governed by the remaining nonadiabatic terms. We found a new weak-coupling region that cannot be accessed by a simple perturbation theory, where the Hall conductivity is proportional to M, with 2M being the exchange splitting of the electron spectrum.

Exchange interaction and tunneling-induced transparency in coupled quantum dots

NASA Astrophysics Data System (ADS)

Borges, H. S.; Alcalde, A. M.; Ulloa, Sergio E.

2014-11-01

We investigate the optical response of quantum dot molecules coherently driven by polarized laser light. Our description includes the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. We consider interdot transitions mediated by hole tunneling between states with the same total angular momentum and between bright and dark exciton states as allowed by spin-flip hopping between the dots in the molecule. Using realistic experimental parameters we demonstrate that the excitonic states coupled by tunneling exhibit a rich and controllable optical response. We show that through the appropriate control of an external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra whenever states of appropriate symmetry are mixed by the carrier tunneling. We explore the relevant parameter space that allows probing this phenomenon in experiments. Controlled variation in applied field and laser detuning would allow the optical characterization of spin-preserving and spin-flip hopping amplitudes in such systems by measuring the width of the tunneling-induced transparency windows.

Atomic physics effects on tokamak edge drift-tearing modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hahm, T.S.

1993-03-01

The effects of ionization and charge exchange on the linear stability of drift-tearing modes are analytically investigated. In particular, the linear instability threshold {Delta}{sup Th}, produced by ion sound wave coupling is modified. In the strongly collisional regime, the ionization breaks up the near cancellation of the perturbed electric field and the pressure gradient along the magnetic field, and increases the threshold. In the semi-collisional regime, both ionization and charge exchange act as drag on the ion parallel velocity, and consequently decrease the threshold by reducing the effectiveness of ion sound wave propagation.

Atomic physics effects on tokamak edge drift-tearing modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hahm, T.S.

1993-03-01

The effects of ionization and charge exchange on the linear stability of drift-tearing modes are analytically investigated. In particular, the linear instability threshold [Delta][sup Th], produced by ion sound wave coupling is modified. In the strongly collisional regime, the ionization breaks up the near cancellation of the perturbed electric field and the pressure gradient along the magnetic field, and increases the threshold. In the semi-collisional regime, both ionization and charge exchange act as drag on the ion parallel velocity, and consequently decrease the threshold by reducing the effectiveness of ion sound wave propagation.

Electric-field control of local ferromagnetism using a magnetoelectric multiferroic.

PubMed

Chu, Ying-Hao; Martin, Lane W; Holcomb, Mikel B; Gajek, Martin; Han, Shu-Jen; He, Qing; Balke, Nina; Yang, Chan-Ho; Lee, Donkoun; Hu, Wei; Zhan, Qian; Yang, Pei-Ling; Fraile-Rodríguez, Arantxa; Scholl, Andreas; Wang, Shan X; Ramesh, R

2008-06-01

Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

Electric-field control of local ferromagnetism using a magnetoelectric multiferroic

NASA Astrophysics Data System (ADS)

Chu, Ying-Hao; Martin, Lane W.; Holcomb, Mikel B.; Gajek, Martin; Han, Shu-Jen; He, Qing; Balke, Nina; Yang, Chan-Ho; Lee, Donkoun; Hu, Wei; Zhan, Qian; Yang, Pei-Ling; Fraile-Rodríguez, Arantxa; Scholl, Andreas; Wang, Shan X.; Ramesh, R.

2008-06-01

Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO3. The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO3 film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co0.9Fe0.1) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

Spin-flop coupling and exchange anisotropy in ferromagnetic/antiferromagnetic bilayers

NASA Astrophysics Data System (ADS)

Xu, Xiao-Yong; Hu, Jing-Guo

2009-03-01

By investigating the antiferromagnetic spin configuration, the exchange anisotropy and the interfacial spin-flop coupling in ferromagnetic/antiferromagnetic (FM/AF) bilayers have been discussed in detail. The results show that there are four possible cases for the AF spins, namely the reversible recovering case, irreversible half-rotating case, irreversible reversing and irreversible half-reversing cases. Moreover, the realization of the cases strongly depends on interface quadratic coupling, interface spin-flop (biquadratic) coupling and AF thickness. The magnetic phase diagram in terms of the AF thickness tAF, the interfacial bilinear coupling J1 and the spin-flop coupling J2 has been constructed. The corresponding critical parameters in which the exchange bias will occur or approach saturation have been also presented. Specially, the small spin-flop exchange coupling may result in an exchange bias without the interfacial bilinear exchange coupling. However, in general, the spin-flop exchange coupling can weaken or eliminate the exchange bias, but always enhances the coercivity greatly.

The effect of external magnetic field on the Raman peaks in manganites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sahu, A. K., E-mail: ajitsahu@seemantaengg.ac.in; Rout, G. C.

2014-04-24

We report here a microscopic theoretical model study exhibiting the effect of external magnetic field on the Raman excitation peaks in the CMR manganite system. The Hamiltonian consists of Jahn-Teller (J-T) distortion in e{sub g} band, the double exchange interaction and the Heisenberg spin-spin interaction. Further the phonons are coupled to e{sub g} band electrons, J-T distorted e{sub g} band and the double exchange interaction. The Raman spectral intensity is calculated from the imaginary part of the phonon Green function. The spectra exhibits three peaks besides a very weak high energy peak. The magnetic field effect on these peaks aremore » reported.« less

High temperature coercive field behavior of Fe-Zr powder

NASA Astrophysics Data System (ADS)

Mishra, Debabrata; Perumal, A.; Srinivasan, A.

2009-04-01

We report the investigation of high temperature coercive field behavior of Fe80Zr20 nanocrystalline alloy powder having two-phase microstructure prepared by mechanical alloying process. Thermomagnetization measurement shows the presence of two different magnetic phase transitions corresponding to the amorphous matrix and nonequilibrium Fe(Zr) solid solution. Temperature dependent coercivity exhibits a sharp increase in its value close to the Curie temperature of the amorphous matrix. This feature is attributed to the loss of intergranular ferromagnetic exchange coupling between the nanocrystallites due to the paramagnetic nature of the amorphous matrix. The temperature dependent coercive field behavior is ascribed to the variations in both the effective anisotropy and the exchange stiffness constant with temperature.

Gravitational wave-Gauge field oscillations

NASA Astrophysics Data System (ADS)

Caldwell, R. R.; Devulder, C.; Maksimova, N. A.

2016-09-01

Gravitational waves propagating through a stationary gauge field transform into gauge field waves and back again. When multiple families of flavor-space locked gauge fields are present, the gravitational and gauge field waves exhibit novel dynamics. At high frequencies, the system behaves like coupled oscillators in which the gravitational wave is the central pacemaker. Due to energy conservation and exchange among the oscillators, the wave amplitudes lie on a multidimensional sphere, reminiscent of neutrino flavor oscillations. This phenomenon has implications for cosmological scenarios based on flavor-space locked gauge fields.

The Effect of Interface Texture on Exchange Biasing in Ni(80)Fe(20)/Ir(20)Mn(80) System.

PubMed

Chen, Yuan-Tsung

2009-01-01

Exchange-biasing phenomenon can induce an evident unidirectional hysteresis loop shift by spin coupling effect in the ferromagnetic (FM)/antiferromagnetic (AFM) interface which can be applied in magnetoresistance random access memory (MRAM) and recording-head applications. However, magnetic properties are the most important to AFM texturing. In this work, top-configuration exchange-biasing NiFe/IrMn(x A) systems have been investigated with three different conditions. From the high-resolution cross-sectional transmission electron microscopy (HR X-TEM) and X-ray diffraction results, we conclude that the IrMn (111) texture plays an important role in exchange-biasing field (H(ex)) and interfacial exchange energy (J(k)). H(ex) and J(k) tend to saturate when the IrMn thickness increases. Moreover, the coercivity (H(c)) dependence on IrMn thickness is explained based on the coupling or decoupling effect between the spins of the NiFe and IrMn layers near the NiFe/IrMn interface. In this work, the optimal values for H(ex) and J(k) are 115 Oe and 0.062 erg/cm(2), respectively.

Observation of giant exchange bias in bulk Mn50Ni42Sn8 Heusler alloy

NASA Astrophysics Data System (ADS)

Sharma, Jyoti; Suresh, K. G.

2015-02-01

We report a giant exchange bias (EB) field of 3520 Oe in bulk Mn50Ni42Sn8 Heusler alloy. The low temperature magnetic state of the martensite phase has been studied by DC magnetization and AC susceptibility measurements. Frequency dependence of spin freezing temperature (Tf) on critical slowing down relation and observation of memory effect in zero field cooling mode confirms the super spin glass (SSG) phase at low temperatures. Large EB is attributed to the strong exchange coupling between the SSG clusters formed by small regions of ferromagnetic order embedded in an antiferromagnetic (AFM) matrix. The temperature and cooling field dependence of EB have been studied and related to the change in unidirectional anisotropy at SSG/AFM interface. The training effect also corroborates with the presence of frozen (SSG) moments at the interface and their role in EB.

Exchange Bias in Layered GdBaCo2O5.5 Cobaltite

NASA Astrophysics Data System (ADS)

Solin, N. I.; Naumov, S. V.; Telegin, S. V.; Korolev, A. V.

2017-12-01

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co2O5 + δ cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo2O5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H EB , blocking temperature T B , exchange coupling energy J i of antiferromagnet-ferromagnet (AFM-FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co3+ and Co4+ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.

Effects of substitution on the exchange stiffness and magnetization of Co films

NASA Astrophysics Data System (ADS)

Eyrich, C.; Zamani, A.; Huttema, W.; Arora, M.; Harrison, D.; Rashidi, F.; Broun, D.; Heinrich, B.; Mryasov, O.; Ahlberg, M.; Karis, O.; Jönsson, P. E.; From, M.; Zhu, X.; Girt, E.

2014-12-01

An antiferromagnetically coupled FM/NM/FM (FM = ferromagnet, NM = normal metal) trilayer structure responds to an external magnetic field by the formation of a magnetic-moment spring within the FM layers. We show that the exchange stiffness (Aex) of an FM layer can be determined by fitting the field-dependent magnetization, M (H ) , of the FM/NM/FM trilayer to a micromagnetic model. Using this method, we have measured the exchange stiffness of thin-film Co alloyed with Cr, Fe, Ni, Pd, Pt, and Ru. The results show that the rate at which a substituent element reduces the exchange stiffness is not directly related to its effect on the magnetization of the alloy. The observed trends have been understood by material-specific modeling based on density functional theory within the local density approximation. The stiffness measurements are in agreement with Brillouin light scattering carried out on thicker Co films.

A state interaction spin-orbit coupling density matrix renormalization group method

NASA Astrophysics Data System (ADS)

Sayfutyarova, Elvira R.; Chan, Garnet Kin-Lic

2016-06-01

We describe a state interaction spin-orbit (SISO) coupling method using density matrix renormalization group (DMRG) wavefunctions and the spin-orbit mean-field (SOMF) operator. We implement our DMRG-SISO scheme using a spin-adapted algorithm that computes transition density matrices between arbitrary matrix product states. To demonstrate the potential of the DMRG-SISO scheme we present accurate benchmark calculations for the zero-field splitting of the copper and gold atoms, comparing to earlier complete active space self-consistent-field and second-order complete active space perturbation theory results in the same basis. We also compute the effects of spin-orbit coupling on the spin-ladder of the iron-sulfur dimer complex [Fe2S2(SCH3)4]3-, determining the splitting of the lowest quartet and sextet states. We find that the magnitude of the zero-field splitting for the higher quartet and sextet states approaches a significant fraction of the Heisenberg exchange parameter.

All-Silicon Switchable Magnetoelectric Effect through Interlayer Exchange Coupling.

PubMed

Liu, Hang; Sun, Jia-Tao; Fu, Hui-Xia; Sun, Pei-Jie; Feng, Y P; Meng, Sheng

2017-07-19

The magnetoelectric (ME) effect originating from the effective coupling between electric field and magnetism is an exciting frontier in nanoscale science such as magnetic tunneling junction (MTJ), ferroelectric/piezoelectric heterojunctions etc. The realization of switchable ME effect under external electric field in d0 semiconducting materials of single composition is needed especially for all-silicon spintronics applications because of its natural compatibility with current industry. We employ density functional theory (DFT) to reveal that the pristine Si(111)-3×3 R30° (Si3 hereafter) reconstructed surfaces of thin films with a thickness smaller than eleven bilayers support a sizeable linear ME effect with switchable direction of magnetic moment under external electric field. This is achieved through the interlayer exchange coupling effect in the antiferromagnetic regime, where the spin-up and spin-down magnetized density is located on opposite surfaces of Si3 thin films. The obtained coefficient for the linear ME effect can be four times larger than that of ferromagnetic Fe films, which fail to have the reversal switching capabilities. The larger ME effect originates from the spin-dependent screening of the spin-polarized Dirac fermion. The prediction will promote the realization of well-controlled and switchable data storage in all-silicon electronics. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Antiferromagnetic Interlayer Exchange Coupling in All-Semiconducting EuS/PbS/EuS Trilayers

NASA Technical Reports Server (NTRS)

Smits, C. J. P.; Filip, A. T.; Swagten, H. J. M.; Koopmans, B.; deJonge, W. J. M.; Chernyshova, M.; Kowalczyk, L.; Grasza, K.; Szczerbakow, A.; Story, T.

2003-01-01

A comprehensive experimental study on the antiferromagnetic interlayer exchange coupling in high quality epitaxial all-semiconducting EuSPbSEuS trilayers is reported. The influence of substrates, the thickness of the non-magnetic PbS spacer layer, and of temperature, was investigated by means of SQUID magnetometry. In trilayers with a PbS thickness between 4 and 12 deg A the low temperature hysteresis loops showed the signature of antiferromagnetic coupling. The value of the interlayer exchange coupling energy was determined by simulating the data with a modified Stoner model, including Zeeman, anisotropy, and exchange coupling energies. An important observation was of a strong dependence of the interlayer exchange coupling energy on temperature, consistent with a power law dependence of the exchange coupling constant on the saturation magnetization of the EuS layers. While no theoretical description is readily available, we conjecture that the observed behavior is due to a dependence of the interlayer exchange coupling energy on the exchange splitting of the EuS conduction band.

Non-equilibrium magnetic interactions in strongly correlated systems

NASA Astrophysics Data System (ADS)

Secchi, A.; Brener, S.; Lichtenstein, A. I.; Katsnelson, M. I.

2013-06-01

We formulate a low-energy theory for the magnetic interactions between electrons in the multi-band Hubbard model under non-equilibrium conditions determined by an external time-dependent electric field which simulates laser-induced spin dynamics. We derive expressions for dynamical exchange parameters in terms of non-equilibrium electronic Green functions and self-energies, which can be computed, e.g., with the methods of time-dependent dynamical mean-field theory. Moreover, we find that a correct description of the system requires, in addition to exchange, a new kind of magnetic interaction, that we name twist exchange, which formally resembles Dzyaloshinskii-Moriya coupling, but is not due to spin-orbit, and is actually due to an effective three-spin interaction. Our theory allows the evaluation of the related time-dependent parameters as well.

Magnetisation switching of ECC grains in microwave-assisted magnetic recording

NASA Astrophysics Data System (ADS)

Greaves, Simon John; Muraoka, Hiroaki; Kanai, Yasushi

2018-05-01

Microwave-assisted magnetic recording was investigated using a planar write head and exchange-coupled composite (ECC) media. When recording on ECC media using a planar head field distribution and the high frequency field generated by a spin torque oscillator it was possible to switch the media magnetisation into the opposite direction to the head field, i.e. the media effectively had a negative coercive field. The conditions for this effect to occur are discussed.

[A review of water and carbon flux partitioning and coupling in SPAC using stable isotope techniques].

PubMed

Xu, Xiao Wu; Yu, Xin Xiao; Jia, Guo Dong; Li, Han Zhi; Lu, Wei Wei; Liu, Zi Qiang

2017-07-18

Soil-vegetation-atmosphere continuum (SPAC) is one of the important research objects in the field of terrestrial hydrology, ecology and global change. The process of water and carbon cycling, and their coupling mechanism are frontier issues. With characteristics of tracing, integration and indication, stable isotope techniques contribute to the estimation of the relationship between carbon sequestration and water consumption in ecosystems. In this review, based on a brief introduction of stable isotope principles and techniques, the applications of stable isotope techniques to water and carbon exchange in SPAC using optical stable isotope techniques were mainly explained, including: partitioning of net carbon exchange into photosynthesis and respiration; partitioning of evapotranspiration into transpiration and evaporation; coupling of water and carbon cycle at the ecosystem scale. Advanced techniques and methods provided long-term and high frequency measurements for isotope signals at the ecosystem scale, but the issues about the precision and accuracy for measurements, partitioning of ecosystem respiration, adaptability for models under non-steady state, scaling up, coupling mechanism of water and carbon cycles, were challenging. The main existing research findings, limitations and future research prospects were discussed, which might help new research and technology development in the field of stable isotope ecology.

Investigation of hurricane Ivan using the coupled ocean-atmosphere-wave-sediment transport (COAWST) model

USGS Publications Warehouse

Zambon, Joseph B.; He, Ruoying; Warner, John C.

2014-01-01

The coupled ocean–atmosphere–wave–sediment transport (COAWST) model is used to hindcast Hurricane Ivan (2004), an extremely intense tropical cyclone (TC) translating through the Gulf of Mexico. Sensitivity experiments with increasing complexity in ocean–atmosphere–wave coupled exchange processes are performed to assess the impacts of coupling on the predictions of the atmosphere, ocean, and wave environments during the occurrence of a TC. Modest improvement in track but significant improvement in intensity are found when using the fully atmosphere–ocean-wave coupled configuration versus uncoupled (e.g., standalone atmosphere, ocean, or wave) model simulations. Surface wave fields generated in the fully coupled configuration also demonstrates good agreement with in situ buoy measurements. Coupled and uncoupled model-simulated sea surface temperature (SST) fields are compared with both in situ and remote observations. Detailed heat budget analysis reveals that the mixed layer temperature cooling in the deep ocean (on the shelf) is caused primarily by advection (equally by advection and diffusion).

Shielding property for thermal equilibrium states in the quantum Ising model

NASA Astrophysics Data System (ADS)

Móller, N. S.; de Paula, A. L.; Drumond, R. C.

2018-03-01

We show that Gibbs states of nonhomogeneous transverse Ising chains satisfy a shielding property. Namely, whatever the fields on each spin and exchange couplings between neighboring spins are, if the field in one particular site is null, then the reduced states of the subchains to the right and to the left of this site are exactly the Gibbs states of each subchain alone. Therefore, even if there is a strong exchange coupling between the extremal sites of each subchain, the Gibbs states of the each subchain behave as if there is no interaction between them. In general, if a lattice can be divided into two disconnected regions separated by an interface of sites with zero applied field, then we can guarantee a similar result only if the surface contains a single site. Already for an interface with two sites we show an example where the property does not hold. When it holds, however, we show that if a perturbation of the Hamiltonian parameters is done in one side of the lattice, then the other side is completely unchanged, with regard to both its equilibrium state and dynamics.

Adiabatic physics of an exchange-coupled spin-dimer system: Magnetocaloric effect, zero-point fluctuations, and possible two-dimensional universal behavior

DOE PAGES

Brambleby, J.; Goddard, P. A.; Singleton, John; ...

2017-01-05

We present the magnetic and thermal properties of the bosonic-superfluid phase in a spin-dimer network using both quasistatic and rapidly changing pulsed magnetic fields. The entropy derived from a heat-capacity study reveals that the pulsed-field measurements are strongly adiabatic in nature and are responsible for the onset of a significant magnetocaloric effect (MCE). In contrast to previous predictions we show that the MCE is not just confined to the critical regions, but occurs for all fields greater than zero at sufficiently low temperatures. We explain the MCE using a model of the thermal occupation of exchange-coupled dimer spin states andmore » highlight that failure to take this effect into account inevitably leads to incorrect interpretations of experimental results. In addition, the heat capacity in our material is suggestive of an extraordinary contribution from zero-point fluctuations and appears to indicate universal behavior with different critical exponents at the two field-induced critical points. Finally, the data at the upper critical point, combined with the layered structure of the system, are consistent with a two-dimensional nature of spin excitations in the system.« less

Electronic and magnetic properties of double perovskite Sr2CoUO6: Heisenberg model

NASA Astrophysics Data System (ADS)

Nid-bahami, A.; Ahmed, S. Sidi; Ait-Tamerd, M.; Zaari, H.; El Kenz, A.; Benyoussef, A.

2018-01-01

This work will be focused on the electronic and magnetic properties of Sr2CoUO6 (SCUO) using ab-initio calculations and Monte Carlo Simulation (MCS). Firstly, we calculate the exchange coupling and the crystal field, then, the electronic and magnetic properties will be studied, using the full-potential linearized augmented plane wave (FP-LAPW) method, as implemented in the Wien2k code. This method employing the generalized gradient approximation (GGA) for exchange-correlation term. The half-metallic ferromagnetic nature implies a potential application of this new compound in spintronics devices. Also, we have presented the results of the band structures and densities of states for the two up and down spin polarizations. The exchange coupling and the crystal field calculated are J = 0 . 567 meV and δ = 0 . 559meV, and total spin magnetic moments is 2.96 μB closed to experimental values 3 μB. Secondly, we have presented the results for the magnetization and the susceptibility as a function of temperature. Finally, we obtain the critical temperature T = 9 . 20 K by MCS in good agreement with the experimental value.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hammond, Glenn Edward; Yang, Xiaofan; Song, Xuehang

The groundwater-surface water interaction zone (GSIZ) plays an important role in riverine and watershed ecosystems as the exchange of waters of variable composition and temperature (hydrologic exchange flows) stimulate microbial activity and associated biogeochemical reactions. Variable temporal and spatial scales of hydrologic exchange flows, heterogeneity of the subsurface environment, and complexity of biogeochemical reaction networks in the GSIZ present challenges to incorporation of fundamental process representations and model parameterization across a range of spatial scales (e.g. from pore-scale to field scale). This paper presents a novel hybrid multiscale simulation approach that couples hydrologic-biogeochemical (HBGC) processes between two distinct length scalesmore » of interest.« less

Micromagnetic Study of Perpendicular Magnetic Recording Media

NASA Astrophysics Data System (ADS)

Dong, Yan

With increasing areal density in magnetic recording systems, perpendicular recording has successfully replaced longitudinal recording to mitigate the superparamagnetic limit. The extensive theoretical and experimental research associated with perpendicular magnetic recording media has contributed significantly to improving magnetic recording performance. Micromagnetic studies on perpendicular recording media, including aspects of the design of hybrid soft underlayers, media noise properties, inter-grain exchange characterization and ultra-high density bit patterned media recording, are presented in this dissertation. To improve the writability of recording media, one needs to reduce the head-to-keeper spacing while maintaining a good texture growth for the recording layer. A hybrid soft underlayer, consisting of a thin crystalline soft underlayer stacked above a non-magnetic seed layer and a conventional amorphous soft underlayer, provides an alternative approach for reducing the effective head-to-keeper spacing in perpendicular recording. Micromagnetic simulations indicate that the media using a hybrid soft underlayer helps enhance the effective field and the field gradient in comparison with conventional media that uses only an amorphous soft underlayer. The hybrid soft underlayer can support a thicker non-magnetic seed layer yet achieve an equivalent or better effective field and field gradient. A noise plateau for intermediate recording densities is observed for a recording layer of typical magnetization. Medium noise characteristics and transition jitter in perpendicular magnetic recording are explored using micromagnetic simulation. The plateau is replaced by a normal linear dependence of noise on recording density for a low magnetization recording layer. We show analytically that a source of the plateau is similar to that producing the Non-Linear-Transition-Shift of signal. In particular, magnetostatic effects are predicted to produce positive correlation of jitter and thus negative correlation of noise at the densities associated with the plateau. One focus for developing perpendicular recording media is on how to extract intergranular exchange coupling and intrinsic anisotropy field dispersion. A micromagnetic numerical technique is developed to effectively separate the effects of intergranular exchange coupling and anisotropy dispersion by finding their correlation to differentiated M-H curves with different initial magnetization states, even in the presence of thermal fluctuation. The validity of this method is investigated with a series of intergranular exchange couplings and anisotropy dispersions for different media thickness. This characterization method allows for an experimental measurement employing a vibrating sample magnetometer (VSM). Bit patterned media have been suggested to extend areal density beyond 1 Tbit/in2. The feasibility of 4 Tbit/in2 bit patterned recording is determined by aspects of write head design and media fabrication, and is estimated by the bit error rate. Micromagnetic specifications including 2.3:1 BAR bit patterned exchange coupled composite media, trailing shield, and side shields are proposed to meet the requirement of 3x10 -4 bit error rate, 4 nm fly height, 5% switching field distribution, 5% timing and 5% jitter errors for 4 Tbit/in2 bit-patterned recording. Demagnetizing field distribution is examined by studying the shielding effect of the side shields on the stray field from the neighboring dots. For recording self-assembled bit-patterned media, the head design writes two staggered tracks in a single pass and has maximum perpendicular field gradients of 580 Oe/nm along the down-track direction and 476 Oe/nm along the cross-track direction. The geometry demanded by self-assembly reduces recording density to 2.9 Tbit/in 2.

Recovery and nonrecovery of the untrained state in an exchange-coupled system

NASA Astrophysics Data System (ADS)

Jutimoosik, Jaru; Yimnirun, Rattikorn; Setzer, Annette; Esquinazi, Pablo; Stahn, Jochen; Paul, Amitesh

2015-06-01

We report depth sensitive investigations of the magnetic interaction between exchange-coupled stacked CoO and ferromagnetic Co bilayers (separated by thick Au layers) as we explore the degree of recovery of the untrained state after the first two field cycles. Such a recovery is expected by field cycling a reorientation field (HRE) along a direction (ΩRE) away from the initial field cooling direction. Measurements as a function of ΩRE and the strength of HRE (along each direction) map the influence of ΩRE on the reversal mechanism in the layers and thereby the degree of recovery. Our results are consistent with the earlier observations in similar systems that was realized with ΩRE=90∘ . We ascribe these partial and/or significant recoveries to the unchanged sense of rotation after initial field cooling of the ferromagnetic magnetization upon each field cycling. Furthermore, in our system, we find that this recovery can be regulated by choosing various other HRE and ΩRE values without changing the rotational sense. The best recipe for recovery is identified for ΩRE=45∘ , that can be achieved partially with HRE=3.0 kOe and remain significant even with HRE=10.0 kOe. In this study we not only understand the fundamental mechanism in the recovery of training, but also instigate its technological prospects by lifting the directional restrictions of the reorientation field.

FAST TRACK COMMUNICATION: Magnetic exchange hardening in polycrystalline GdN thin films

NASA Astrophysics Data System (ADS)

Senapati, K.; Fix, T.; Vickers, M. E.; Blamire, M. G.; Barber, Z. H.

2010-08-01

We report the observation of intrinsic exchange hardening in polycrystalline GdN thin films grown at room temperature by magnetron sputtering. We find, in addition to the ferromagnetic phase, that a fraction of GdN crystallizes in a structural polymorphic form which orders antiferromagnetically. The relative fraction of these two phases was controlled by varying the relative abundance of reactive species in the sputtering plasma by means of the sputtering power and N2 partial pressure. An exchange bias of ~ 30 Oe was observed at 10 K. The exchange coupling between the ferromagnetic and the antiferromagnetic phases resulted in an order of magnitude enhancement in the coercive field in these films.

Study of switching behavior of exchange-coupled nanomagnets by transverse magnetization metrology

NASA Astrophysics Data System (ADS)

Dey, Himadri S.; Csaba, Gyorgy; Bernstein, Gary H.; Porod, Wolfgang

2017-05-01

We investigate the static switching modes of nanomagnets patterned from antiferromagnetically exchange-coupled magnetic multilayers, and compare them to nanomagnets having only dipole coupling between the ferromagnetic layers. Vibrating sample magnetometry experiments, supported by micromagnetic simulations, reveal two distinct switching mechanisms between the exchange-coupled and only dipole-coupled nanomagnets. The exchange-coupled nanomagnets exhibit gradual switching of the layers, dictated by the strong antiferromagnetic exchange coupling present between the layers. However, the layers of the only dipole-coupled nanomagnets show abrupt nucleation/growth type switching. A comprehensive understanding of the switching modes of such layered and patterned systems can add new insight into the reversal mechanisms of similar systems employed for spintronic and magneto-logic device applications.

Coupled oscillations of vortex cores confined in a ferromagnetic elliptical disk

NASA Astrophysics Data System (ADS)

Hata, Hiroshi; Goto, Minori; Yamaguchi, Akinobu; Sato, Tomonori; Nakatani, Yoshinobu; Nozaki, Yukio

2014-09-01

By solving the Thiele equation with simultaneous application of a radio-frequency (rf) magnetic field (hrf) and an rf spin current (jsp), the dynamic susceptibility of exchange-coupled vortices in response to hrf and jsp was obtained. It was found that the four eigenmodes expected for two vortices trapped in a magnetic elliptical disk were coupled to different components of hrf and jsp. As a consequence, orthogonal hrf and jsp (which are simultaneously generated by the application of an rf current to an elliptical disk) can excite two modes with different eigenfrequencies. This result suggests that a fieldlike nonadiabatic torque caused by an rf spin current can be spectroscopically distinguished from the one caused by the rf magnetic field.

High-field Overhauser dynamic nuclear polarization in silicon below the metal-insulator transition.

PubMed

Dementyev, Anatoly E; Cory, David G; Ramanathan, Chandrasekhar

2011-04-21

Single crystal silicon is an excellent system to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high-doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of (29)Si in n-doped silicon crystals, with doping concentrations in the range of (1-3) × 10(17) cm(-3). In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the (29)Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is a rate-limiting step. Since only about 2% of the silicon nuclei are located within 1 Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of 244 ± 84 resulting in a silicon polarization of 10.4 ± 3.4% following 2 h of microwave irradiation.

Exchange coupling and NOL magnetism consideration in Co 1-xFe x specular spin-valves

NASA Astrophysics Data System (ADS)

Doi, M.; Izumi, M.; Abe, Y.; Fukuzawa, H.; Fuke, H. N.; Iwasaki, H.; Sahashi, M.

2005-02-01

To confirm the origin of the exchange coupling through the nano-oxide layer (NOL), the detailed analyses of the magnetization curve and the temperature dependence on magnetization for the Co1-xFex-NOL samples with various oxidation processes are investigated. The field cooling effect is observed for NOL-inserted samples, which suggests the existence of the antiferromagnetic (AFM) component in the NOL. The characteristic temperature (blocking temperature) increases with increase in the Fe content for Co1-xFex(x=0.08,0.17,0.26)-NOL. These results support that NOL pinning appears higher than room temperature. Further, the twisted coupling through the NOL observed for the Co0.50Fe0.50-NOL is discussed from the viewpoint of the magnetic roughness due to the ferrimagnetic nature of the very thin NOL in specular spin valve (SPSV).

Spin effects in transport through single-molecule magnets in the sequential and cotunneling regimes

NASA Astrophysics Data System (ADS)

Misiorny, Maciej; Weymann, Ireneusz; Barnaś, Józef

2009-06-01

We analyze the stationary spin-dependent transport through a single-molecule magnet weakly coupled to external ferromagnetic leads. Using the real-time diagrammatic technique, we calculate the sequential and cotunneling contributions to current, tunnel magnetoresistance, and Fano factor in both linear and nonlinear response regimes. We show that the effects of cotunneling are predominantly visible in the blockade regime and lead to enhancement of tunnel magnetoresistance (TMR) above the Julliere value, which is accompanied with super-Poissonian shot noise due to bunching of inelastic cotunneling processes through different virtual spin states of the molecule. The effects of external magnetic field and the role of type and strength of exchange interaction between the LUMO level and the molecule’s spin are also considered. When the exchange coupling is ferromagnetic, we find an enhanced TMR, while in the case of antiferromagnetic coupling we predict a large negative TMR effect.

The nature of the exchange coupling between high-spin Fe(III) heme o3 and CuBII in Escherichia coli quinol oxidase, cytochrome bo3: MCD and EPR studies.

PubMed

Cheesman, Myles R; Oganesyan, Vasily S; Watmough, Nicholas J; Butler, Clive S; Thomson, Andrew J

2004-04-07

Fully oxidized cytochrome bo3 from Escherichia coli has been studied in its oxidized and several ligand-bound forms using electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopies. In each form, the spin-coupled high-spin Fe(III) heme o3 and CuB(II) ion at the active site give rise to similar fast-relaxing broad features in the dual-mode X-band EPR spectra. Simulations of dual-mode spectra are presented which show that this EPR can arise only from a dinuclear site in which the metal ions are weakly coupled by an anisotropic exchange interaction of J 1 cm-1. A variable-temperature and magnetic field (VTVF) MCD study is also presented for the cytochrome bo3 fluoride and azide derivatives. New methods are used to extract the contribution to the MCD of the spin-coupled active site in the presence of strong transitions from low-spin Fe(III) heme b. Analysis of the MCD data, independent of the EPR study, also shows that the spin-coupling within the active site is weak with J approximately 1 cm-1. These conclusions overturn a long-held view that such EPR signals in bovine cytochrome c oxidase arise from an S' = 2 ground state resulting from strong exchange coupling (J > 10(2) cm-1) within the active site.

Interface mixing and its impact on exchange coupling in exchange biased systems

DOE PAGES

Manna, P. K.; Skoropata, E.; Ting, Y-W; ...

2016-10-05

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni 80Fe 20/CoO(t CoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. But, in our trilayers the interface ferrites were found not to be ferro-or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealedmore » that (i) interlayer exchange coupling was present between Ni 80Fe 20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. Here, we also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent.« less

Evaluating the Performance of the ff99SB Force Field Based on NMR Scalar Coupling Data

PubMed Central

Wickstrom, Lauren; Okur, Asim; Simmerling, Carlos

2009-01-01

Abstract Force-field validation is essential for the identification of weaknesses in current models and the development of more accurate models of biomolecules. NMR coupling and relaxation methods have been used to effectively diagnose the strengths and weaknesses of many existing force fields. Studies using the ff99SB force field have shown excellent agreement between experimental and calculated order parameters and residual dipolar calculations. However, recent studies have suggested that ff99SB demonstrates poor agreement with J-coupling constants for short polyalanines. We performed extensive replica-exchange molecular-dynamics simulations on Ala3 and Ala5 in TIP3P and TIP4P-Ew solvent models. Our results suggest that the performance of ff99SB is among the best of currently available models. In addition, scalar coupling constants derived from simulations in the TIP4P-Ew model show a slight improvement over those obtained using the TIP3P model. Despite the overall excellent agreement, the data suggest areas for possible improvement. PMID:19651043

Ground-state phase diagram in the Kugel-Khomskii model with finite spin-orbit interactions

NASA Astrophysics Data System (ADS)

Koga, Akihisa; Nakauchi, Shiryu; Nasu, Joji

2018-05-01

We study ground-state properties in the Kugel-Khomskii model on the two-dimensional honeycomb lattice. Using the cluster mean-field approximations, we deal with the exchange and spin-orbit couplings on an equal footing. We then discuss the stability of the ferromagnetically ordered states against the nonmagnetic state, which is adiabatically connected to the quantum spin liquid state realized in a strong spin-orbit coupling limit.

Highly anisotropic exchange interactions in a trigonal bipyramidal cyanide-bridged Ni(II)3Os(III)2 cluster.

PubMed

Palii, Andrei V; Reu, Oleg S; Ostrovsky, Sergei M; Klokishner, Sophia I; Tsukerblat, Boris S; Hilfiger, Matthew; Shatruk, Michael; Prosvirin, Andrey; Dunbar, Kim R

2009-06-25

This article is a part of our efforts to control the magnetic anisotropy in cyanide-based exchange-coupled systems with the eventual goal to obtain single-molecule magnets with higher blocking temperatures. We give the theoretical interpretation of the magnetic properties of the new pentanuclear complex {[Ni(II)(tmphen)(2)](3)[Os(III)(CN)(6)](2)} x 6 CH(3)CN (Ni(II)(3)Os(III)(2) cluster). Because the system contains the heavy Os(III) ions, spin-orbit coupling considerably exceeds the contributions from the low-symmetry crystal field and exchange coupling. The magnetic properties of the Ni(II)(3)Os(III)(2) cluster are described in the framework of a highly anisotropic pseudo-spin Hamiltonian that corresponds to the limit of strong spin-orbital coupling and takes into account the complex molecular structure. The model provides a good fit to the experimental data and allows the conclusion that the trigonal axis of the bipyramidal Ni(II)(3)Os(III)(2) cluster is a hard axis of magnetization. This explains the fact that in contrast with the isostructural trigonal bipyramidal Mn(III)(2)Mn(II)(3) cluster, the Ni(II)(3)Os(III)(2) system does not exhibit the single-molecule magnetic behavior.

Modelling the Auroral Magnetosphere-Ionosphere Coupling System at Jupiter

NASA Astrophysics Data System (ADS)

Bunce, E. J.; Cowley, S.; Provan, G.

2016-12-01

The magnetosphere-ionosphere coupling system at Jupiter is a topic of central significance in understanding the fundamental properties of its large-scale plasma environment. Theoretical discussion of this topic typically considers the properties of the field-aligned current systems that form part of a large-scale magnetosphere-ionosphere coupling current system associated with momentum exchange between the ionosphere and the magnetosphere, communicated via the magnetic field. The current system associated with the main oval is believed to be related to centrifugally-driven outward radial transport of iogenic plasma that leads to sub-corotation in the middle magnetosphere. In addition to the magnetosphere-ionosphere coupling current system, upward-directed field-aligned currents may flow at the open-closed field line boundary due to the shear between outer closed field lines and open field lines, which may relate to emission poleward of the main oval. An axi-symmetric model of the plasma flow in the jovian system, the related coupling currents, and the consequent auroral precipitation based on these combined ideas was initially devised to represent typical steady-state conditions for the system and later extended to consider auroral effects resulting from sudden compressions of the magnetosphere. More recently, the model has been extended along model magnetic field lines into the magnetosphere in order to relate them to in situ observations from the NASA Juno spacecraft at Jupiter. The field-aligned coupling currents associated with the modelled current systems produce a readily-observable azimuthal field signature that bends the field lines out of magnetic meridians. Here we show the computed azimuthal fields produced by our model auroral current system throughout the region between the ionosphere and the magnetic equator, and illustrate the results by evaluation of various model parameters (e.g. field-aligned current density, accelerating voltages, accelerated energy flux) along the Juno orbits.

Dual Control of Giant Field-like Spin Torque in Spin Filter Tunnel Junctions

PubMed Central

Tang, Y. -H.; Chu, F. -C.; Kioussis, Nicholas

2015-01-01

We predict a giant field-like spin torque, , in spin-filter (SF) barrier tunnel junctions in sharp contrast to existing junctions based on nonmagnetic passive barriers. We demonstrate that has linear bias behavior, is independent of the SF thickness, and has odd parity with respect to the SF’s exchange splitting. Thus, it can be selectively controlled via external bias or external magnetic field which gives rise to sign reversal of via magnetic field switching. The underlying mechanism is the interlayer exchange coupling between the noncollinear magnetizations of the SF and free ferromagnetic electrode via the nonmagnetic insulating (I) spacer giving rise to giant spin-dependent reflection at the SF/I interface. These findings suggest that the proposed field-like-spin-torque MRAM may provide promising dual functionalities for both ‘reading’ and ‘writing’ processes which require lower critical current densities and faster writing and reading speeds. PMID:26095146

Perpendicular exchange coupling effects in ferrimagnetic TbFeCo/GdFeCo hard/soft structures

NASA Astrophysics Data System (ADS)

Wang, Ke; Wang, Yahong; Ling, Fujin; Xu, Zhan

2018-04-01

Bilayers consisting of magnetically hard TbFeCo and soft GdFeCo alloy were fabricated. Exchange-spring and sharp switching in a step-by-step fashion were observed in the TbFeCo/GdFeCo hard/soft bilayers with increasing GdFeCo thickness. A perpendicular exchange bias field of several hundred Oersteds is observed from the shift of minor loops pinned by TbFeCo layer. The perpendicular exchange energy is derived to be in the range of 0.18-0.30 erg/cm2. The exchange energy is shown to increase with the thickness of GdFeCo layer in the bilayers, which can be attributed to the enhanced perpendicular anisotropy of GdFeCo layer in our experimental range.

Comparison of the function of exchange in couples of similar and differing physical attractiveness.

PubMed

Murstein, Bernard I; Reif, Jeffrey A; Syracuse-Siewert, Gia

2002-08-01

Couples (22 young, married, 18 young unmarried) completed the Exchange Orientation Scale, which measures how much individuals believe equality of exchange should characterize their social relationships, as well as the Norman Personality Trait Scale, which gives measures for the self and ideal-self. Couples also rated their sexual satisfaction, their partners' physical attractiveness, their own attractiveness, and had their photographs evaluated for attractiveness by disinterested raters. Subgroups were formed of physically attractive couples, couples in which the members were of disparate attractiveness, and couples in which the individuals perceived themselves as equal to their partners in attractiveness or as inferior. In accordance with exchange theory, the hypotheses were (1) unmarried men would show higher exchange scores than married men, (2) attractive men in disparately attractive couples would show greater exchange scores than attractive men in both-attractive couples, (3) attractive members of disparately attractive couples would possess lower self-acceptance and (4) experience greater sexual satisfaction than attractive members of both-attractive couples. Results supported all hypotheses in varying extents.

Hyperfine spin interactions between polarons and nuclei in organic light emitting diodes: Magneto-EL measurements

NASA Astrophysics Data System (ADS)

Crooker, S. A.; Kelley, M. R.; Martinez, N.; Nie, W.; Mohite, A. D.; Smith, D. L.; Tretiak, S.; Ruden, P. P.

2014-03-01

Considerable attention in recent years has focused on the effects of applied magnetic fields on the conductance, photocurrent, electroluminescence (EL), and photoluminescence of nominally nonmagnetic organic semiconductor materials and devices. These magnetic field effects have proven useful in revealing the underlying physical mechanisms and relevant spin interactions that influence the electrical and optical properties in these organic systems (e.g., hyperfine coupling, exchange interactions, and spin-orbit coupling). Here we study the field-dependent properties of organic light-emitting diode (OLEDs) based on MTDATA/LiF/Bphen layered structures, in which exciplex recombination at the interface dominates the EL spectra. Small applied magnetic fields (~10 mT) are found to boost the net EL yield by up to 10%, due to a suppression of the mixing between singlet and triplet polaron pairs which, in turn, arises from hyperfine spin coupling of the polarons to the underlying nuclei of the host molecules. We discuss the dependence of these field-induced effects on the LiF barrier thickness, device bias, and on the orientation of the applied magnetic field, as well as the mechanisms responsible.

Brillouin Light Scattering from Magnetic Excitations in Superparamagnetic and Ferromagnetic Co-Al-O Granular Films

NASA Astrophysics Data System (ADS)

Yoshihara, Akira; Maeda, Toshiteru; Kawamura, Satoshi; Nakamura, Shintaro; Nojima, Tsutomu; Takeda, Yoshihiko; Ohnuma, Shigehiro

2018-04-01

A systematic study of Brillouin light scattering (BLS) from superparamagnetic (SPM) and ferromagnetic (FM) Co-Al-O granular films was performed under magnetic fields of up to 4.6 kOe in the standard backscattering geometry at room temperature. The SPM and FM boundary, defined as the Co composition at which the exchange field vanishes, was found to be located at xC(Co) = 59.3 ± 1.3 at. %. From FM films we observed a pair of bulk spin-wave peaks on both the positive- and negative-frequency sides and a surface localized Damon-Eshbach peak only on the positive-frequency side under the present scattering conditions. From SPM films, a pair of broader but propagative excitation peaks with asymmetric intensity were observed on both frequency sides in a spectrum. We performed a numerical analysis of the BLS spectrum by employing the theory developed by Camley and Mills (CM) while retaining dipole and exchange couplings for FM films and only dipole coupling for SPM films. The CM theory successfully reproduced the observed spectrum for both SPM and FM films. The SPM spectrum exhibits a singlet-doublet peak structure similarly to an FM SW spectrum. The SPM peak stems from the dipole-coupled larger-amplitude precession motion of the granule magnetic moment around the external-field-induced magnetization.

Observation of giant exchange bias in bulk Mn{sub 50}Ni{sub 42}Sn{sub 8} Heusler alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sharma, Jyoti; Suresh, K. G., E-mail: suresh@iitb.ac.in

2015-02-16

We report a giant exchange bias (EB) field of 3520 Oe in bulk Mn{sub 50}Ni{sub 42}Sn{sub 8} Heusler alloy. The low temperature magnetic state of the martensite phase has been studied by DC magnetization and AC susceptibility measurements. Frequency dependence of spin freezing temperature (T{sub f}) on critical slowing down relation and observation of memory effect in zero field cooling mode confirms the super spin glass (SSG) phase at low temperatures. Large EB is attributed to the strong exchange coupling between the SSG clusters formed by small regions of ferromagnetic order embedded in an antiferromagnetic (AFM) matrix. The temperature and coolingmore » field dependence of EB have been studied and related to the change in unidirectional anisotropy at SSG/AFM interface. The training effect also corroborates with the presence of frozen (SSG) moments at the interface and their role in EB.« less

Quantum dynamics of a two-atom-qubit system

NASA Astrophysics Data System (ADS)

Van Hieu, Nguyen; Bich Ha, Nguyen; Linh, Le Thi Ha

2009-09-01

A physical model of the quantum information exchange between two qubits is studied theoretically. The qubits are two identical two-level atoms, the physical mechanism of the quantum information exchange is the mutual dependence of the reduced density matrices of two qubits generated by their couplings with a multimode radiation field. The Lehmberg-Agarwal master equation is exactly solved. The explicit form of the mutual dependence of two reduced density matrices is established. The application to study the entanglement of two qubits is discussed.

Topological Defects in Double Exchange Materials and Anomalous Hall Resistance.

NASA Astrophysics Data System (ADS)

Calderón, M. J.; Brey, L.

2000-03-01

Recently it has been proposed that the anomalous Hall effect observed in Double Exchange materials is due to Berry phase effects caused by carrier hopping in a nontrivial spins background (J.Ye et al.) Phys.Rev.Lett. 83, 3737 1999.In order to study this possibility we have performed Monte Carlo simulations of the Double Exchange model and we have computed, as a function of the temperature, the number of topological defects in the system and the internal gauge magnetic field associated with these defects. In the simplest Double Exchange model the gauge magnetic field is random, and its average value is zero. The inclusion in the problem of spin-orbit coupling privileges the opposite direction of the magnetization and an anomalous Hall resistance (AHR) effect arises. We have computed the AHR, and we have obtained its temperature dependence. In agreement with previous experiments we obtain that AHR increases exponentially at low temperature and presents a maximum at a temperature slightly higher than the critical temperature.

Evaluation of the exchange interaction and crystal fields in a prototype Dy2 SMM

NASA Astrophysics Data System (ADS)

Zhang, Qing; Sarachik, Myriam; Baker, Michael; Chen, Yizhang; Kent, Andrew; Pineda, Eufemio; McInnes, Eric

In order to gain an understanding of the INS and magnetization data obtained for Dy2, the simplest member of a newly synthesized family of dysprosium-based molecular magnets, we report on calculations of the magnetic behavior of a Dy2 cluster with the formula [hqH2][Dy2(hq)4(NO3)3].MeOH. The molecular complex contains one high symmetry Dy(III) ion and one low symmetry Dy(III) ion. Our calculations suggest that exchange coupling between the two ions controls the behavior of the magnetization at low temperature, while the crystal field of the low symmetry Dy(III) ion controls the behavior at higher temperature. A point charge electrostatic model, based on crystallographic coordinates, provides a starting point for the determination of the crystal field. Parameters in these calculations are adjusted to provide best fits to inelastic neutron scattering data (INS) and low temperature magnetometry: the INS measurements access crystal field energies and low temperature magnetization probes the Dy-Dy exchange interaction. Work supported by ARO W911NF-13-1-1025 (CCNY) and NSF-DMR-1309202 (NYU).

Theory of proximity-induced exchange coupling in graphene on hBN/(Co, Ni)

NASA Astrophysics Data System (ADS)

Zollner, Klaus; Gmitra, Martin; Frank, Tobias; Fabian, Jaroslav

2016-10-01

Graphene, being essentially a surface, can borrow some properties of an insulating substrate (such as exchange or spin-orbit couplings) while still preserving a great degree of autonomy of its electronic structure. Such derived properties are commonly labeled as proximity. Here we perform systematic first-principles calculations of the proximity exchange coupling, induced by cobalt (Co) and nickel (Ni) in graphene, via a few (up to three) layers of hexagonal boron nitride (hBN). We find that the induced spin splitting of the graphene bands is of the order of 10 meV for a monolayer of hBN, decreasing in magnitude but alternating in sign by adding each new insulating layer. We find that the proximity exchange can be giant if there is a resonant d level of the transition metal close to the Dirac point. Our calculations suggest that this effect could be present in Co heterostructures, in which a d level strongly hybridizes with the valence-band orbitals of graphene. Since this hybridization is spin dependent, the proximity spin splitting is unusually large, about 10 meV even for two layers of hBN. An external electric field can change the offset of the graphene and transition-metal orbitals and can lead to a reversal of the sign of the exchange parameter. This we predict to happen for the case of two monolayers of hBN, enabling electrical control of proximity spin polarization (but also spin injection) in graphene/hBN/Co structures. Nickel-based heterostructures show weaker proximity effects than cobalt heterostructures. We introduce two phenomenological models to describe the first-principles data. The minimal model comprises the graphene (effective) pz orbitals and can be used to study transport in graphene with proximity exchange, while the pz-d model also includes hybridization with d orbitals, which is important to capture the giant proximity exchange. Crucial to both models is the pseudospin-dependent exchange coupling, needed to describe the different spin splittings of the valence and conduction bands.

A state interaction spin-orbit coupling density matrix renormalization group method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sayfutyarova, Elvira R.; Chan, Garnet Kin-Lic

We describe a state interaction spin-orbit (SISO) coupling method using density matrix renormalization group (DMRG) wavefunctions and the spin-orbit mean-field (SOMF) operator. We implement our DMRG-SISO scheme using a spin-adapted algorithm that computes transition density matrices between arbitrary matrix product states. To demonstrate the potential of the DMRG-SISO scheme we present accurate benchmark calculations for the zero-field splitting of the copper and gold atoms, comparing to earlier complete active space self-consistent-field and second-order complete active space perturbation theory results in the same basis. We also compute the effects of spin-orbit coupling on the spin-ladder of the iron-sulfur dimer complex [Fe{submore » 2}S{sub 2}(SCH{sub 3}){sub 4}]{sup 3−}, determining the splitting of the lowest quartet and sextet states. We find that the magnitude of the zero-field splitting for the higher quartet and sextet states approaches a significant fraction of the Heisenberg exchange parameter.« less

Electric-field-induced modification in Curie temperature of Co monolayer on Pt(111)

NASA Astrophysics Data System (ADS)

Nakamura, Kohji; Oba, Mikito; Akiyama, Toru; Ito, Tomonori; Weinert, Michael

2015-03-01

Magnetism induced by an external electric field (E-field) has received much attention as a potential approach for controlling magnetism at the nano-scale with the promise of ultra-low energy power consumption. Here, the E-field-induced modification of the Curie temperature for a prototypical transition-metal thin layer of a Co monolayer on Pt(111) is investigated by first-principles calculations by using the full-potential linearized augmented plane wave method that treats spin-spiral structures in an E-field. An applied E-field modifies the magnon (spin-spiral formation) energies by a few meV, which leads to a modification of the exchange pair interaction parameters within the classical Heisenberg model. With inclusion of the spin-orbit coupling (SOC), the magnetocrystalline anisotropy and the Dzyaloshinskii-Morita interaction are obtained by the second variation SOC method. An E-field-induced modification of the Curie temperature is demonstrated by Monte Carlo simulations, in which a change in the exchange interaction is found to play a key role.

Surface and exchange-bias effects in compacted CaMnO3-δ nanoparticles

NASA Astrophysics Data System (ADS)

Markovich, V.; Fita, I.; Wisniewski, A.; Puzniak, R.; Mogilyansky, D.; Titelman, L.; Vradman, L.; Herskowitz, M.; Gorodetsky, G.

2008-02-01

Magnetic properties of compacted 50nm CaMnO3-δ (CMO) nanoparticles have been investigated. Measurements of ac-susceptibility exhibit upon cooling two magnetic transitions at Ttilde 270K accompanied by a small spontaneous magnetic moment and a para-antiferromagnetic (AFM) transition at TN˜120K , observed previously in bulk CMO. Asymmetric magnetization hysteresis loops observed in applied magnetic fields H≤90kOe are attributed to an exchange coupling between the antiferromagnetic core and the ferromagnetic (FM) shell of the CMO nanoparticles. This work provides the observation of exchange bias effect in manganite nanoparticles with inverted AFM-core-FM-shell structure, as compared to the typical FM-core-AFM-shell. Effects of surface and exchange anisotropy are also discussed.

Development and applications of a Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System

NASA Astrophysics Data System (ADS)

Warner, J. C.; Armstrong, B. N.; He, R.; Zambon, J. B.; Olabarrieta, M.; Voulgaris, G.; Kumar, N.; Haas, K. A.

2012-12-01

Understanding processes responsible for coastal change is important for managing both our natural and economic coastal resources. Coastal processes respond from both local scale and larger regional scale forcings. Understanding these processes can lead to significant insight into how the coastal zone evolves. Storms are one of the primary driving forces causing coastal change from a coupling of wave and wind driven flows. Here we utilize a numerical modeling approach to investigate these dynamics of coastal storm impacts. We use the Coupled Ocean - Atmosphere - Wave - Sediment Transport (COAWST) Modeling System that utilizes the Model Coupling Toolkit to exchange prognostic variables between the ocean model ROMS, atmosphere model WRF, wave model SWAN, and the Community Sediment Transport Modeling System (CSTMS) sediment routines. The models exchange fields of sea-surface temperature, ocean currents, water levels, bathymetry, wave heights, lengths, periods, bottom orbital velocities, and atmospheric surface heat and momentum fluxes, atmospheric pressure, precipitation, and evaporation. Data fields are exchanged using regridded flux conservative sparse matrix interpolation weights computed from the SCRIP spherical coordinate remapping interpolation package. We describe the modeling components and the model field exchange methods. As part of the system, the wave and ocean models run with cascading, refined, spatial grids to provide increased resolution, scaling down to resolve nearshore wave driven flows simulated by the vortex force formulation, all within selected regions of a larger, coarser-scale coastal modeling system. The ocean and wave models are driven by the atmospheric component, which is affected by wave dependent ocean-surface roughness and sea surface temperature which modify the heat and momentum fluxes at the ocean-atmosphere interface. We describe the application of the modeling system to several regions of multi-scale complexity to identify the significance of larger scale forcing cascading down to smaller scales and to investigate the interactions of the coupled system with increasing degree of model-model interactions. Three examples include the impact of Hurricane Ivan in 2004 in the Gulf of Mexico, Hurricane Ida in 2009 that evolved into a tropical storm on the US East coast, and passage of strong cold fronts across the US southeast. Results identify that hurricane intensity is extremely sensitive to sea-surface temperature, with a reduction in intensity when the atmosphere is coupled to the ocean model due to rapid cooling of the ocean from the surface through the mixed layer. Coupling of the ocean to the atmosphere also results in decreased boundary layer stress and coupling of the waves to the atmosphere results in increased sea-surface stress. Wave results are sensitive to both ocean and atmospheric coupling due to wave-current interactions with the ocean and wave-growth from the atmospheric wind stress. Sediment resuspension at regional scale during the hurricane is controlled by shelf width and wave propagation during hurricane approach. Results from simulation of passage of cold fronts suggest that synoptic meteorological systems can strongly impact surf zone and inner shelf response, therefore act as a strong driver for long term littoral sediment transport. We will also present some of the challenges faced to develop the modeling system.

Temperature dependence of the NMR spin-lattice relaxation rate for spin-1/2 chains

NASA Astrophysics Data System (ADS)

Coira, E.; Barmettler, P.; Giamarchi, T.; Kollath, C.

2016-10-01

We use recent developments in the framework of a time-dependent matrix product state method to compute the nuclear magnetic resonance relaxation rate 1 /T1 for spin-1/2 chains under magnetic field and for different Hamiltonians (XXX, XXZ, isotropically dimerized). We compute numerically the temperature dependence of the 1 /T1 . We consider both gapped and gapless phases, and also the proximity of quantum critical points. At temperatures much lower than the typical exchange energy scale, our results are in excellent agreement with analytical results, such as the ones derived from the Tomonaga-Luttinger liquid (TLL) theory and bosonization, which are valid in this regime. We also cover the regime for which the temperature T is comparable to the exchange coupling. In this case analytical theories are not appropriate, but this regime is relevant for various new compounds with exchange couplings in the range of tens of Kelvin. For the gapped phases, either the fully polarized phase for spin chains or the low-magnetic-field phase for the dimerized systems, we find an exponential decrease in Δ /(kBT ) of the relaxation time and can compute the gap Δ . Close to the quantum critical point our results are in good agreement with the scaling behavior based on the existence of free excitations.

Solvable multistate model of Landau-Zener transitions in cavity QED

DOE PAGES

Sinitsyn, Nikolai; Li, Fuxiang

2016-06-29

We consider the model of a single optical cavity mode interacting with two-level systems (spins) driven by a linearly time-dependent field. When this field passes through values at which spin energy level splittings become comparable to spin coupling to the optical mode, a cascade of Landau-Zener (LZ) transitions leads to co-flips of spins in exchange for photons of the cavity. We derive exact transition probabilities between different diabatic states induced by such a sweep of the field.

"Advances in Coupled Air Quality, Farm Management and Biogeochemistry to address bidirectional ammonia flux"

EPA Science Inventory

A cropland farm management modeling system for regional air quality and field-scale applications of bi-directional ammonia exchange was presented at ITM XXI. The goal of this research is to improve estimates of nitrogen deposition to terrestrial and aquatic ecosystems and ambien...

Intermixing enables strong exchange coupling in nanocomposites: Magnetism through the interfacial ferrite in γ -Fe2O3/NiO

NASA Astrophysics Data System (ADS)

Skoropata, E.; Su, T. T.; Ouyang, H.; Freeland, J. W.; van Lierop, J.

2017-07-01

γ -Fe2O3 particles, surface modified with NiO crystallites, form a unique nanocomposite that points to how to tune strong interfacial exchange coupling. We find that Ni2 + migrates into the octahedral sites of the γ -Fe2O3 nanoparticle surface, and this NiFe2O4 -like layer permits effective magnetic coupling of Ni and Fe sites that strengthens the interface exchange. A large increase in coercivity coinciding with a loss of exchange bias is achieved by this strong interfacial coupling that results in a Ni2 + moment reversal in the NiO with the γ -Fe2O3 . This work reveals the importance of intermixing in, and possibility to use, such an exchange coupling regime to alter substantially the coercivity and hence control an important property of exchange-coupled nanocomposite magnets.

Evidence of exchange-coupled behavior in chromium-cobalt ferrite nanoparticles

NASA Astrophysics Data System (ADS)

Tanbir, Kamar; Sharma, Lalit Kumar; Aakash; Singh, Rakesh Kumar; Choubey, Ravi Kant; Mukherjee, Samrat

2018-06-01

Cr doped cobalt ferrite nanoparticles were synthesized with the generic formula Co1-xCrxFe2O4 (x = 0, 0.05, 0.15, 0.25) through standard chemical co-precipitation method. XRD studies confirmed the pure spinel cubic structure belonging to Fd 3 bar m space group. From the Williamson-Hall plots, crystallite sizes were found to lie within the range (42 ± 1) nm for the different doped samples. The lattice parameter was found to decrease linearly with increase in the concentration of Cr3+ ion. The magnetic behavior of the samples was determined by M-H studies at 300 K, field cooled (5 T) at 5 K and temperature dependent studies. The M-H at 300 K show soft magnetic behavior whereas the M-H plots at 5 K predict the existence of in-homogeneity of the exchange interactions due to strong exchange coupling between the spins at the core and the surface of the nanoparticles.

Er 3+-Yb 3+ co-doped glass waveguide amplifiers using ion exchange and field-assisted annealing

NASA Astrophysics Data System (ADS)

Zhang, X. Z.; Liu, K.; Mu, S. K.; Tan, C. Z.; Zhang, D.; Pun, E. Y. B.; Zhang, D. M.

2006-12-01

Er 3+-Yb 3+ co-doped waveguide amplifiers fabricated using thermal two-step ion-exchange are demonstrated. K +-Na + ion-exchange process was first carried out in pure KNO 3 molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The effective buried depth is estimated to be ˜3.4 μm for the buried FAA waveguides. With the use of cut-back method, the fiber-to-guide coupling loss of ˜4.38 dB, the waveguide loss of ˜2.27 dB/cm, and Er 3+ absorption loss ˜5.7 dB were measured for a ˜1.24-cm-long waveguide. Peak relative gain of ˜7.0 dB is obtained for a ˜1.24-cm-long waveguide. The potential for the fabrication of compact optical amplifiers operating in the range of 1520-1580 nm is also demonstrated.

Antiferromagnetic structure of exchange-coupled L a0.7S r0.3Fe O3 thin films studied using angle-dependent x-ray absorption spectroscopy

NASA Astrophysics Data System (ADS)

Jia, Yue; Chopdekar, Rajesh V.; Shafer, Padraic; Arenholz, Elke; Liu, Zhiqi; Biegalski, Michael D.; Takamura, Yayoi

2017-12-01

The magnetic structure of exchange-coupled antiferromagnetic (AF) layers in epitaxial L a0.7S r0.3Mn O3 (LSMO)/L a0.7S r0.3Fe O3 (LSFO) superlattices grown on (111)-oriented SrTi O3 substrates was studied using angle-dependent x-ray absorption spectroscopy utilizing linearly polarized x rays. We demonstrate the development of the measurement protocols needed to determine the orientation of the LSFO antiferromagnetic spin axis and how it responds to an applied magnetic field due to exchange interactions with an adjacent ferromagnetic layer. A small energy difference exists between two types of AF order: the majority of the AF moments cant out-of-the-plane of the film along the 110 or 100 directions depending on the LSFO layer thickness. In response to an applied magnetic field, these canted moments are aligned with a single 110 or 100 direction that maintains a nearly perpendicular orientation relative to the LSMO sublayer magnetization. The remaining AF moments lie within the (111 ) plane and these in-plane moments can be reoriented to an arbitrary in-plane direction to lie parallel to the LSMO sublayer magnetization. These results demonstrate that the magnetic order of AF thin films and heterostructures is far more complex than in bulk LSFO and can be tuned with orientation, thickness, and applied magnetic field.

Dark energy from the motions of neutrinos

NASA Astrophysics Data System (ADS)

Simpson, Fergus; Jimenez, Raul; Pena-Garay, Carlos; Verde, Licia

2018-06-01

Ordinarily, a scalar field may only play the role of dark energy if it possesses a potential that is either extraordinarily flat or extremely fine-tuned. Here we demonstrate that these restrictions are lifted when the scalar field undergoes persistent energy exchange with another fluid. In this scenario, the field is prevented from reversing its direction of motion, and instead may come to rest while displaced from the local minimum of its potential. Therefore almost any scalar potential is capable of initiating a prolonged phase of cosmic acceleration. If the rate of energy transfer is modulated via a derivative coupling, the field undergoes a rapid process of freezing, after which the field's equation of state mimicks that of a cosmological constant. We present a physically motivated realisation in the form of a neutrino-majoron coupling, which avoids the dynamical instabilities associated with mass-varying neutrino models. Finally we discuss possible means by which this model could be experimentally verified.

Microscopic description of exciton polaritons in direct two-band semiconductors

NASA Astrophysics Data System (ADS)

Nguyen, Van Trong; Mahler, Günter

1999-07-01

Based on a quantum electrodynamical formulation, a microscopic description of exciton polaritons in a two-band semiconductor is presented. We show that the interband exchange Coulomb interaction, responsible for the coupling of the exciton with the longitudinal part of the induced field, should be treated on equal footing together with the coupling to the transverse part of the induced field (the photon field). The constitutive relation is established to connect the current density with the total electric field of polaritons. The classical Maxwell equations are derived from the quantum representation of photons to get a closed system of equations. The temporal evolution for an initial excited exciton state is studied in detail and an anisotropic polariton vacuum Rabi splitting is shown to occur. A number of up-to-now unresolved discrepancies in the literature are clarified.

Coupled granular/continuous medium for thermally stable perpendicular magnetic recording

NASA Astrophysics Data System (ADS)

Sonobe, Y.; Weller, D.; Ikeda, Y.; Takano, K.; Schabes, M. E.; Zeltzer, G.; Do, H.; Yen, B. K.; Best, M. E.

2001-10-01

We studied coupled granular/continuous (CGC) perpendicular media consisting of a continuous multilayer structure and a granular layer. The addition of Co/Pt multilayers decreased the nucleation field from 200 to -1800 Oe and increased the squareness from 0.9 to 1.0. The moment decay at room temperature was significantly reduced from -4.8% to -0.05% per decade. At elevated temperatures, strong exchange coupling between a granular layer and a continuous layer is needed for thermal stability. The exchange-coupled continuous layer reduces thermal demagnetization as it effectively increases the grain size, tightens the grain distribution, and prevents the reversal of individual grains. Magnetic Force Microscope image showed a larger magnetic cluster size for the CGC structure. Compared to the CoCr 18Pt 12 medium, the CGC medium had 2.3 dB higher output. However, the noise for the CGC medium increased with the recording density, while the noise for the CoCr 18Pt 12 medium remained constant from 4 to 15 kfc/mm. Further optimization and noise reduction are still required for future high density recording.

Quantum stream instability in coupled two-dimensional plasmas

NASA Astrophysics Data System (ADS)

Akbari-Moghanjoughi, M.

2014-08-01

In this paper the quantum counter-streaming instability problem is studied in planar two-dimensional (2D) quantum plasmas using the coupled quantum hydrodynamic (CQHD) model which incorporates the most important quantum features such as the statistical Fermi-Dirac electron pressure, the electron-exchange potential and the quantum diffraction effect. The instability is investigated for different 2D quantum electron systems using the dynamics of Coulomb-coupled carriers on each plasma sheet when these plasmas are both monolayer doped graphene or metalfilm (corresponding to 2D Dirac or Fermi electron fluids). It is revealed that there are fundamental differences between these two cases regarding the effects of Bohm's quantum potential and the electron-exchange on the instability criteria. These differences mark yet another interesting feature of the effect of the energy band dispersion of Dirac electrons in graphene. Moreover, the effects of plasma number-density and coupling parameter on the instability criteria are shown to be significant. This study is most relevant to low dimensional graphene-based field-effect-transistor (FET) devices. The current study helps in understanding the collective interactions of the low-dimensional coupled ballistic conductors and the nanofabrication of future graphene-based integrated circuits.

Enhanced interlayer exchange coupling in antiferromagnetically coupled ultrathin (Co70Fe30/Pd) multilayers

NASA Astrophysics Data System (ADS)

Meng, Zhaoliang; Qiu, Jinjun; Han, Guchang; Teo, Kie Leong

2015-12-01

We report the studies of magnetization reversal and magnetic interlayer coupling in synthetic antiferromagnetic (SAF) [Pd/Co70Fe30]9/Ru(tRu)/Pd(tPd)/[Co70Fe30/Pd]9 structure as functions of inserted Pd layer (tPd) and Ru layer (tRu) thicknesses. We found the exchange coupling field (Hex) and perpendicular magnetic anisotropy (PMA) can be controlled by both the tPd and tRu, The Hex shows a Ruderman-Kittel-Kasuya-Yosida-type oscillatory decay dependence on tRu and a maximum interlayer coupling strength Jex = 0.522 erg/cm2 is achieved at tPd + tRu ≈ 0.8 nm in the as-deposited sample. As it is known that a high post-annealing stability of SAF structure is required for magnetic random access memory applications, the dependence of Hex and PMA on the post-annealing temperature (Ta) is also investigated. We found that both high PMA of the top Co70Fe30/Pd multilayer is maintained and Hex is enhanced with increasing Ta up to 350 °C for tRu > 0.7 nm in our SAF structure.

Dynamics of the Transition Corona

NASA Technical Reports Server (NTRS)

Masson, Sophie; McCauley, Patrick; Golub, Leon; Reeves, Katharine K.; DeLuca, Edward E.

2014-01-01

Magnetic reconnection between the open and closed magnetic fields in the corona is believed to play a crucial role in the corona/heliosphere coupling. At large scale, the exchange of open/closed connectivity is expected to occur in pseudo-streamer (PS) structures. However, there is neither clear observational evidence of how such coupling occurs in PSs, nor evidence for how the magnetic reconnection evolves. Using a newly developed technique, we enhance the off-limb magnetic fine structures observed with the Atmospheric Imaging Assembly and identify a PS-like feature located close to the northern coronal hole. We first identify that the magnetic topology associated with the observation is a PS, null-point (NP) related topology bounded by the open field. By comparing the magnetic field configuration with the extreme ultraviolet (EUV) emission regions, we determined that most of the magnetic flux associated with plasma emission are small loops below the PS basic NP and open field bounding the PS topology. In order to interpret the evolution of the PS, we referred to a three-dimensional MHD interchange reconnection modeling the exchange of connectivity between small closed loops and the open field. The observed PS fine structures follow the dynamics of the magnetic field before and after reconnecting at the NP obtained by the interchange model. Moreover, the pattern of the EUV plasma emission is the same as the shape of the expected plasma emission location derived from the simulation. These morphological and dynamical similarities between the PS observations and the results from the simulation strongly suggest that the evolution of the PS, and in particular the opening/closing of the field, occurs via interchange/slipping reconnection at the basic NP of the PS. Besides identifying the mechanism at work in the large-scale coupling between the open and closed fields, our results highlight that interchange reconnection in PSs is a gradual physical process that differs from the impulsive reconnection of the solar-jet model.

Evaluation of Simulated Marine Aerosol Production Using the WaveWatchIII Prognostic Wave Model Coupled to the Community Atmosphere Model within the Community Earth System Model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Long, M. S.; Keene, William C.; Zhang, J.

2016-11-08

Primary marine aerosol (PMA) is emitted into the atmosphere via breaking wind waves on the ocean surface. Most parameterizations of PMA emissions use 10-meter wind speed as a proxy for wave action. This investigation coupled the 3 rd generation prognostic WAVEWATCH-III wind-wave model within a coupled Earth system model (ESM) to drive PMA production using wave energy dissipation rate – analogous to whitecapping – in place of 10-meter wind speed. The wind speed parameterization did not capture basin-scale variability in relations between wind and wave fields. Overall, the wave parameterization did not improve comparison between simulated versus measured AOD ormore » Na +, thus highlighting large remaining uncertainties in model physics. Results confirm the efficacy of prognostic wind-wave models for air-sea exchange studies coupled with laboratory- and field-based characterizations of the primary physical drivers of PMA production. No discernible correlations were evident between simulated PMA fields and observed chlorophyll or sea surface temperature.« less

(1)H-(13)C Hetero-nuclear dipole-dipole couplings of methyl groups in stationary and magic angle spinning solid-state NMR experiments of peptides and proteins.

PubMed

Wu, Chin H; Das, Bibhuti B; Opella, Stanley J

2010-02-01

(13)C NMR of isotopically labeled methyl groups has the potential to combine spectroscopic simplicity with ease of labeling for protein NMR studies. However, in most high resolution separated local field experiments, such as polarization inversion spin exchange at the magic angle (PISEMA), that are used to measure (1)H-(13)C hetero-nuclear dipolar couplings, the four-spin system of the methyl group presents complications. In this study, the properties of the (1)H-(13)C hetero-nuclear dipolar interactions of (13)C-labeled methyl groups are revealed through solid-state NMR experiments on a range of samples, including single crystals, stationary powders, and magic angle spinning of powders, of (13)C(3) labeled alanine alone and incorporated into a protein. The spectral simplifications resulting from proton detected local field (PDLF) experiments are shown to enhance resolution and simplify the interpretation of results on single crystals, magnetically aligned samples, and powders. The complementarity of stationary sample and magic angle spinning (MAS) measurements of dipolar couplings is demonstrated by applying polarization inversion spin exchange at the magic angle and magic angle spinning (PISEMAMAS) to unoriented samples. Copyright 2009 Elsevier Inc. All rights reserved.

Effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties in d2-d3 mixed-valence dimers

NASA Astrophysics Data System (ADS)

Yang, Xiaohua; Hu, Haiquan; Chen, Zhida

The effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties of d2-d3 systems is discussed. The temperature-dependent magnetic moment was calculated with the semiclassical adiabatic approach. The results show that the vibronic coupling from the out-of-phase breathing vibration on the metal sites (Piepho, Krausz, and Schatz [PKS] model) and the vibronic coupling from the stretching vibration between the metal sites (P model) favor the localization and delocalization of the "extra" electron in mixed-valence dimers, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange, and vibronic coupling. The results obtained by analyzing d2-d3 systems can be generalized to other full delocalized dinuclear mixed valence systems with a unique transferable electron.

Tailoring transport properties of phase-separated manganite films with ordered magnetic nanostructures

DOE PAGES

Vlaminck, V.; Yanez, W.; Hoffman, J.; ...

2016-08-02

Here, the magnetotransport properties of thin manganite films (La 0.7Ca 0.3MnO 3) coupled with arrays of permalloy (Py) nanodots deposited on the surface of the film are studied as a function of temperature, magnetic field, and the size of the dots. In the presence of the magnetic dots, a reduction of the electrical resistivity is observed, especially at the insulator-to-metal transition, as well as a shift of the transition peak towards higher temperatures. This indicates that, due to local interface exchange coupling, highly conductive ferromagnetic domains are nucleated in the manganite film underneath the Py nanodots. The use of amore » simplified resistor network model allows us to estimate the size of the metallic regions induced by exchange coupling. At low temperatures, these regions extend ~70 nm beyond the edge of the nanodots, a length scale comparable to the correlation length of the ferromagnetic clusters in the phase-separated state of La 0.7Ca 0.3MnO 3.« less

The influence of ligand field effects on the magnetic exchange of high-spin Co(II)-semiquinonate complexes.

PubMed

Bencini, Alessandro; Beni, Alessandra; Costantino, Ferdinando; Dei, Andrea; Gatteschi, Dante; Sorace, Lorenzo

2006-02-07

[Co(Me(4)cyclam)(tropolonate)](PF(6)) was synthesised and structurally characterised. Its electronic and W-band EPR spectra have been analysed by means of the angular overlap calculation of the Spin Hamiltonian parameters that provided also a satisfactory reproduction of the temperature dependence of the magnetic susceptibility. The present results can be interpreted assuming a pseudo-octahedral character for the Co(II) center. This prompted us to reconsider the model formerly used for the analysis of the magnetic coupling between hs-Co(II) and the paramagnetic o-semiquinonate ligand in the corresponding derivatives [Co(Me(4)cyclam)(PhenSQ)](PF(6)) and [Co(Me(4)cyclam)(DTBSQ)](PF(6)). These results indicate that the effect of the magnetic coupling is active only below 50 K and that a more refined model of exchange coupling between Co(II) and semiquinonato ligands is needed to quantitatively analyze the magnetic behaviour of this class of systems.

Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings

DOE PAGES

Pica, G.; Lovett, B. W.; Bhatt, R. N.; ...

2016-01-14

A scaled quantum computer with donor spins in silicon would benefit from a viable semiconductor framework and a strong inherent decoupling of the qubits from the noisy environment. Coupling neighboring spins via the natural exchange interaction according to current designs requires gate control structures with extremely small length scales. In this work, we present a silicon architecture where bismuth donors with long coherence times are coupled to electrons that can shuttle between adjacent quantum dots, thus relaxing the pitch requirements and allowing space between donors for classical control devices. An adiabatic SWAP operation within each donor/dot pair solves the scalabilitymore » issues intrinsic to exchange-based two-qubit gates, as it does not rely on subnanometer precision in donor placement and is robust against noise in the control fields. In conclusion, we use this SWAP together with well established global microwave Rabi pulses and parallel electron shuttling to construct a surface code that needs minimal, feasible local control.« less

Exchange coupling in permalloy/BiFeO3 heterostructures

NASA Astrophysics Data System (ADS)

Heron, John; Wang, Chen; Carlton, David; Nowakowski, Mark; Gajek, Martin; Awschalom, David; Bokor, Jeff; Ralph, Dan; Ramesh, R.

2010-03-01

BiFeO3 is a ferroelectric and antiferromagnetic multiferroic with the ferroelectric and antiferromagnetic order parameters coupled at room temperature. This coupling results in the reorientation of the ferroelectric and magnetic domains as applied voltages switch the electric polarization. Previous studies using ferromagnet/BiFeO3 heterostructures have shown that the anisotropy of the ferromagnetic layer can be tuned by the ferroelectric domain structure of the BiFeO3 film [1, 2]. The physical mechanism driving this exchange bias with BiFeO3 is still under investigation. We use patterned permalloy structures, with varying aspect ratios, on BiFeO3 thin films to investigate the physics of this interaction. The results of our studies using MFM, PEEM, and MOKE to understand this mechanism as a means to electric field control of magnetic structures will be presented. [4pt] [1] H. Bea et al., Physical Review Letters 100, 017204 (2008).[0pt] [2] L.W. Martin et al., Nanoletters 8, 2050 (2008).

Oceanic response to tropical cyclone `Phailin' in the Bay of Bengal

NASA Astrophysics Data System (ADS)

Pant, V.; Prakash, K. R.

2016-02-01

Vertical mixing largely explains surface cooling induced by Tropical Cyclones (TCs). However, TC-induced upwelling of deeper waters plays an important role as it partly balances the warming of subsurface waters induced by vertical mixing. Below 100 m, vertical advection results in cooling that persists for a few days after the storm. The present study investigates the integrated ocean response to tropical cyclone `Phaillin' (10-14 October 2013) in the Bay of Bengal (BoB) through both coupled and stand-alone ocean-atmosphere models. Two numerical experiments with different coupling configurations between Regional Ocean Modelling System (ROMS) and Weather Research and Forecasting (WRF) were performed to investigate the impact of Phailin cyclone on the surface and sub-surface oceanic parameters. In the first experiment, ocean circulation model ROMS observe surface wind forcing from a mesoscale atmospheric model (WRF with nested damin setup), while rest forcing parameters are supplied to ROMS from NCEP data. In the second experiment, all surface forcing data to ROMS directly comes from WRF. The modeling components and data fields exchanged between atmospheric and oceanic models are described. The coupled modeling system is used to identify model sensitivity by exchanging prognostic variable fields between the two model components during simulation of Phallin cyclone (10-14 October 2013) in the BoB.In general, the simulated Phailin cyclone track and intensities agree well with observations in WRF simulations. Further, the inter-comparison between stand-alone and coupled model simulations validated against observations highlights better performance of coupled modeling system in simulating the oceanic conditions during the Phailin cyclone event.

Multiferroic and magnetoelectric nanocomposites for data processing

NASA Astrophysics Data System (ADS)

Kleemann, Wolfgang

2017-06-01

Recent progress in preparing and understanding composite magnetoelectrics is highlighted. Apart from optimized standard solutions novel methods of switching magnetism with electric fields and vice versa with focus on magnetoelectric (ME) data processing in multiferroic and magnetoelectric nanocomposites deserve particular interest. First, we report on the patented MERAM, which uses the electric field control of exchange bias in a layered composite via an epitaxial magnetoelectric Cr2O3 layer exchange coupled to a Pt/Co/Pt trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic field in a composite of CoFe2O4 nanopillars embedded in a vertically poled BaTiO3 thick film produces a regular surface polarization pattern with rectangular local symmetry. Its possible use for data processing is discussed. Third, in the relaxor ferroelectric single-phase compound (BiFe0.9Co0.1O3)0.4-(Bi1/2K1/2TiO3)0.6 polar nanoregions emerging from ferrimagnetic Bi(Fe,Co)O3 regions embedded in a Bi1/2K1/2TiO3 relaxor component transform into ferroelectric clusters and simultaneously enable congruent magnetic clusters. The local polarization and magnetization couple with record-high direct and converse magnetoelectric coupling coefficients, α  ≈  1.0  ×  10-5 s m-1. These ‘multiferroic’ clusters are promising for applications in data storage or processing devices.

The COSMO-CLM 4.8 regional climate model coupled to regional ocean, land surface and global earth system models using OASIS3-MCT: description and performance

NASA Astrophysics Data System (ADS)

Will, Andreas; Akhtar, Naveed; Brauch, Jennifer; Breil, Marcus; Davin, Edouard; Ho-Hagemann, Ha T. M.; Maisonnave, Eric; Thürkow, Markus; Weiher, Stefan

2017-04-01

We developed a coupled regional climate system model based on the CCLM regional climate model. Within this model system, using OASIS3-MCT as a coupler, CCLM can be coupled to two land surface models (the Community Land Model (CLM) and VEG3D), the NEMO-MED12 regional ocean model for the Mediterranean Sea, two ocean models for the North and Baltic seas (NEMO-NORDIC and TRIMNP+CICE) and the MPI-ESM Earth system model.We first present the different model components and the unified OASIS3-MCT interface which handles all couplings in a consistent way, minimising the model source code modifications and defining the physical and numerical aspects of the couplings. We also address specific coupling issues like the handling of different domains, multiple usage of the MCT library and exchange of 3-D fields.We analyse and compare the computational performance of the different couplings based on real-case simulations over Europe. The usage of the LUCIA tool implemented in OASIS3-MCT enables the quantification of the contributions of the coupled components to the overall coupling cost. These individual contributions are (1) cost of the model(s) coupled, (2) direct cost of coupling including horizontal interpolation and communication between the components, (3) load imbalance, (4) cost of different usage of processors by CCLM in coupled and stand-alone mode and (5) residual cost including i.a. CCLM additional computations.Finally a procedure for finding an optimum processor configuration for each of the couplings was developed considering the time to solution, computing cost and parallel efficiency of the simulation. The optimum configurations are presented for sequential, concurrent and mixed (sequential+concurrent) coupling layouts. The procedure applied can be regarded as independent of the specific coupling layout and coupling details.We found that the direct cost of coupling, i.e. communications and horizontal interpolation, in OASIS3-MCT remains below 7 % of the CCLM stand-alone cost for all couplings investigated. This is in particular true for the exchange of 450 2-D fields between CCLM and MPI-ESM. We identified remaining limitations in the coupling strategies and discuss possible future improvements of the computational efficiency.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Hakjoon; Bac, Seul-Ki; Lee, Sangyeop

We report direct experimental determination of next-nearest-neighbor (NNN) interlayer exchange coupling (IEC) in antiferromagnetically coupled GaMnAs/GaAs:Be multilayers. Magnetoresistance in such multilayers shows step-like transitions as a function of applied magnetic field that corresponds to abrupt changes of spin configuration. By adjusting the field range, one obtains minor hysteresis loops that allow one to determine spin configurations occurring in the multilayer, which in turn can be used to obtain the ratio of NNN IEC to NN IEC. By using this method on a series of GaMnAs/GaAs:Be with different numbers of GaMnAs layers, we obtain this ratio to be 0.23, in goodmore » agreement with theoretical predictions.« less

Inverse spin Hall and spin rectification effects in NiFe/FeMn exchange-biased thin films

NASA Astrophysics Data System (ADS)

Garcia, W. J. S.; Seeger, R. L.; da Silva, R. B.; Harres, A.

2017-11-01

Materials presenting high spin-orbit coupling are able to convert spin currents in charge currents. The phenomenon, known as inverse spin Hall effect, promises to revolutionize spintronic technology enabling the electrical detection of spin currents. It has been observed in a variety of systems, usually non-magnetic metals. We study the voltage emerging in exchange biased Ta/NiFe/FeMn/Ta thin films near the ferromagnetic resonance. Measured signals are related to both inverse spin Hall and spin rectification effects, and two distinct protocols were employed to separate their contributions.The curve shift due to the exchange bias effect may enable high frequency applications without an external applied magnetic field.

Magnetic exchange coupling through superconductors: A trilayer study

NASA Astrophysics Data System (ADS)

Sá de Melo, C. A.

2000-11-01

The possibility of magnetic exchange coupling between two ferromagnets (F) separated by a superconductor (S) spacer is analyzed using the functional integral method. For this coupling to occur three prima facie conditions need to be satisfied. First, an indirect exchange coupling between the ferromagnets must exist when the superconductor is in its normal state. Second, superconductivity must not be destroyed due to the proximity to ferromagnetic boundaries. Third, roughness of the F/S interfaces must be small. Under these conditions, when the superconductor is cooled to below its critical temperature, the magnetic coupling changes. The appearance of the superconducting gap introduces a new length scale (the coherence length of the superconductor) and modifies the temperature dependence of the indirect exchange coupling existent in the normal state. The magnetic coupling is oscillatory both above and below the the critical temperature of the superconductor, as well as strongly temperature-dependent. However, at low temperatures the indirect exchange coupling decay length is controlled by the coherence length of the superconductor, while at temperatures close to and above the critical temperature of the superconductor the magnetic coupling decay length is controlled by the thermal length.

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

NASA Astrophysics Data System (ADS)

Dospial, M.; Plusa, D.; Ślusarek, B.

2012-03-01

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity. The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd2Fe14B grains.

Spin heat capacity of monolayer and AB-stacked bilayer MoS2 in the presence of exchange magnetic field

NASA Astrophysics Data System (ADS)

Hoi, Bui Dinh; Yarmohammadi, Mohsen; Mirabbaszadeh, Kavoos

2017-04-01

Dirac theory and Green's function technique are carried out to compute the spin dependent band structures and corresponding electronic heat capacity (EHC) of monolayer (ML) and AB-stacked bilayer (BL) molybdenum disulfide (MoS2) two-dimensional (2D) crystals. We report the influence of induced exchange magnetic field (EMF) by magnetic insulator substrates on these quantities for both structures. The spin-up (down) subband gaps are shifted with EMF from conduction (valence) band to valence (conduction) band at both Dirac points in the ML because of the spin-orbit coupling (SOC) which leads to a critical EMF in the K point and EHC returns to its initial states for both spins. In the BL case, EMF results split states and the decrease (increase) behavior of spin-up (down) subband gaps has been observed at both K and K‧ valleys which is due to the combined effect of SOC and interlayer coupling. For low and high EMFs, EHC of BL MoS2 does not change for spin-up subbands while increases for spin-down subbands.

Spin Waves and Transport Properties in Ferromagnetic Co-Al-O and Fe-Al-O Granular Films: A Brillouin Scattering Study

NASA Astrophysics Data System (ADS)

Yoshihara, Akira; Ohnuma, Shigehiro; Fujimori, Hiroyasu; Nakamura, Shintaro; Nojima, Tsutomu

2008-09-01

A systematic Brillouin light scattering (BLS) study on long-wavelength spin waves (SWs) in ferromagnetic TM-Al-O (TM=Co, Fe) nano-granular films with thickness of >1 μm was performed under magnetic fields of up to 4 kOe at room temperature. BLS spectra consist of a pair of bulk SW peaks on both frequency sides and a surface localized SW peak only on the positive frequency side in this study. These SW frequencies as a function of the magnetic field can be fully reproduced by the magnetostatic frequency formula developed for a semi-infinite uniform ferromagnetic medium with an exchange coupling and an in-plane uniaxial magnetic anisotropy. We determined a set of the magnetic constants including the exchange field HE for each film. Combining the exchange field HE with the electrical resistivity ρ for each film at room temperature, we found an inverse-square law given by ρ=a(HE)-2 for both the Co and Fe granular films with aFe=30.3 μΩ\\cdotcm\\cdot(kOe)2 and aCo=22.1 μΩ\\cdotcm\\cdot(kOe)2, respectively.

Magnetic excitations in praseodymium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Houmann, J.G.; Rainford, B.D.; Jensen, J.

1979-08-01

The magnetic excitations in a single crystal of dhcp Pr have been studied by inelastic neutron scattering. The excitations on the hexagonal sites, and their dependence on magnetic fields up to 43 kOe applied in the basal plane, have been analyzed in terms of a Hamiltonian in which exchange, crystal-field, and magnetoelastic interactions are included. The exchange is found to be strongly anisotropic, and this anisotropy is manifested directly in a splitting of most branches of the dispersion relations. By considering a variety of magnetic properties, we have been able to determine the crystal-field level scheme for the hexagonal sitesmore » fairly unambiguously. The first excited level is 3.5 meV above the ground state. The value of the magnetoelastic coupling deduced from the excitations is in good agreement with values obtained from other measurements. A field-dependent interaction with the phonons has been observed, and a pronounced broadening of the acoustic excitations of long wavelength is ascribed to the influence of the conduction electrons. The first excited state on the cubic ions is about 8.3 meV above the ground state. The corresponding excitations show a pronounced dispersion, but the exchange anisotropy is of less importance than for the hexagonal sites.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Manna, P. K.; Skoropata, E.; Ting, Y-W

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni 80Fe 20/CoO(t CoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. But, in our trilayers the interface ferrites were found not to be ferro-or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealedmore » that (i) interlayer exchange coupling was present between Ni 80Fe 20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. Here, we also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent.« less

Molecular and electronic structure of terminal and alkali metal-capped uranium(V) nitride complexes

PubMed Central

King, David M.; Cleaves, Peter A.; Wooles, Ashley J.; Gardner, Benedict M.; Chilton, Nicholas F.; Tuna, Floriana; Lewis, William; McInnes, Eric J. L.; Liddle, Stephen T.

2016-01-01

Determining the electronic structure of actinide complexes is intrinsically challenging because inter-electronic repulsion, crystal field, and spin–orbit coupling effects can be of similar magnitude. Moreover, such efforts have been hampered by the lack of structurally analogous families of complexes to study. Here we report an improved method to U≡N triple bonds, and assemble a family of uranium(V) nitrides. Along with an isoelectronic oxo, we quantify the electronic structure of this 5f1 family by magnetometry, optical and electron paramagnetic resonance (EPR) spectroscopies and modelling. Thus, we define the relative importance of the spin–orbit and crystal field interactions, and explain the experimentally observed different ground states. We find optical absorption linewidths give a potential tool to identify spin–orbit coupled states, and show measurement of UV···UV super-exchange coupling in dimers by EPR. We show that observed slow magnetic relaxation occurs via two-phonon processes, with no obvious correlation to the crystal field. PMID:27996007

Collective dynamics and entanglement of two distant atoms embedded into single-negative index material.

PubMed

Fang, Wei; Li, Gao-Xiang; Yang, Yaping; Ficek, Zbigniew

2017-02-06

We study the dynamics of two two-level atoms embedded near to the interface of paired meta-material slabs, one of negative permeability and the other of negative permittivity. This combination generates a strong surface plasmon field at the interface between the meta-materials. It is found that the symmetric and antisymmetric modes of the two-atom system couple to the plasmonic field with different Rabi frequencies. Including the Ohmic losses of the materials we find that the Rabi frequencies exhibit threshold behaviour which distinguish between the non-Markovian (memory preserving) and Markovian (memoryless) regimes of the evolution. Moreover, it is found that significantly different dynamics occur for the resonant and an off-resonant couplings of the plasmon field to the atoms. In the case of the resonant coupling, the field does not appear as a dissipative reservoir to the atoms. We adopt the image method and show that the dynamics of the two atoms coupled to the plasmon field are analogous to the dynamics of a four-atom system in a rectangular configuration. A large and long living entanglement mediated by the plasmonic field in both Markovian and non-Markovian regimes of the evolution is predicted. We also show that a simultaneous Markovian and non-Markovian regime of the evolution may occur in which the memory effects exist over a finite evolution time. In the case of an off-resonant coupling of the atoms to the plasmon field, the atoms interact with each other by exchanging virtual photons which results in the dynamics corresponding to those of two atoms coupled to a common reservoir. In addition, the entanglement is significantly enhanced.

Brillouin Light Scattering study of the rotatable magnetic anisotropy in exchange biased bilayers of Ni81 Fe19 Ir20 Mn80

NASA Astrophysics Data System (ADS)

Rodríguez, Roberto; Oliveira, Alexandre; Estrada, Francisco; Santos, Obed; Azevedo, Antonio; Rezende, Sergio

It is known that when a ferromagnet (FM) is in atomic contact with an antiferromagnet (AF) the exchange coupling between the FM and AF spins at the interface induces a unidirectional anisotropy in the ferromagnetic film. This effect is known as exchange bias (EB). Despite the large amount of research on this topic there are still several aspects of the EB mechanism that are not well understood. One of this aspects is the origin of the rotatable anisotropy in polycrystalline AFs. By means of Brillouin Light Scattering (BLS) measurements, we investigated the dependence of the rotatable anisotropy field HRA and exchange field HE with the magnitude of the external magnetic field (Ho) in FM/AM bilayers of Ni81Fe19(10nm)/Ir20Mn80(tAF) . We developed an algorithm to numerically fit the in-plane angular dependence of the magnon frequency, at a fixed value of Ho measured by BLS. From the fit parameters we were able to investigate HRA and HE dependency on Ho. The results reveal that HRA value depends on Ho, so we argue that AF grain distribution at the interface is partially modified by the applied field strength. Contrary to this, the relation between HE and Ho is not straightforward, remaining constant at high values of Ho.

Enhancement of exchange coupling interaction of NdFeB/MnBi hybrid magnets

NASA Astrophysics Data System (ADS)

Nguyen, Truong Xuan; Nguyen, Khanh Van; Nguyen, Vuong Van

2018-03-01

MnBi ribbons were fabricated by melt - spinning with subsequent annealing. The MnBi ribbons were ground and mixed with NdFeB commercial Magnequench powders (MQA). The hybrid powder mixtures were subjected thrice to the annealing and ball-milling route. The hybrid magnets (100 - x)NdFeB/xMnBi, x=0, 30, 40, 50 and 100 wt% were in-mold aligned in an 18 kOe magnetic field and warm compacted at 290 °C by 2000 psi uniaxial pressure for 10 min. An enhancement of the exchange coupling of NdFeB/MnBi hybrid magnets was obtained by optimizing the magnets' microstructures via annealing and ball-milling processes. The magnetic properties of prepared NdFeB/MnBi hybrid magnets were studied and discussed in details.

Exploring the protein folding free energy landscape: coupling replica exchange method with P3ME/RESPA algorithm.

PubMed

Zhou, Ruhong

2004-05-01

A highly parallel replica exchange method (REM) that couples with a newly developed molecular dynamics algorithm particle-particle particle-mesh Ewald (P3ME)/RESPA has been proposed for efficient sampling of protein folding free energy landscape. The algorithm is then applied to two separate protein systems, beta-hairpin and a designed protein Trp-cage. The all-atom OPLSAA force field with an explicit solvent model is used for both protein folding simulations. Up to 64 replicas of solvated protein systems are simulated in parallel over a wide range of temperatures. The combined trajectories in temperature and configurational space allow a replica to overcome free energy barriers present at low temperatures. These large scale simulations reveal detailed results on folding mechanisms, intermediate state structures, thermodynamic properties and the temperature dependences for both protein systems.

Thermoacoustic refrigerators and engines comprising cascading stirling thermodynamic units

DOEpatents

Backhaus, Scott; Swift, Greg

2013-06-25

The present invention includes a thermoacoustic assembly and method for improved efficiency. The assembly has a first stage Stirling thermal unit comprising a main ambient heat exchanger, a regenerator and at least one additional heat exchanger. The first stage Stirling thermal unit is serially coupled to a first end of a quarter wavelength long coupling tube. A second stage Stirling thermal unit comprising a main ambient heat exchanger, a regenerator, and at least one additional heat exchanger, is serially coupled to a second end of the quarter wavelength long coupling tube.

Exchange Coupling Interactions from the Density Matrix Renormalization Group and N-Electron Valence Perturbation Theory: Application to a Biomimetic Mixed-Valence Manganese Complex.

PubMed

Roemelt, Michael; Krewald, Vera; Pantazis, Dimitrios A

2018-01-09

The accurate description of magnetic level energetics in oligonuclear exchange-coupled transition-metal complexes remains a formidable challenge for quantum chemistry. The density matrix renormalization group (DMRG) brings such systems for the first time easily within reach of multireference wave function methods by enabling the use of unprecedentedly large active spaces. But does this guarantee systematic improvement in predictive ability and, if so, under which conditions? We identify operational parameters in the use of DMRG using as a test system an experimentally characterized mixed-valence bis-μ-oxo/μ-acetato Mn(III,IV) dimer, a model for the oxygen-evolving complex of photosystem II. A complete active space of all metal 3d and bridge 2p orbitals proved to be the smallest meaningful starting point; this is readily accessible with DMRG and greatly improves on the unrealistic metal-only configuration interaction or complete active space self-consistent field (CASSCF) values. Orbital optimization is critical for stabilizing the antiferromagnetic state, while a state-averaged approach over all spin states involved is required to avoid artificial deviations from isotropic behavior that are associated with state-specific calculations. Selective inclusion of localized orbital subspaces enables probing the relative contributions of different ligands and distinct superexchange pathways. Overall, however, full-valence DMRG-CASSCF calculations fall short of providing a quantitative description of the exchange coupling owing to insufficient recovery of dynamic correlation. Quantitatively accurate results can be achieved through a DMRG implementation of second order N-electron valence perturbation theory (NEVPT2) in conjunction with a full-valence metal and ligand active space. Perspectives for future applications of DMRG-CASSCF/NEVPT2 to exchange coupling in oligonuclear clusters are discussed.

Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative.

PubMed

Chow, Chun Y; Bolvin, Hélène; Campbell, Victoria E; Guillot, Régis; Kampf, Jeff W; Wernsdorfer, Wolfgang; Gendron, Frédéric; Autschbach, Jochen; Pecoraro, Vincent L; Mallah, Talal

2015-07-01

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga 4 Ln 2 (shi 3- ) 4 (Hshi 2- ) 2 (H 2 shi - ) 2 (C 5 H 5 N) 4 (CH 3 OH) x (H 2 O) x ]· x C 5 H 5 N· x CH 3 OH· x H 2 O (where H 3 shi = salicylhydroxamic acid and Ln = Gd III 1 ; Tb III 2 ; Dy III 3 ; Er III 4 ; Y III 5 ; Y III 0.9 Dy III 0.1 6 ). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled Dy III ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy 2 than for the Er 2 complex.

Phase transitions and magnetization of the mixed-spin Ising–Heisenberg double sawtooth frustrated ladder

NASA Astrophysics Data System (ADS)

Arian Zad, Hamid; Ananikian, Nerses

2018-04-01

The mixed spin-(1,1/2) Ising–Heisenberg double sawtooth ladder containing a mixture of both spin-1 and spin-1/2 nodal atoms, and the spin-1/2 interstitial dimers are approximately solved by the transfer-matrix method. Here, we study in detail the ground-state phase diagrams, also influences of the bilinear exchange coupling on the rungs and cyclic four-spin exchange interaction in square plaquette of each block on the magnetization and magnetic susceptibility of the suggested ladder at low temperature. Such a double sawtooth ladder may be found in a Shastry-Sutherland lattice-type. In spite of the spin ordering of odd and even blocks being different from each other, due to the commutation relation between all different block Hamiltonians, phase diagrams, magnetization behavior and thermodynamic properties of the model are the same for odd and even blocks. We show that at low temperature, both exchange couplings can change the quality and quantity of the magnetization plateaus versus the magnetic field changes. Specially, we find a new magnetization plateau M/Ms= 5/6 for this model. Besides, we examine the magnetic susceptibility and specific heat of the model in detail. It is proven that behaviors of the magnetization and the magnetic susceptibility coincide at low temperature. The specific heat displays diverse temperature dependencies, which include a Schottky-type peak at a special temperature interval. We observe that with increase of the bilinear exchange coupling on the rungs, second peak temperature dependence grows.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Jae Wook; Artyukhin, Sergei; Mun, Eun Deok

In this paper, we report the discovery of a metamagnetic phase transition in a polar antiferromagnet Ni 3TeO 6 that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of 0.3 μC/cm 2, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behindmore » the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. Finally, the resultant spin structure is rather counterintuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.« less

Exchange bias training relaxation in spin glass/ferromagnet bilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chi, Xiaodan; Du, An; Rui, Wenbin

2016-04-25

A canonical spin glass (SG) FeAu layer is fabricated to couple to a soft ferromagnet (FM) FeNi layer. Below the SG freezing temperature, exchange bias (EB) and training are observed. Training in SG/FM bilayers is insensitive to cooling field and may suppress the EB or change the sign of the EB field from negative to positive at specific temperatures, violating from the simple power-law or the single exponential function derived from the antiferromagnet based systems. In view of the SG nature, we employ a double decay model to distinguish the contributions from the SG bulk and the SG/FM interface tomore » training. Dynamical properties during training under different cooling fields and at different temperatures are discussed, and the nonzero shifting coefficient in the time index as a signature of slowing-down decay for SG based systems is interpreted by means of a modified Monte Carlo Metropolis algorithm.« less

Exchange bias in multiferroic Ca3Mn2O7 effected by Dzyaloshinskii-Moriya interaction

NASA Astrophysics Data System (ADS)

Sahlot, Pooja; Jana, Anupam; Awasthi, A. M.

2018-04-01

Ruddlesden-Popper manganite Ca3Mn2O7 has been synthesized in single phase orthorhombic structure with Cmcm space group. Temperature dependent magnetization M(T) shows that Ca3Mn2O7 undergoes long range antiferromagnetic (AFM) transition below 123 K, with weak ferromagnetism (WFM) at lower temperatures. Field dependent magnetization M(H) confirms WFM character below ˜110 K in the AFM-base magnetic structure. Detailed analysis of the zero field cooled magnetic hysteresis loops reveals a measurable exchange bias (EB) effect in the sample. EB is attributed to the high anisotropy in the sample and the presence of Dzyaloshinskii-Moriya (D-M) interaction, responsible for the canted interfacial-spins that couple "FM-clusters" with the "AFM-matrix". Temperature dependence of horizontal shifts of the M(H) loops in terms of the coercive fields (Hc±) and vertical shifts in terms of the remnant magnetizations (Mr±) is presented.

MBE growth and FMR, BLS and MOKE studies of exchange coupling in Fe whisker/Cr/Fe(001) and in Fe/Cu/Fe(001) 'loose spin' structures

NASA Astrophysics Data System (ADS)

Heinrich, B.; From, M.; Cochran, J. F.; Kowalewski, M.; Atlan, D.; Celinski, Z.; Myrtle, K.

1995-02-01

The exchange coupling has been studied in structures which consist of two ferromagnetic layers separated by non-ferromagnetic spacers (trilayers). The exchange coupling was measured using FMR and BLS techniques in the temperature range 77-400 K. Two systems were investigated: (a) Fe whisker/Cr/Fe(001) and (b) Fe/Cr/Fe(001). The oscillatory thickness dependence of the exchange coupling through a spin-density wave Cr spacer will be discussed and compared with recent data obtained by other groups. Cu interlayers were deposited either in a pure form, or a single monolayer of {Cu}/{Fe} alloy ('loose spins') was inserted between two pure bcc Cu(001) layers. Several such 'loose spin' structures were engineered to test the behavior of 'loose spin' structures. It was found that the presence of Fe impurity atoms has a strong tendency to decrease the direct bilinear exchange coupling. The contribution of 'loose spins' to the exchange coupling can be made significant, and even dominant, by a suitable choice of the RKKY coupling energy between the 'loose spins' and the surrounding ferromagnetic layers.

Instability of the heliopause driven by charge exchange interactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Avinash, K.; Zank, G. P.; Dasgupta, B.

2014-08-20

The stability of the heliopause that separates the tenuous hot magnetized heliosheath plasma from the dense cool local interstellar magnetized plasma is examined using a fully general model that includes all the essential physical processes. Charge exchange coupling between plasma protons and primary interstellar neutral atoms provides an effective gravity that drives Rayleigh-Taylor (RT)-like instabilities. The velocity difference or shear between the heliosheath and interstellar flows, when coupled to energetic neutral atoms (ENAs), drives a Kelvin-Helmholtz (KH)-like instability on the heliopause. The shoulder region of the heliopause is unstable to a new instability that has characteristics of a mixed RT-KH-likemore » mode. The instabilities are not stabilized by typical values of the magnetic fields in the inner and outer heliosheath (OHS). ENAs play an essential role in driving the KH-like instability, which is fully stabilized in their absence by magnetic fields. The nonlinear phase of these instabilities is briefly discussed. We also discuss the possibility that RT-like or mixed KH-RT-like instabilities drag outer heliosheath/very local interstellar medium (OHS/VLISM) magnetic field lines into the inner heliosheath (IHS) with the VLISM flow, and the possibility that IHS and VLISM magnetic field lines experience reconnection. Such reconnection may (1) greatly enhance the mixing of plasmas across the heliopause and (2) provide open magnetic field lines that allow easy ingress of galactic cosmic rays into the heliosphere and corresponding easy loss of anomalous cosmic rays from the heliosphere.« less

School Organization and Institutional Change: Exchange and Power in Loosely Coupled Systems.

ERIC Educational Resources Information Center

Talbert, Joan E.

This paper presents an exchange-theory view of school authority relations in order to identify patterns of coupling, or interdependencies, within school organizations and to analyze the potential for tighter coupling of administrative and teaching subsystems. The analysis proceeds from an argument that the social-exchange view of administrative…

Heisenberg necklace model in a magnetic field

DOE PAGES

Tsvelik, A. M.; Zaliznyak, I. A.

2016-08-26

Here, we study the low-energy sector of the Heisenberg necklace model. Using the field-theory methods, we estimate how the coupling of the electronic spins with the paramagnetic Kondo spins affects the overall spin dynamics and evaluate its dependence on a magnetic field. We are motivated by the experimental realizations of the spin-1/2 Heisenberg chains in SrCuO 2 and Sr 2CuO 3 cuprates, which remain one-dimensional Luttinger liquids down to temperatures much lower than the in-chain exchange coupling J. We also consider the perturbation of the energy spectrum caused by the interaction γ with nuclear spins (I=3/2) present on the samemore » sites. We find that the resulting necklace model has a characteristic energy scale, Λ~J 1/3(γI) 2/3, at which the coupling between (nuclear) spins of the necklace and the spins of the Heisenberg chain becomes strong. Furthermore, this energy scale is insensitive to a magnetic field B. For μBB>Λ we find two gapless bosonic modes that have different velocities, whose ratio at strong fields approaches a universal number, 2√+1.« less

Ultrastrong light-matter coupling in electrically doped microcavity organic light emitting diodes

NASA Astrophysics Data System (ADS)

Mazzeo, M.; Genco, A.; Gambino, S.; Ballarini, D.; Mangione, F.; Di Stefano, O.; Patanè, S.; Savasta, S.; Sanvitto, D.; Gigli, G.

2014-06-01

The coupling of the electromagnetic field with an electronic transition gives rise, for strong enough light-matter interactions, to hybrid states called exciton-polaritons. When the energy exchanged between light and matter becomes a significant fraction of the material transition energy an extreme optical regime called ultrastrong coupling (USC) is achieved. We report a microcavity embedded p-i-n monolithic organic light emitting diode working in USC, employing a thin film of squaraine dye as active layer. A normalized coupling ratio of 30% has been achieved at room temperature. These USC devices exhibit a dispersion-less angle-resolved electroluminescence that can be exploited for the realization of innovative optoelectronic devices. Our results may open the way towards electrically pumped polariton lasers.

Ultrafast control of strong light-matter coupling

NASA Astrophysics Data System (ADS)

Lange, Christoph; Cancellieri, Emiliano; Panna, Dmitry; Whittaker, David M.; Steger, Mark; Snoke, David W.; Pfeiffer, Loren N.; West, Kenneth W.; Hayat, Alex

2018-01-01

We dynamically modulate strong light-matter coupling in a GaAs/AlGaAs microcavity using intense ultrashort laser pulses tuned below the interband exciton energy, which induce a transient Stark shift of the cavity polaritons. For 225-fs pulses, shorter than the cavity Rabi cycle period of 1000 fs, this shift decouples excitons and cavity photons for the duration of the pulse, interrupting the periodic energy exchange between photonic and electronic states. For 1500-fs pulses, longer than the Rabi cycle period, however, the Stark shift does not affect the strong coupling. The two regimes are marked by distinctly different line shapes in ultrafast reflectivity measurements—regardless of the Stark field intensity. The crossover marks the transition from adiabatic to diabatic switching of strong light-matter coupling.

Magnetic exchange coupling through superconductors : a trilayer study.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sa de Melo, C. A. R.; Materials Science Division

1997-09-08

The possibility of magnetic exchange coupling between two ferromagnets (F) separated by a superconductor (S) spacer is analyzed using the functional integral method. For this coupling to occur three prima facie conditions need to be satisfied. First, an indirect exchange coupling between the ferromagnets must exist when the superconductor is in its normal state. Second, superconductivity must not be destroyed due to the proximity to ferromagnetic boundaries. Third, roughness of the F/S interfaces must be small. Under these conditions, when the superconductor is cooled to below its critical temperature, the magnetic coupling changes. The appearance of the superconducting gap introducesmore » a new length scale (the coherence length of the superconductor) and modifies the temperature dependence of the indirect exchange coupling existent in the normal state. The magnetic coupling is oscillatory both above and below the critical temperature of the superconductor, as well as strongly temperature-dependent. However, at low temperatures the indirect exchange coupling decay length is controlled by the coherence length of the superconductor, while at temperatures close to and above the critical temperature of the superconductor the magnetic coupling decay length is controlled by the thermal length.« less

Strain manipulation of Majorana fermions in graphene armchair nanoribbons

NASA Astrophysics Data System (ADS)

Wang, Zhen-Hua; Castro, Eduardo V.; Lin, Hai-Qing

2018-01-01

Graphene nanoribbons with armchair edges are studied for externally enhanced but realistic parameter values: enhanced Rashba spin-orbit coupling due to proximity to a transition-metal dichalcogenide, such as WS2, and enhanced Zeeman field due to exchange coupling with a magnetic insulator, such as EuS under an applied magnetic field. The presence of s -wave superconductivity, induced either by proximity or by decoration with alkali-metal atoms, such as Ca or Li, leads to a topological superconducting phase with Majorana end modes. The topological phase is highly sensitive to the application of uniaxial strain with a transition to the trivial state above a critical strain well below 0.1%. This sensitivity allows for real-space manipulation of Majorana fermions by applying nonuniform strain profiles. Similar manipulation is also possible by applying an inhomogeneous Zeeman field or chemical potential.

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers

PubMed Central

Gilbert, Dustin A.; Liao, Jung-Wei; Kirby, Brian J.; Winklhofer, Michael; Lai, Chih-Huang; Liu, Kai

2016-01-01

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the ΔM method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices. PMID:27604428

Recognition of exchange striction as the origin of magnetoelectric coupling in multiferroics

NASA Astrophysics Data System (ADS)

Yahia, G.; Damay, F.; Chattopadhyay, S.; Balédent, V.; Peng, W.; Elkaim, E.; Whitaker, M.; Greenblatt, M.; Lepetit, M.-B.; Foury-Leylekian, P.

2017-05-01

The magnetoelectric coupling, a phenomenon inducing magnetic (electric) polarization by application of an external electric (magnetic) field and first conjectured by Curie in 1894, is observed in most of the multiferroics and used for many applications in various fields such as data storage or sensing. However, its microscopic origin is a long-standing controversy in the scientific community. An intense revival of interest developed in the beginning of the 21st century due to the emergence of multiferroic frustrated magnets in which the ferroelectricity is magnetically induced and which present an inherent strong magnetoelectric coupling. The Dzyaloshinskii-Moriya interaction (DMI) well accounts for such ferroelectricity in systems with a noncollinear magnetic order such as the RMnO3 manganites. The DMI effect is, however, inadequate for systems presenting ferroelectricity induced by quasicollinear spin arrangements such as the prominent RMn2O5 manganites. Among different microscopic mechanisms proposed to resolve this incompatibility, the exchange-striction model stands as the most invoked candidate. In this scenario, the polar atomic displacements originate from the release of a frustration caused by the magnetic order. Despite its theoretical description 15 years ago, this mechanism had yet to be unambiguously validated experimentally. The breakthrough finally comes from SmMn2O5 presenting a unique magnetic order revealed by powder neutron diffraction. The unique orientation of its magnetic moment establishes the missing element that definitely validates the exchange striction as the effective mechanism for the spin-induced ferroelectricity in this series. More generally, this is a proof of concept that validates this model on actual systems, facilitating the development of a new generation of multiferroics with unrivaled magnetoelectric properties.

Oscillations in exchange coupling across a nonmagnetic metallic layer

NASA Astrophysics Data System (ADS)

Edwards, D. M.; Mathon, J.

1991-02-01

The exchange coupling between two strong itinerant ferromagnets separated by N atomic planes of a nonmagnetic metal is calculated using a Hubbard-type model. It is shown that for certain positions of the Fermi level the variation of the exchange coupling with N exhibits oscillations of long period. The amplitude of the oscillations falls of as 1/ N2 and agrees in order of magnitude with the exchange coupling observed by Parkin et al. in Co/Ru and Fe/Cr multilayers. Further agreement is the finding that antiparallel alignment of the ferromagnetic layers is favoured for small N. The relationship between the coupling found here and one of RKKY type is discussed.

RKKY interaction in a chirally coupled double quantum dot system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heine, A. W.; Tutuc, D.; Haug, R. J.

2013-12-04

The competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction is investigated in a double quantum dots system, coupled via a central open conducting region. A perpendicular magnetic field induces the formation of Landau Levels which in turn give rise to the so-called Kondo chessboard pattern in the transport through the quantum dots. The two quantum dots become therefore chirally coupled via the edge channels formed in the open conducting area. In regions where both quantum dots exhibit Kondo transport the presence of the RKKY exchange interaction is probed by an analysis of the temperature dependence. The thus obtainedmore » Kondo temperature of one dot shows an abrupt increase at the onset of Kondo transport in the other, independent of the magnetic field polarity, i.e. edge state chirality in the central region.« less

Assessment of the removal of side nanoparticulated populations generated during one-pot synthesis by asymmetric flow field-flow fractionation coupled to elemental mass spectrometry.

PubMed

Bouzas-Ramos, Diego; García-Cortes, Marta; Sanz-Medel, Alfredo; Encinar, Jorge Ruiz; Costa-Fernández, José M

2017-10-13

Coupling of asymmetric flow field-flow fractionation (AF4) to an on-line elemental detection (inductively coupled plasma-mass spectrometry, ICP-MS) has been recently proposed as a powerful diagnostic tool for characterization of the bioconjugation of CdSe/ZnS core-shell Quantum Dots (QDs) to antibodies. Such approach has been used herein to demonstrate that cap exchange of the native hydrophobic shell of core/shell QDs with the bidentate dihydrolipoic acid ligands directly removes completely the eventual side nanoparticulated populations generated during simple one-pot synthesis, which can ruin the subsequent final bioapplication. The critical assessment of the chemical and physical purity of the surface-modified QDs achieved allows to explain the transmission electron microscopy findings obtained for the different nanoparticle surface modification assayed. Copyright © 2017 Elsevier B.V. All rights reserved.

Successive Magnetic-Field-Induced Transitions and Colossal Magnetoelectric Effect in Ni 3 TeO 6

DOE PAGES

Kim, Jae Wook; Artyukhin, Sergei; Mun, Eun Deok; ...

2015-09-24

In this paper, we report the discovery of a metamagnetic phase transition in a polar antiferromagnet Ni 3TeO 6 that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of 0.3 μC/cm 2, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behindmore » the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. Finally, the resultant spin structure is rather counterintuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.« less

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers.

PubMed

Baruth, A; Adenwalla, S

2011-09-21

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

Critical thickness investigation of magnetic properties in exchange-coupled bilayers

NASA Astrophysics Data System (ADS)

Rodríguez-Suárez, R. L.; Vilela-Leão, L. H.; Bueno, T.; Oliveira, A. B.; de Almeida, J. R. L.; Landeros, P.; Rezende, S. M.; Azevedo, A.

2011-06-01

We present a systematic investigation of the magnetic properties of two series of polycrystalline ferromagnetic-antiferromagnetic bilayers (FM-AF) of Ni81Fe19(10nm)/Ir20Mn80(tAF) grown by dc magnetron sputtering. One series was grown at an oblique angle of 50° and the other one was grown at 0°. Ferromagnetic resonance (FMR) was used to measure the exchange bias field HE, the rotatable anisotropy field HRA, and the FMR linewidth ΔH as a function of the antiferromagnetic layer thickness tAF. Three relaxation channels due to isotropic Gilbert damping, anisotropic two-magnon scattering, and mosaicity effects are simultaneously distinguished through the angular dependence of the FMR linewidth. In the regime of small IrMn layer thicknesses, not enough to establish the exchange bias anisotropy, the FMR linewidth shows a sharp peak due to the contribution of the two-magnon scattering mechanism. The results presented here are of general importance for understanding the dynamics of magnetization in the FM-AF structures.

Unravelling the tunable exchange bias-like effect in magnetostatically-coupled two dimensional hybrid (hard/soft) composites

NASA Astrophysics Data System (ADS)

Hierro-Rodriguez, A.; Teixeira, J. M.; Rodriguez-Rodriguez, G.; Rubio, H.; Vélez, M.; Álvarez-Prado, L. M.; Martín, J. I.; Alameda, J. M.

2015-06-01

Hybrid 2D hard-soft composites have been fabricated by combining soft (Co73Si27) and hard (NdCo5) magnetic materials with in-plane and out-of-plane magnetic anisotropies, respectively. They have been microstructured in a square lattice of CoSi anti-dots with NdCo dots within the holes. The magnetic properties of the dots allow us to introduce a magnetostatic stray field that can be controlled in direction and sense by their last saturating magnetic field. The magnetostatic interactions between dot and anti-dot layers induce a completely tunable exchange bias-like shift in the system’s hysteresis loops. Two different regimes for this shift are present depending on the lattice parameter of the microstructures. For large parameters, dipolar magnetostatic decay is observed, while for the smaller one, the interaction between the adjacent anti-dot’s characteristic closure domain structures enhances the exchange bias-like effect as clarified by micromagnetic simulations.

Observation of magnetization and exchange bias reversals in NdFe0.5Cr0.5O3

NASA Astrophysics Data System (ADS)

Sharannia, M. P.; De, Santanu; Singh, Ripandeep; Das, A.; Nirmala, R.; Santhosh, P. N.

2017-05-01

Polycrystalline NdFe0.5Cr0.5O3 has orthorhombic structure with Pnma space group and is magnetically ordered at room temperature as confirmed by neutron diffraction. The magnetic structure involves CxGyFz type ordering of Fe3+/Cr3+ ions. NdFe0.5Cr0.5O3 shows magnetization reversal and sign reversal of exchange bias at 16 K. Nd3+ moments that get induced by the internal field of |Fe+Cr| sublattice couple antiferromagnetically with the ferromagnetic component of |Fe+Cr| sublattice. Nd3+ moments overcome the |Fe+Cr| moments at 16 K below which the material shows negative magnetization and positive exchange bias.

Water flux characterization through hydraulic head and temperature data assimilation: Numerical modeling and sandbox experiments

NASA Astrophysics Data System (ADS)

Ju, Lei; Zhang, Jiangjiang; Chen, Cheng; Wu, Laosheng; Zeng, Lingzao

2018-03-01

Spatial distribution of groundwater recharge/discharge fluxes has an important impact on mass and energy exchanges in shallow streambeds. During the last two decades, extensive studies have been devoted to the quantification of one-dimensional (1-D) vertical exchange fluxes. Nevertheless, few studies were conducted to characterize two-dimensional (2-D) heterogeneous flux fields that commonly exist in real-world cases. In this study, we used an iterative ensemble smoother (IES) to quantify the spatial distribution of 2-D exchange fluxes by assimilating hydraulic head and temperature measurements. Four assimilation scenarios corresponding to different potential field applications were tested. In the first three scenarios, the heterogeneous hydraulic conductivity fields were first inferred from hydraulic head and/or temperature measurements, and then the flux fields were derived through Darcy's law using the estimated conductivity fields. In the fourth scenario, the flux fields were estimated directly from the temperature measurements, which is more efficient and especially suitable for the situation that a complete knowledge of flow boundary conditions is unavailable. We concluded that, the best estimation could be achieved through jointly assimilating hydraulic head and temperature measurements, and temperature data were superior to the head data when they were used independently. Overall, the IES method provided more robust and accurate vertical flux estimations than those given by the widely used analytical solution-based methods. Furthermore, IES gave reasonable uncertainty estimations, which were unavailable in traditional methods. Since temperature can be accurately monitored with high spatial and temporal resolutions, the coupling of heat tracing techniques and IES provides promising potential in quantifying complex exchange fluxes under field conditions.

Electric Field Controlled Magnetism in BiFeO3/Ferromagnet Films

NASA Astrophysics Data System (ADS)

Holcomb, M. B.; Chu, Y. H.; Martin, L. W.; Gajek, M.; Seidel, J.; Ramesh, R.; Scholl, A.; Fraile-Rodriguez, A.

2008-03-01

Electric field control of magnetism is a hot technological topic at the moment due to its potential to revolutionize today's devices. Magnetoelectric materials, those having both electric and magnetic order and the potential for coupling between the two, are a promising avenue to approach electric control. BiFeO3, both a ferroelectric and an antiferromagnet, is the only single phase room temperature magnetoelectric that is currently known. In addition to other possibilities, its multiferroic nature has potential in the very active field of exchange bias, where an antiferromagnetic thin film pins the magnetic direction of an adjoining ferromagnetic layer. Since this antiferromagnet is electrically tunable, this coupling could allow electric-field control of the ferromagnetic magnetization. Direction determination of antiferromagnetic domains in BFO has recently been shown using linear and circular dichroism studies. Recently, this technique has been extended to look at the magnetic domains of a ferromagnetic grown on top of BFO. The clear magnetic changes induced by application of electric fields reveal the possibility of electric control.

A Longitudinal Study of Premarital Couples: A Social Exchange Perspective.

ERIC Educational Resources Information Center

Markman, Howard J.

The attributes of couples planning to marry can affect their future relationship satisfaction. To study this phenomenon, a social exchange model was applied to a sample of couples planning to marry to assess the predictive validity of a measure of positive exchange. The longitudinal design of the two-and-a-half year investigation provided direct…

Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes

NASA Astrophysics Data System (ADS)

Ma, X.; Fang, F.; Li, Q.; Zhu, J.; Yang, Y.; Wu, Y. Z.; Zhao, H. B.; Lüpke, G.

2015-10-01

Optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.

Enhanced magnetic properties in ZnCoAlO caused by exchange-coupling to Co nanoparticles

NASA Astrophysics Data System (ADS)

Feng, Qi; Dizayee, Wala; Li, Xiaoli; Score, David S.; Neal, James R.; Behan, Anthony J.; Mokhtari, Abbas; Alshammari, Marzook S.; Al-Qahtani, Mohammed S.; Blythe, Harry J.; Chantrell, Roy W.; Heald, Steve M.; Xu, Xiao-Hong; Fox, A. Mark; Gehring, Gillian A.

2016-11-01

We report the results of a sequence of magnetisation and magneto-optical studies on laser ablated thin films of ZnCoAlO and ZnCoO that contain a small amount of metallic cobalt. The results are compared to those expected when all the magnetization is due to isolated metallic clusters of cobalt and with an oxide sample that is almost free from metallic inclusions. Using a variety of direct magnetic measurements and also magnetic circular dichroism we find that there is ferromagnetism within both the oxide and the metallic inclusions, and furthermore that these magnetic components are exchange-coupled when aluminium is included. This enhances both the coercive field and the remanence. Hence the presence of a controlled quantity of metallic nanoparticles in ZnAlO can improve the magnetic response of the oxide, thus giving great advantages for applications in spintronics.

Ground-state information geometry and quantum criticality in an inhomogeneous spin model

NASA Astrophysics Data System (ADS)

Ma, Yu-Quan

2015-09-01

We investigate the ground-state Riemannian metric and the cyclic quantum distance of an inhomogeneous quantum spin-1/2 chain in a transverse field. This model can be diagonalized by using a general canonical transformation to the fermionic Hamiltonian mapped from the spin system. The ground-state Riemannian metric is derived exactly on a parameter manifold ring S1, which is introduced by performing a gauge transformation to the spin Hamiltonian through a twist operator. The cyclic ground-state quantum distance and the second derivative of the ground-state energy are studied in different exchange coupling parameter regions. Particularly, we show that, in the case of exchange coupling parameter Ja = Jb, the quantum ferromagnetic phase can be characterized by an invariant quantum distance and this distance will decay to zero rapidly in the paramagnetic phase. Project supported by the National Natural Science Foundation of China (Grant Nos. 11404023 and 11347131).

Enhanced magnetic properties in ZnCoAlO caused by exchange-coupling to Co nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feng, Qi; Dizayee, Wala; Li, Xiaoli

2016-11-01

We report the results of a sequence of magnetisation and magneto-optical studies on laser ablated thin films of ZnCoAlO and ZnCoO that contain a small amount of metallic cobalt. The results are compared to those expected when all the magnetization is due to isolated metallic clusters of cobalt and with an oxide sample that is almost free from metallic inclusions. Using a variety of direct magnetic measurements and also magnetic circular dichroism we find that there is ferromagnetism within both the oxide and the metallic inclusions, and furthermore that these magnetic components are exchange-coupled when aluminium is included. This enhancesmore » both the coercive field and the remanence. Hence the presence of a controlled quantity of metallic nanoparticles in ZnAlO can improve the magnetic response of the oxide, thus giving great advantages for applications in spintronics.« less

Exchange interactions of CaMnO3 in the bulk and at the surface

NASA Astrophysics Data System (ADS)

Keshavarz, S.; Kvashnin, Y. O.; Rodrigues, D. C. M.; Pereiro, M.; Di Marco, I.; Autieri, C.; Nordström, L.; Solovyev, I. V.; Sanyal, B.; Eriksson, O.

2017-03-01

We present electronic and magnetic properties of CaMnO3 (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard U calculations and extract the effective exchange parameters (Ji j's) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower-energy magnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W. E. Pickett, Phys. Rev. Lett. 83, 4184 (1999), 10.1103/PhysRevLett.83.4184] suggested that this coupling is ferromagnetic and gives rise to the spin-flip (SF) process on the surface of CMO. In our work, we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same G -type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.

Exchange coupling in the complex magnetic multilayers

NASA Astrophysics Data System (ADS)

Uzdin, V. M.; Adamowicz, L.; Kocinski, P.

1996-06-01

Exchange coupling in the complex magnetic sandwich structures containing nonmagnetic (NM) and ferromagnetic (FM) layers composed of two different ferromagnetic metals has been studied within the framework of the quantum wells model. The strength of the exchange coupling in the multilayer structure with thin layers of a second ferromagnetic material inserted at the interface of FM/NM/FM sandwich was calculated at various physical situations. In one case the exponential dependence of the exchange coupling on the thickness of the interface ferromagnetic layer has been obtained in striking resemblance to the Parkin experimental results for magnetoresistance (S. S. P. Parkin, Phys. Rev. Lett., 71 (1993) 1641).

Tuning the exchange bias in NiFe/Fe-oxide bilayers by way of different Fe-oxide based mixtures made with an ion-beam deposition technique.

PubMed

Lin, K W; Kol, P H; Guo, Z Y; Ouyang, H; van Lierop, J

2007-01-01

We have investigated the structural and magnetic properties of ion-beam deposited polycrystalline NiFe (25 nm)/Fe-oxide (35 nm) bilayers. A film prepared with an assist beam O2 to Ar gas ratio of 0% during deposition had a bottom layer that consisted of pure b.c.c. Fe (a = 2.87 A) whereas films prepared with 19%O2/Ar and 35%O2/Ar had either Fe3O4 (a = 8.47 angstroms) or alpha-Fe2O3 (a = 5.04 angstroms, c = 13.86 angstroms) bottom layers, respectively. Cross-sectional transmission electron microscopy revealed a smooth interface between the top nano-columnar NiFe and bottom nano-columnar Fe-oxide layer for all films. At room temperature, the observed coercivity (Hc approximately 25 Oe) for a film prepared with 19% O2/Ar indicates the existence of a magnetically hard ferrimagnetic Fe3O4 phase that is enhancing the plain NiFe (Hc approximately 2 Oe) by way of exchange coupling. A significant amount of exchange bias is observed below 50 K, and at 10 K the size of exchange bias hysteresis loops shift increases with increasing oxygen in the films. Furthermore, the strongest exchange coupling (H(ex) approximately 135 Oe at 10 K) is with alpha-Fe2O3 (35% O2/Ar) as the bottom film layer. This indicates that the pure antiferromagnetic phases work better than ferrimagnetic phases when in contact with ferromagnetic NiFe. H(ex) (T) is well described by an effective AF domain wall energy that creates an exchange field with a (1 - T/T(crit)) temperature dependence. Hc (T) exhibits three distinct regimes of constant temperature that may indicate the existence of different AF spin populations that couple to the FM layer at different temperatures.

Collider detection of dark matter electromagnetic anapole moments

NASA Astrophysics Data System (ADS)

Alves, Alexandre; Santos, A. C. O.; Sinha, Kuver

2018-03-01

Dark matter that interacts with the Standard Model by exchanging photons through higher multipole interactions occurs in a wide range of both strongly and weakly coupled hidden sector models. We study the collider detection prospects of these candidates, with a focus on Majorana dark matter that couples through the anapole moment. The study is conducted at the effective field theory level with the mono-Z signature incorporating varying levels of systematic uncertainties at the high-luminosity LHC. The projected collider reach on the anapole moment is then compared to the reach coming from direct detection experiments like LZ. Finally, the analysis is applied to a weakly coupled completion with leptophilic dark matter.

Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.

PubMed

Mergenthaler, Matthias; Liu, Junjie; Le Roy, Jennifer J; Ares, Natalia; Thompson, Amber L; Bogani, Lapo; Luis, Fernando; Blundell, Stephen J; Lancaster, Tom; Ardavan, Arzhang; Briggs, G Andrew D; Leek, Peter J; Laird, Edward A

2017-10-06

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet

NASA Astrophysics Data System (ADS)

Mergenthaler, Matthias; Liu, Junjie; Le Roy, Jennifer J.; Ares, Natalia; Thompson, Amber L.; Bogani, Lapo; Luis, Fernando; Blundell, Stephen J.; Lancaster, Tom; Ardavan, Arzhang; Briggs, G. Andrew D.; Leek, Peter J.; Laird, Edward A.

2017-10-01

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

Magnetosonic solitons in semiconductor plasmas in the presence of quantum tunneling and exchange correlation effects

NASA Astrophysics Data System (ADS)

Hussain, S.; Mahmood, S.

2018-01-01

Low frequency magnetosonic wave excitations are investigated in semiconductor hole-electron plasmas. The quantum mechanical effects such as Fermi pressure, quantum tunneling, and exchange-correlation of holes and electrons in the presence of the magnetic field are considered. The two fluid quantum magnetohydrodynamic model is used to study magnetosonic wave dynamics, while electric and magnetic fields are coupled via Maxwell equations. The dispersion relation of the magnetosonic wave in electron-hole semiconductor plasma propagating in the perpendicular direction of the magnetic field is obtained, and its dispersion effects are discussed. The Korteweg-de Vries equation (KdV) for magnetosonic solitons is derived by employing the reductive perturbation method. For numerical analysis, the plasma parameters are taken from the semiconductors such as GaAs, GaSb, GaN, and InP already existing in the literature. It is found that the phase velocity of the magnetosonic wave is increased with the inclusion of exchange-correlation force in the model. The soliton dip structures of the magnetosonic wave in GaN semiconductor plasma are obtained, which satisfy the quantum plasma conditions for electron and hole fluids. The magnetosonic soliton dip structures move with speed less than the magnetosonic wave phase speed in the lab frame. The effects of exchange-correlation force in the model and variations of magnetic field intensity and electron/hole density on the magnetosonic wave dip structures are also investigated numerically for illustration.

Control of Low-Field Hysteresis Loop Shift of Spin Valves

NASA Astrophysics Data System (ADS)

Chernyshova, T. A.; Milyaev, M. A.; Naumova, L. I.; Proglyado, V. V.; Maksimova, I. K.; Pavlova, A. Yu.; Blagodatkov, D. V.; Ustinov, V. V.

2017-12-01

Spin valves that comprise synthetic antiferromagnet as a component of pinned layer and an exchange-coupled ferromagnet/Ru/ferromagnet structure in the free layer have been prepared by magnetron sputtering. Microobjects have been formed from spin valves by optical and electron-beam lithography. It has been shown that the shift of the low-field magnetoresistance hysteresis loop decreases as the thicknes of the Ru spacer in the free layer of spin valve increases. The almost hysteresis-free odd-field dependences of the magnetoresistance were obtained for micron-sized samples; in this case, the sensitivity is 0.2%/Oe.

Magnetic Exchange Couplings from Semilocal Functionals Evaluated Nonself-Consistently on Hybrid Densities: Insights on Relative Importance of Exchange, Correlation, and Delocalization.

PubMed

Phillips, Jordan J; Peralta, Juan E

2012-09-11

Semilocal functionals generally yield poor magnetic exchange couplings for transition-metal complexes, typically overpredicting in magnitude the experimental values. Here we show that semilocal functionals evaluated nonself-consistently on densities from hybrid functionals can yield magnetic exchange couplings that are greatly improved with respect to their self-consistent semilocal values. Furthermore, when semilocal functionals are evaluated nonself-consistently on densities from a "half-and-half" hybrid, their errors with respect to experimental values can actually be lower than those from self-consistent calculations with standard hybrid functionals such as PBEh or TPSSh. This illustrates that despite their notoriously poor performance for exchange couplings, for many systems semilocal functionals are capable of delivering accurate relative energies for magnetic states provided that their electron delocalization error is corrected. However, while self-consistent calculations with hybrids uniformly improve results for all complexes, evaluating nonself-consistently with semilocal functionals does not give a balanced improvement for both ferro- and antiferromagnetically coupled complexes, indicating that there is more at play with the overestimation problem than simply the delocalization error. Additionally, we show that for some systems the conventional wisdom of choice of exchange functional mattering more than correlation does not hold. This combined with results from the nonself-consistent calculations provide insight on clarifying the relative roles of exchange, correlation, and delocalization in calculating magnetic exchange coupling parameters in Kohn-Sham Density Functional Theory.

Critical exponents and universal magnetic behavior of noncentrosymmetric Fe0.6Co0.4Si.

PubMed

Samatham, S Shanmukharao; Suresh, K G

2018-05-31

The critical magnetic properties of a non-centrosymmetric B20 cubic helimagnet Fe 0.6 Co 0.4 Si are investigated using magnetization isotherms. It belongs to the 3D-Heisenberg universality class with short range magnetic coupling as inferred from the self-consistent critical exponents [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] in combination with exchange interaction [Formula: see text]. Itinerant magnetic nature of the compound is realized by the Rhodes-Wholfarth analysis. Field-induced weak first (para[Formula: see text]helical) to second (para[Formula: see text]field-polarized) order transition is reported to occur at low critical field due to the weak spin-orbit coupling arising from the weak Dzyaloshinksii-Moriya interactions. Our study suggests the distinct phenomenological magnetic structures for Fe-based cubic magnets (Fe 1-x Co x Si and FeGe) and MnSi which cause contrasting physical properties.

Mid-infrared polaritonic coupling between boron nitride nanotubes and graphene.

PubMed

Xu, Xiaoji G; Jiang, Jian-Hua; Gilburd, Leonid; Rensing, Rachel G; Burch, Kenneth S; Zhi, Chunyi; Bando, Yoshio; Golberg, Dmitri; Walker, Gilbert C

2014-11-25

Boron nitride (BN) is considered to be a promising substrate for graphene-based devices in part because its large band gap can serve to insulate graphene in layered heterostructures. At mid-infrared frequencies, graphene supports surface plasmon polaritons (SPPs), whereas hexagonal-BN (h-BN) is found to support surface phonon polaritons (SPhPs). We report on the observation of infrared polaritonic coupling between graphene SPPs and boron nitride nanotube (BNNT) SPhPs. Infrared scattering type scanning near-field optical microscopy is used to obtain spatial distribution of the two types of polaritons at the nanoscale. The observation suggests that those polaritons interact at the nanoscale in a one-dimensional/two-dimensional (1D/2D) geometry, exchanging energy in a nonplanar configuration at the nanoscale. Control of the polaritonic interaction is achieved by adjustment of the graphene Fermi level through voltage gating. Our observation suggests that boron nitride nanotubes and graphene can interact at mid-infrared frequencies and coherently exchange their energies at the nanoscale through the overlap of mutual electric near field of surface phonon polaritons and surface plasmon polaritons. Such interaction enables the design of nano-optical devices based on BNNT-graphene polaritonics in the mid-infrared range.

Kinetic Monte Carlo simulations of the effect of the exchange control layer thickness in CoPtCrB/CoPtCrSiO granular media

NASA Astrophysics Data System (ADS)

Almudallal, Ahmad M.; Mercer, J. I.; Whitehead, J. P.; Plumer, M. L.; van Ek, J.

2018-05-01

A hybrid Landau Lifshitz Gilbert/kinetic Monte Carlo algorithm is used to simulate experimental magnetic hysteresis loops for dual layer exchange coupled composite media. The calculation of the rate coefficients and difficulties arising from low energy barriers, a fundamental problem of the kinetic Monte Carlo method, are discussed and the methodology used to treat them in the present work is described. The results from simulations are compared with experimental vibrating sample magnetometer measurements on dual layer CoPtCrB/CoPtCrSiO media and a quantitative relationship between the thickness of the exchange control layer separating the layers and the effective exchange constant between the layers is obtained. Estimates of the energy barriers separating magnetically reversed states of the individual grains in zero applied field as well as the saturation field at sweep rates relevant to the bit write speeds in magnetic recording are also presented. The significance of this comparison between simulations and experiment and the estimates of the material parameters obtained from it are discussed in relation to optimizing the performance of magnetic storage media.

Controllable Interfacial Coupling Effects on the Magnetic Dynamic Properties of Perpendicular [Co/Ni]5/Cu/TbCo Composite Thin Films.

PubMed

Tang, Minghong; Zhao, Bingcheng; Zhu, Weihua; Zhu, Zhendong; Jin, Q Y; Zhang, Zongzhi

2018-02-07

Dynamic magnetic properties in perpendicularly exchange-coupled [Co/Ni] 5 /Cu (t Cu = 0-2 nm)/TbCo structures show strong dependences on the interfacial antiferromagnetic strength J ex , which is controlled by the Cu interlayer thickness. The precession frequency f and effective damping constant α eff of a [Co/Ni] 5 multilayer differ distinctly for parallel (P) and antiparallel (AP) magnetization orientation states. For samples with a thin t Cu , f of the AP state is apparently higher, whereas α eff is lower than that in the P state, owing to the unidirectional exchange bias effect (H EB ) from the TbCo layer. The differences in f and α eff between the two states gradually decrease with increasing t Cu . By using a uniform precession model including an additional H EB term, the field-dependent frequency curves can be well-fitted, and the fitted H EB value is in good agreement with the experimental data. Moreover, the saturation damping constant α 0 displays a nearly linear correlation with J ex . It decreases significantly with J ex and eventually approaches a constant value of 0.027 at t Cu = 2 nm where J ex vanishes. These results provide a better understanding and effective control of magnetization dynamics in exchange-coupled composite structures for spintronic applications.

Magnetic nanopantograph in the SrCu2(BO3)2 Shastry–Sutherland lattice

PubMed Central

Radtke, Guillaume; Saúl, Andrés; Dabkowska, Hanna A.; Salamon, Myron B.; Jaime, Marcelo

2015-01-01

Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu^ superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO. PMID:25646467

Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films

NASA Astrophysics Data System (ADS)

Rawat, Naveen; Pan, Zhenwen; Lamarche, Cody J.; Wetherby, Anthony; Waterman, Rory; Tokumoto, Takahisa; Cherian, Judy G.; Headrick, Randall L.; McGill, Stephen A.; Furis, Madalina I.

2015-11-01

The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ - d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of -4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials.

Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films

PubMed Central

Rawat, Naveen; Pan, Zhenwen; Lamarche, Cody J.; Wetherby, Anthony; Waterman, Rory; Tokumoto, Takahisa; Cherian, Judy G.; Headrick, Randall L.; McGill, Stephen A.; Furis, Madalina I.

2015-01-01

The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ − d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of −4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials. PMID:26559337

Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films.

PubMed

Rawat, Naveen; Pan, Zhenwen; Lamarche, Cody J; Wetherby, Anthony; Waterman, Rory; Tokumoto, Takahisa; Cherian, Judy G; Headrick, Randall L; McGill, Stephen A; Furis, Madalina I

2015-11-12

The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ - d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of -4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials.

Interlayer exchange coupling in complex magnetic multilayers

NASA Astrophysics Data System (ADS)

Xiang-dong, Zhang; Lie-ming, Li; Bo-zang, Li; Fu-cho, Pu

1998-07-01

We extend the hole confinement model of Edwards et al. to the problem of two kinds of complex magnetic sandwich structures. One is the magnetic sandwich covered on both sides by nonmagnetic films (case 1) and the other is that covered by magnetic films (case 2). The interlayer exchange coupling and the angular dependence of coupling energy in the two cases are investigated systematically. For case 1, our results show that the magnetic and outer nonmagnetic films influence significantly the oscillation behavior of exchange coupling and the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers. Our results also show that the nonoscillatory component of the coupling generally varies with the thickness of magnetic (outer nonmagnetic) films and the results in the case where the thickness of both magnetic (outer nonmagnetic) films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic (outer nonmagnetic) films is fixed while the other is varied, which is qualitatively in agreement with the experimental measurements. For case 2, the exponential dependence of exchange coupling on the thickness of the intermagnetic layer has been obtained, similar to the Parkin's experimental results for giant magnetoresistance.

Exact Mapping from Many-Spin Hamiltonians to Giant-Spin Hamiltonians.

PubMed

Ghassemi Tabrizi, Shadan; Arbuznikov, Alexei V; Kaupp, Martin

2018-03-26

Thermodynamic and spectroscopic data of exchange-coupled molecular spin clusters (e.g. single-molecule magnets) are routinely interpreted in terms of two different models: the many-spin Hamiltonian (MSH) explicitly considers couplings between individual spin centers, while the giant-spin Hamiltonian (GSH) treats the system as a single collective spin. When isotropic exchange coupling is weak, the physical compatibility between both spin Hamiltonian models becomes a serious concern, due to mixing of spin multiplets by local zero-field splitting (ZFS) interactions ('S-mixing'). Until now, this effect, which makes the mapping MSH→GSH ('spin projection') non-trivial, had only been treated perturbationally (up to third order), with obvious limitations. Here, based on exact diagonalization of the MSH, canonical effective Hamiltonian theory is applied to construct a GSH that exactly matches the energies of the relevant (2S+1) states comprising an effective spin multiplet. For comparison, a recently developed strategy for the unique derivation of effective ('pseudospin') Hamiltonians, now routinely employed in ab initio calculations of mononuclear systems, is adapted to the problem of spin projection. Expansion of the zero-field Hamiltonian and the magnetic moment in terms of irreducible tensor operators (or Stevens operators) yields terms of all ranks k (up to k=2S) in the effective spin. Calculations employing published MSH parameters illustrate exact spin projection for the well-investigated [Ni(hmp)(dmb)Cl] 4 ('Ni 4 ') single-molecule magnet, which displays weak isotropic exchange (dmb=3,3-dimethyl-1-butanol, hmp - is the anion of 2-hydroxymethylpyridine). The performance of the resulting GSH in finite field is assessed in terms of EPR resonances and diabolical points. The large tunnel splitting in the M=± 4 ground doublet of the S=4 multiplet, responsible for fast tunneling in Ni 4 , is attributed to a Stevens operator with eightfold rotational symmetry, marking the first quantification of a k=8 term in a spin cluster. The unique and exact mapping MSH→GSH should be of general importance for weakly-coupled systems; it represents a mandatory ultimate step for comparing theoretical predictions (e.g. from quantum-chemical calculations) to ZFS, hyperfine or g-tensors from spectral fittings. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Modified ion exchange separation for tungsten isotopic measurements from kimberlite samples using multi-collector inductively coupled plasma mass spectrometry.

PubMed

Sahoo, Yu Vin; Nakai, Shun'ichi; Ali, Arshad

2006-03-01

Tungsten isotope composition of a sample of deep-seated rock can record the influence of core-mantle interaction of the parent magma. Samples of kimberlite, which is known as a carrier of diamond, from the deep mantle might exhibit effects of core-mantle interaction. Although tungsten isotope anomaly was reported for kimberlites from South Africa, a subsequent investigation did not verify the anomaly. The magnesium-rich and calcium-rich chemical composition of kimberlite might engender difficulty during chemical separation of tungsten for isotope analyses. This paper presents a simple, one-step anion exchange technique for precise and accurate determination of tungsten isotopes in kimberlites using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Large quantities of Ca and Mg in kimberlite samples were precipitated and removed with aqueous H(2)SO(4). Highly pure fractions of tungsten for isotopic measurements were obtained following an anion exchange chromatographic procedure involving mixed acids. That procedure enabled efficient removal of high field strength elements (HFSE), such as Hf, Zr and Ti, which are small ions that carry strong charges and develop intense electrostatic fields. The tungsten yields were 85%-95%. Advantages of this system include less time and less use of reagents. Precise and accurate isotopic measurements are possible using fractions of tungsten that are obtained using this method. The accuracy and precision of these measurements were confirmed using various silicate standard rock samples, JB-2, JB-3 and AGV-1.

Two-dimensional chiral asymmetry in unidirectional magnetic anisotropy structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perna, P., E-mail: paolo.perna@imdea.org; Guerrero, R.; Niño, M. A.

2016-05-15

We investigate the symmetry-breaking effects of magnetic nanostructures that present unidirectional (one-fold) magnetic anisotropy. Angular and field dependent transport and magnetic properties have been studied in two different exchange-biased systems, i.e. ferromagnetic (FM)/ antiferromagnetic (AFM) bilayer and spin-valve structures. We experimentally show the direct relationships between the magnetoresistance (MR) response and the magnetization reversal pathways for any field value and direction. We demonstrate that even though the MR signals are related to different transport phenomena, namely anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR), chiral asymmetries are found around the magnetization hard-axis direction, in both cases originated from the one-fold symmetrymore » of the interfacial exchange coupling. Our results indicate that the chiral asymmetry of transport and magnetic behaviors are intrinsic of systems with an unidirectional contribution.« less

Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode.

PubMed

Verhagen, E; Deléglise, S; Weis, S; Schliesser, A; Kippenberg, T J

2012-02-01

Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities. If the optomechanical coupling is 'quantum coherent'--that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate--quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 ± 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links.

Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes

DOE PAGES

Ma, X.; Fang, F.; Li, Q.; ...

2015-10-28

In this study, optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recoverymore » time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.« less

Monte Carlo study of a ferrimagnetic mixed-spin (2, 5/2) system with the nearest and next-nearest neighbors exchange couplings

NASA Astrophysics Data System (ADS)

Bi, Jiang-lin; Wang, Wei; Li, Qi

2017-07-01

In this paper, the effects of the next-nearest neighbors exchange couplings on the magnetic and thermal properties of the ferrimagnetic mixed-spin (2, 5/2) Ising model on a 3D honeycomb lattice have been investigated by the use of Monte Carlo simulation. In particular, the influences of exchange couplings (Ja, Jb, Jan) and the single-ion anisotropy(Da) on the phase diagrams, the total magnetization, the sublattice magnetization, the total susceptibility, the internal energy and the specific heat have been discussed in detail. The results clearly show that the system can express the critical and compensation behavior within the next-nearest neighbors exchange coupling. Great deals of the M curves such as N-, Q-, P- and L-types have been discovered, owing to the competition between the exchange coupling and the temperature. Compared with other theoretical and experimental works, our results have an excellent consistency with theirs.

Electrically protected resonant exchange qubits in triple quantum dots.

PubMed

Taylor, J M; Srinivasa, V; Medford, J

2013-08-02

We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates.

Developing a polymeric sensor to monitor intracellular conditions

NASA Astrophysics Data System (ADS)

Mudarri, Timothy C.; Leo, Donald J.; Wood, Brett C.; Shires, Peter K.

2004-07-01

Ionic electroactive polymers have been developed as mechanical sensors or actuators, taking advantage of the electromechanical coupling of the materials. This research attempts to take advantage of the chemomechanical and chemoelectrical coupling by characterizing the transient response as the polymer undergoes an ion exchange, thus using the polymer for ionic sensing. Nafion is a biocompatible material, and an implantable polymeric ion sensor which has applications in the biomedical field for bone healing research. An ion sensor and a strain gauge could determine the effects of motion allowed at the fracture site, thus improving rehabilitation procedures for bone fractures. The charge sensitivity of the material and the capacitance of the material were analyzed to determine the transient response. Both measures indicate a change when immersed in ionic salt solutions. It is demonstrated that measuring the capacitance is the best indicator of an ion exchange. Relative to a flat response in deionized water (+/-2%), the capacitance of the polymer exhibits an exponential decay of ~25% of its peak when placed in a salt solution. A linear correlation between the time constant of the decay and the ionic size of the exchanging ion was developed that could reasonably predict a diffusing ion. Tests using an energy dispersive spectrometer (EDS) indicate that 90% of the exchange occurs in the first 20 minutes, shown by both capacitance decay and an atomic level scan. The diffusion rate time constant was found to within 0.3% of the capacitance time constant, confirming the ability of capacitance to measure ion exchange.

Coupled Atmosphere-Wave-Ocean Modeling of Tropical Cyclones: Progress, Challenges, and Ways Forward

NASA Astrophysics Data System (ADS)

Chen, Shuyi

2015-04-01

It has long been recognized that air-sea interaction plays an important role in tropical cyclones (TC) intensity change. However, most current numerical weather prediction (NWP) models are deficient in predicting TC intensity. The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in TCs push the surface-exchange parameters for temperature, water vapor, and momentum into untested regimes. Parameterizations of air-sea fluxes in NWP models are often crude and create "manmade" energy source/sink that does not exist, especially in the absence of a fully interactive ocean in the model. The erroneous surface heat, moisture, and momentum fluxes can cause compounding errors in the model (e.g., precipitation, water vapor, boundary layer properties). The energy source (heat and moisture fluxes from the ocean) and sink (surface friction and wind-induced upper ocean cooling) are critical to TC intensity. However, observations of air-sea fluxes in TCs are very limited, especially in extreme high wind conditions underneath of the eyewall region. The Coupled Boundary Layer Air-Sea Transfer (CBLAST) program was designed to better understand the air-sea interaction, especially in high wind conditions, which included laboratory and coupled model experiments and field campaign in 2003-04 hurricane seasons. Significant progress has been made in better understanding of air-sea exchange coefficients up to 30 m/s, i.e., a leveling off in drag coefficient and relatively invariant exchange coefficient of enthalpy with wind speed. More recently, the Impact of Typhoon on the Ocean in the Pacific (ITOP) field campaign in 2010 has provided an unprecedented data set to study the air-sea fluxes in TCs and their impact on TC structure and intensity. More than 800 GPS dropsondes and 900 AXBTs/AXCTs as well as drifters, floats, and moorings were deployed in TCs, including Typhoons Fanapi and Malakas, and Supertyphoon Megi with a record peak wind speed of more than 80 m/s. It is found that the air-sea fluxes are quite asymmetric around a storm with complex features representing various air-sea interaction processes in TCs. A unique observation in Typhoon Fanapi is the development of a stable boundary layer in the near-storm cold wake region, which has a direct impact on TC inner core structure and intensity. Despite of the progress, challenges remain. Air-sea momentum exchange in wind speed greater than 30-40 m/s is largely unresolved. Directional wind-wave stress and wave-current stress are difficult to determine from observations. Effects of sea spray on the air-sea fluxes are still not well understood. This talk will provide an overview on progress made in recent years, challenges we are facing, and ways forward. An integrated coupled observational and atmosphere-wave-ocean modeling system is urgently needed, in which coupled model development and targeted observations from field campaign and lab measurements together form the core of the research and prediction system. Another important aspect is that fully coupled models provide explicit, integrated impact forecasts of wind, rain, waves, ocean currents and surges in TCs and winter storms, which are missing in most current NWP models. It requires a new strategy for model development, evaluation, and verification. Ensemble forecasts using high-resolution coupled atmosphere-wave-ocean models can provide probabilistic forecasts and quantitative uncertainty estimates, which also allow us to explore new methodologies to verify probabilistic impact forecasts and evaluate model physics using a stochastic approach. Examples of such approach in TCs including Superstorm Sandy will be presented.

Study of interlayer coupling between FePt and FeCoB thin films through MgO spacer layer

NASA Astrophysics Data System (ADS)

Singh, Sadhana; Kumar, Dileep; Gupta, Mukul; Reddy, V. Raghvendra

2017-05-01

Interlayer exchange coupling between hard-FePt and soft-FeCoB magnetic layers has been studied with increasing thickness of insulator MgO spacer layer in FePt/MgO/FeCoB sandwiched structure. A series of the samples were prepared in identical condition using ion beam sputtering method and characterized for their magnetic and structural properties using magneto-optical Kerr effect (MOKE) and X-ray reflectivity measurements. The nature of coupling between FePt and FeCoB was found to be ferromagnetic which decreases exponentially with increasing thickness of MgO layer. At very low thickness of MgO layer, both layers were found strongly coupled thus exhibiting coherent magnetization reversal. At higher thickness, both layers were found decoupled and magnetization reversal occurred at different switching fields. Strong coupling at very low thickness is attributed to pin holes in MgO layer which lead to direct coupling whereas on increasing thickness, coupling may arise due to magneto-static interactions.

Is {sup 276}U a doubly magic nucleus?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liliani, N., E-mail: netta.liliani@gmail.com; Sulaksono, A.

2016-04-19

We investigate a possible new doubly magic heavy nucleus by using a relativistic mean-field (RMF) model with the addition of a cross interaction term of omega-rho mesons and an electromagnetic exchange term. We propose that {sup 276}U is a doubly magic nucleus. The evidence for {sup 276}U being a doubly magic nucleus is shown through the two-nucleon gaps, the single-particle energies, and the neutron skin thickness of the nucleus. We have also found that the prediction of {sup 276}U as a doubly magic nucleus by the RMF model is not affected by the inclusion of isoscalar-isovector and electromagnetic exchange couplings.

Layer and doping tunable ferromagnetic order in two-dimensional Cr S2 layers

NASA Astrophysics Data System (ADS)

Wang, Cong; Zhou, Xieyu; Pan, Yuhao; Qiao, Jingsi; Kong, Xianghua; Kaun, Chao-Cheng; Ji, Wei

2018-06-01

Interlayer coupling is of vital importance for manipulating physical properties, e.g., electronic band gap, in two-dimensional materials. However, tuning magnetic properties in these materials is yet to be addressed. Here, we found the in-plane magnetic orders of Cr S2 mono and few layers are tunable between striped antiferromagnetic (sAFM) and ferromagnetic (FM) orders by manipulating charge transfer between Cr t2 g and eg orbitals. Such charge transfer is realizable through interlayer coupling, direct charge doping, or substituting S with Cl atoms. In particular, the transferred charge effectively reduces a portion of Cr4 + to Cr3 +, which, together with delocalized S p orbitals and their resulting direct S-S interlayer hopping, enhances the double-exchange mechanism favoring the FM rather than sAFM order. An exceptional interlayer spin-exchange parameter was revealed over -10 meV , an order of magnitude stronger than available results of interlayer magnetic coupling. It addition, the charge doping could tune Cr S2 between p - and n -doped magnetic semiconductors. Given these results, several prototype devices were proposed for manipulating magnetic orders using external electric fields or mechanical motion. These results manifest the role of interlayer coupling in modifying magnetic properties of layered materials and shed considerable light on manipulating magnetism in these materials.

Orbit-spin coupling and the circulation of the Martian atmosphere

NASA Astrophysics Data System (ADS)

Shirley, James H.

2017-07-01

The physical origins of the observed interannual variability of weather and climate on Mars are poorly understood. In this paper we introduce a deterministic physical mechanism that may account for much of the variability of the circulation of the Mars atmosphere on seasonal and longer timescales. We focus on a possible coupling between the planetary orbital angular momentum and the angular momentum of the planetary rotation. We suspect that the planetary atmosphere may participate in an exchange of momentum between these two reservoirs. Nontrivial changes in the circulation of the atmosphere are likely to occur, as the atmospheric system gains and loses angular momentum, during this exchange. We derive a coupling expression linking orbital and rotational motions that produces an acceleration field varying with position and with time on and within a subject body. The spatially and temporally varying accelerations may interfere constructively or destructively with large-scale flows of geophysical fluids that are established and maintained by other means. This physical hypothesis predicts cycles of intensification and relaxation of circulatory flows of atmospheres on seasonal and longer timescales that are largely independent of solar forcing. The predictions of this hypothesis may be tested through numerical modeling. Examples from investigations of the atmospheric circulation of Mars are provided to illustrate qualitative features and quantitative aspects of the coupling mechanism proposed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hammond, Glenn Edward; Bao, J; Huang, M

Hyporheic exchange is a critical mechanism shaping hydrological and biogeochemical processes along a river corridor. Recent studies on quantifying the hyporheic exchange were mostly limited to local scales due to field inaccessibility, computational demand, and complexity of geomorphology and subsurface geology. Surface flow conditions and subsurface physical properties are well known factors on modulating the hyporheic exchange, but quantitative understanding of their impacts on the strength and direction of hyporheic exchanges at reach scales is absent. In this study, a high resolution computational fluid dynamics (CFD) model that couples surface and subsurface flow and transport is employed to simulate hyporheicmore » exchanges in a 7-km long reach along the main-stem of the Columbia River. Assuming that the hyporheic exchange does not affect surface water flow conditions due to its negligible magnitude compared to the volume and velocity of river water, we developed a one-way coupled surface and subsurface water flow model using the commercial CFD software STAR-CCM+. The model integrates the Reynolds-averaged Navier-Stokes (RANS) equation solver with a realizable κ-ε two-layer turbulence model, a two-layer all y + wall treatment, and the volume of fluid (VOF) method, and is used to simulate hyporheic exchanges by tracking the free water-air interface as well as flow in the river and the subsurface porous media. The model is validated against measurements from acoustic Doppler current profiler (ADCP) in the stream water and hyporheic fluxes derived from a set of temperature profilers installed across the riverbed. The validated model is then employed to systematically investigate how hyporheic exchanges are influenced by surface water fluid dynamics strongly regulated by upstream dam operations, as well as subsurface structures (e.g. thickness of riverbed and subsurface formation layers) and hydrogeological properties (e.g. permeability). The results suggest that the thickness of riverbed alluvium layer is the dominant factor for reach-scale hyporheic exchanges, followed by the alluvium permeability, the depth of the underlying impermeable layer, and the assumption of hydrostatic pressure.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kharel, P.; Herran, J.; Lukashev, P.

Recent discovery of a new class of materials, spin-gapless semiconductors (SGS), has attracted considerable attention in the last few years, primarily due to potential applications in the emerging field of spin-based electronics (spintronics). Here, we investigate structural, electronic, and magnetic properties of one potential SGS compound, MnCrVAl, using various experimental and theoretical techniques. Our calculations show that this material exhibits ≈ 0.5 eV band gap for the majority-spin states, while for the minority-spin it is nearly gapless. The calculated magnetic moment for the completely ordered structure is 2.9 μB/f.u., which is different from our experimentally measured value of almost zero.more » Here, this discrepancy is explained by the structural disorder. In particular, A2 type disorder, where Mn or Cr atoms exchange their positions with Al atoms, results in induced antiferromagnetic exchange coupling, which, at a certain level of disorder, effectively reduces the total magnetic moment to zero. This is consistent with our x-ray diffraction measurements which indicate the presence of A2 disorder in all of our samples. In addition, we also show that B2 disorder does not result in antiferromagnetic exchange coupling and therefore does not significantly reduce the total magnetic moment.« less

Composite multi-qubit gates dynamically corrected against charge noise and magnetic field noise for singlet-triplet qubits

NASA Astrophysics Data System (ADS)

Kestner, Jason; Barnes, Edwin; Wang, Xin; Bishop, Lev; Das Sarma, Sankar

2013-03-01

We use previously described single-qubit SUPCODE pulses on both intra-qubit and inter-qubit exchange couplings, integrated with existing strategies such as BB1, to theoretically construct a CNOT gate that is robust against both charge noise and magnetic field gradient fluctuations. We show how this allows scalable, high-fidelity implementation of arbitrary multi-qubit operations using singlet-triplet spin qubits in the presence of experimentally realistic noise. This work is supported by LPS-NSA-CMTC, IARPA-MQCO and CNAM.

Instabilities and turbulence in highly ionized plasmas in a magnetic field

NASA Technical Reports Server (NTRS)

Jennings, W. C.

1972-01-01

Physical mechanisms were considered which are responsible for plasma turbulence and the establishment of necessary conditions for energy exchange and transfer in the frequency spectrum. In addition, work was performed to better understand the drift instability in the highly inhomogeneous Rensselaer arc, and methods to suppress this instability using feedback stabilization techniques. Correlation techniques were refined to study plasma turbulence, the diffusion wave technique for monitoring cross-field diffusion was extended to include regimes of high turbulence levels, and a technique for coupling stabilizing RF power to the Rensselaer arc was developed.

Air-Sea Enthalpy and Momentum Exchange at Major Hurricane Wind Speeds

DTIC Science & Technology

2010-06-01

momentum fluxes. Hurricane simulations using the Navy Coupled Ocean / Atmosphere Mesoscale Prediction System are also sensitive to the surface flux and sea... Atmospheric Research NWP Numerical Weather Prediction NOAA National Oceanic and Atmospheric Administration PTH Pressure, Temperature, relative Humidity RE87... Oceanic and Atmospheric Administration for organizing the CBLAST field program and collecting the data used for this study. xx THIS PAGE

Assessing the exchange coupling in binuclear lanthanide(iii) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative† †Electronic supplementary information (ESI) available: Additional magnetic data, additional figures and computational details. CCDC 1020818–1020822. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc01029b

PubMed Central

Chow, Chun Y.; Bolvin, Hélène; Campbell, Victoria E.; Guillot, Régis; Kampf, Jeff W.; Wernsdorfer, Wolfgang; Gendron, Frédéric; Autschbach, Jochen

2015-01-01

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga4Ln2(shi3–)4(Hshi2–)2(H2shi–)2(C5H5N)4(CH3OH)x(H2O)x]·xC5H5N·xCH3OH·xH2O (where H3shi = salicylhydroxamic acid and Ln = GdIII1; TbIII2; DyIII3; ErIII4; YIII5; YIII0.9DyIII0.16). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled DyIII ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy2 than for the Er2 complex. PMID:29218180

Barrier versus tilt exchange gate operations in spin-based quantum computing

NASA Astrophysics Data System (ADS)

Shim, Yun-Pil; Tahan, Charles

2018-04-01

We present a theory for understanding the exchange interaction between electron spins in neighboring quantum dots, either by changing the detuning of the two quantum dots or independently tuning the tunneling barrier between quantum dots. The Hubbard model and a more realistic confining-potential model are used to investigate how the tilting and barrier control affect the effective exchange coupling and thus the gate fidelity in both the detuning and symmetric regimes. We show that the exchange coupling is less sensitive to the charge noise through tunnel barrier control (while allowing for exchange coupling operations on a sweet spot where the exchange interaction has zero derivative with respect to the detuning). Both GaAs and Si quantum dots are considered, and we compare our results with experimental data showing qualitative agreements. Our results answer the open question of why barrier gates are preferable to tilt gates for exchange-based gate operations.

Spin-orbit coupling induced two-electron relaxation in silicon donor pairs

NASA Astrophysics Data System (ADS)

Song, Yang; Das Sarma, S.

2017-09-01

We unravel theoretically a key intrinsic relaxation mechanism among the low-lying singlet and triplet donor-pair states in silicon, an important element in the fast-developing field of spintronics and quantum computation. Despite the perceived weak spin-orbit coupling (SOC) in Si, we find that our discovered relaxation mechanism, combined with the electron-phonon and interdonor interactions, drives the transitions in the two-electron states over a large range of donor coupling regimes. The scaling of the relaxation rate with interdonor exchange interaction J goes from J5 to J4 at the low to high temperature limits. Our analytical study draws on the symmetry analysis over combined band, donor envelope, and valley configurations. It uncovers naturally the dependence on the donor-alignment direction and triplet spin orientation, and especially on the dominant SOC source from donor impurities. While a magnetic field is not necessary for this relaxation, unlike in the single-donor spin relaxation, we discuss the crossover behavior with increasing Zeeman energy in order to facilitate comparison with experiments.

Bi-quadratic interlayer exchange coupling in Co{sub 2}MnSi/Ag/Co{sub 2}MnSi pseudo spin-valve

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goripati, Hari S.; Hono, K.; Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-0047

2011-12-15

Bi-quadratic interlayer exchange coupling is found below 100 K in a Co{sub 2}MnSi/Ag/Co{sub 2}MnSi current-perpendicular-to-plane pseudo spin valves. The bi-quadratic coupling constant J{sub 2} was estimated to be {approx}-0.30 erg/cm{sup 2} at 5 K and the strong temperature dependence of the coupling strength points its likely origin to the ''loose spin'' model. Application of current of {approx}2 x 10{sup 7} A/cm{sup 2} below 100 K leads to an increase in the magnetoresistance (MR), indicating current induced antiparallel alignment of the two magnetic layers. These results strongly suggest that the presence of the bi-quadratic interlayer exchange coupling causes the reduction ofmore » the magnetoresistance at low temperature and illustrates the importance of understanding the influence of interlayer exchange coupling on magnetization configuration in magnetic nanostructures.« less

Spin-state transfer in laterally coupled quantum-dot chains with disorders

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang Song; Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026; Bayat, Abolfazl

2010-08-15

Quantum dot arrays are a promising medium for transferring quantum information between two distant points without resorting to mobile qubits. Here we study the two most common disorders, namely hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that antiferromagnetic chains are much more resistive than the ferromagnetic ones against both kind of disorders. Even whenmore » time modulation of a coupling and optimal control is employed to improve the transmission, the antiferromagnetic chain performs much better. We have assumed the quasistatic approximation for hyperfine interaction and time-dependent fluctuations in the exchange couplings. Particularly for studying exchange coupling fluctuations we have considered the static disorder, white noise, and 1/f noise.« less

Chemical trend of exchange coupling in diluted magnetic II-VI semiconductors: Ab initio calculations

NASA Astrophysics Data System (ADS)

Chanier, T.; Virot, F.; Hayn, R.

2009-05-01

We have calculated the chemical trend of magnetic exchange parameters ( Jdd , Nα , and Nβ ) of Zn-based II-VI semiconductors ZnA ( A=O , S, Se, and Te) doped with Co or Mn. We show that a proper treatment of electron correlations by the local spin-density approximation (LSDA)+U method leads to good agreement between experimental and theoretical values of the nearest-neighbor exchange coupling Jdd between localized 3d spins in contrast to the LSDA method. The exchange couplings between localized spins and doped electrons in the conduction band Nα are in good agreement with experiment as well. But the values for Nβ (coupling to doped holes in the valence band) indicate a crossover from weak coupling (for A=Te and Se) to strong coupling (for A=O ) and a localized hole state in ZnO:Mn. This hole localization explains the apparent discrepancy between photoemission and magneto-optical data for ZnO:Mn.

High-field magnetization and magnetic phase diagram of α -Cu2V2O7

NASA Astrophysics Data System (ADS)

Gitgeatpong, G.; Suewattana, M.; Zhang, Shiwei; Miyake, A.; Tokunaga, M.; Chanlert, P.; Kurita, N.; Tanaka, H.; Sato, T. J.; Zhao, Y.; Matan, K.

2017-06-01

High-field magnetization of the spin-1 /2 antiferromagnet α -Cu2V2O7 was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the a axis), two distinct transitions were observed at Hc 1=6.5 T and Hc 2=18.0 T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for Hc 1

Correlational and thermodynamic properties of finite-temperature electron liquids in the hypernetted-chain approximation.

PubMed

Tanaka, Shigenori

2016-12-07

Correlational and thermodynamic properties of homogeneous electron liquids at finite temperatures are theoretically analyzed in terms of dielectric response formalism with the hypernetted-chain (HNC) approximation and its modified version. The static structure factor and the local-field correction to describe the strong Coulomb-coupling effects beyond the random-phase approximation are self-consistently calculated through solution to integral equations in the paramagnetic (spin unpolarized) and ferromagnetic (spin polarized) states. In the ground state with the normalized temperature θ=0, the present HNC scheme well reproduces the exchange-correlation energies obtained by quantum Monte Carlo (QMC) simulations over the whole fluid phase (the coupling constant r s ≤100), i.e., within 1% and 2% deviations from putative best QMC values in the paramagnetic and ferromagnetic states, respectively. As compared with earlier studies based on the Singwi-Tosi-Land-Sjölander and modified convolution approximations, some improvements on the correlation energies and the correlation functions including the compressibility sum rule are found in the intermediate to strong coupling regimes. When applied to the electron fluids at intermediate Fermi degeneracies (θ≈1), the static structure factors calculated in the HNC scheme show good agreements with the results obtained by the path integral Monte Carlo (PIMC) simulation, while a small negative region in the radial distribution function is observed near the origin, which may be associated with a slight overestimation for the exchange-correlation hole in the HNC approximation. The interaction energies are calculated for various combinations of density and temperature parameters ranging from strong to weak degeneracy and from weak to strong coupling, and the HNC values are then parametrized as functions of r s and θ. The HNC exchange-correlation free energies obtained through the coupling-constant integration show reasonable agreements with earlier results including the PIMC-based fitting over the whole fluid region at finite degeneracies in the paramagnetic state. In contrast, a systematic difference between the HNC and PIMC results is observed in the ferromagnetic state, which suggests a necessity of further studies on the exchange-correlation free energies from both aspects of analytical theory and simulation.

Coupling Solute and Fine Particle Transport with Sand Bed Morphodynamics within a Field Experiment

NASA Astrophysics Data System (ADS)

Phillips, C. B.; Ortiz, C. P.; Schumer, R.; Jerolmack, D. J.; Packman, A. I.

2017-12-01

Fine suspended particles are typically considered to pass through streams and rivers as wash load without interacting with the bed, however experiments have demonstrated that hyporheic flow causes advective exchange of fine particles with the stream bed, yielding accumulation of fine particle deposits within the bed. Ultimately, understanding river morphodynamics and ecosystem dynamics requires coupling both fine particle and solute transport with bed morphodynamics. To better understand the coupling between these processes we analyze a novel dataset from a controlled field experiment conducted on Clear Run, a 2nd order sand bed stream located within the North Carolina coastal plain. Data include concentrations of continuously injected conservative solutes and fine particulate tracers measured at various depths within the stream bed, overhead time lapse images of bed forms, stream discharge, and geomorphological surveys of the stream. We use image analysis of bed morphodynamics to assess exchange, retention, and remobilization of solutes and fine particles during constant discharge and a short duration experimental flood. From the images, we extract a time series of bedform elevations and scour depths for the duration of the experiment. The high-resolution timeseries of bed elevation enables us to assess coupling of bed morphodynamics with both the solute and fine particle flux during steady state mobile bedforms prior to the flood and to changing bedforms during the flood. These data allow the application of a stochastic modeling framework relating bed elevation fluctuations to fine particle residence times. This combined experimental and modeling approach ultimately informs our ability to predict not only the fate of fine particulate matter but also associated nutrient and carbon dynamics within streams and rivers.

Quantitative analysis of dinuclear manganese(II) EPR spectra

NASA Astrophysics Data System (ADS)

Golombek, Adina P.; Hendrich, Michael P.

2003-11-01

A quantitative method for the analysis of EPR spectra from dinuclear Mn(II) complexes is presented. The complex [(Me 3TACN) 2Mn(II) 2(μ-OAc) 3]BPh 4 ( 1) (Me 3TACN= N, N', N''-trimethyl-1,4,7-triazacyclononane; OAc=acetate 1-; BPh 4=tetraphenylborate 1-) was studied with EPR spectroscopy at X- and Q-band frequencies, for both perpendicular and parallel polarizations of the microwave field, and with variable temperature (2-50 K). Complex 1 is an antiferromagnetically coupled dimer which shows signals from all excited spin manifolds, S=1 to 5. The spectra were simulated with diagonalization of the full spin Hamiltonian which includes the Zeeman and zero-field splittings of the individual manganese sites within the dimer, the exchange and dipolar coupling between the two manganese sites of the dimer, and the nuclear hyperfine coupling for each manganese ion. All possible transitions for all spin manifolds were simulated, with the intensities determined from the calculated probability of each transition. In addition, the non-uniform broadening of all resonances was quantitatively predicted using a lineshape model based on D- and r-strain. As the temperature is increased from 2 K, an 11-line hyperfine pattern characteristic of dinuclear Mn(II) is first observed from the S=3 manifold. D- and r-strain are the dominate broadening effects that determine where the hyperfine pattern will be resolved. A single unique parameter set was found to simulate all spectra arising for all temperatures, microwave frequencies, and microwave modes. The simulations are quantitative, allowing for the first time the determination of species concentrations directly from EPR spectra. Thus, this work describes the first method for the quantitative characterization of EPR spectra of dinuclear manganese centers in model complexes and proteins. The exchange coupling parameter J for complex 1 was determined ( J=-1.5±0.3 cm-1; H ex=-2J S1· S2) and found to be in agreement with a previous determination from magnetization. The phenomenon of exchange striction was found to be insignificant for 1.

Exchange-coupling of hard and soft magnetic sublattices and magnetic anomalies in mixed spinel NiFe0.75Cr1.25O4 nanoparticles

NASA Astrophysics Data System (ADS)

Lyubutin, I. S.; Starchikov, S. S.; Baskakov, A. O.; Gervits, N. E.; Lin, Chun-Rong; Tseng, Yaw-Teng; Lee, Wen-Jen; Shih, Kun-Yauh

2018-04-01

A set of single-crystalline nanoparticles (NPs) of nickel-chromium ferrite NiFe0.75Cr1.25O4 with a cubic spinel structure were synthesized and investigated. The NPs size can be varied from about 5 to 50 nm by the final annealing of the precursor at different temperatures. The distribution of cations over the tetrahedral (A) and the octahedral [B] sites (Fe0.75 Ni0.25) [Ni0.75 Cr1.25] O4 was established from the magnetic and Mössbauer measurements. In large NPs, the magnetic structure at low temperatures is close to the collinear antiferromagnetic (AFM) structure of the Neel type; and the total magnetic moment Mtot of the ferrite coincides with the direction of the B-sublattice moment. Several size-dependent magnetic anomalies were revealed. Three types of magnetic ions present in the A- and B- sublattices cause the competition of AFM and FM exchange interactions resulting in the highly frustrated magnetic ordering and the occurrence of canted magnetic structure in the octahedral B-sublattice. The frustrated structure is very flexible and significantly subjected to temperature and applied field. It results in several magnetic anomalies observed, including the occurrence of magnetic compensation, abnormal behavior of ZFC and FC magnetization curves and hysteresis loops. It was shown that magnetic anomalies can be explained in terms of exchange coupling of "soft" and "hard" magnetic B- and A-sublattices. This effect in the (Fe0.75 Ni0.25) [Ni0.75Cr1.25] O4 NPs can be considered as an atomic-scale analog of a similar effect observed in two-phase exchange-coupled alloys developed for permanent magnets and for the perpendicular recoding media.

Magnetic exchange couplings from noncollinear perturbation theory: dinuclear CuII complexes.

PubMed

Phillips, Jordan J; Peralta, Juan E

2014-08-07

To benchmark the performance of a new method based on noncollinear coupled-perturbed density functional theory [J. Chem. Phys. 138, 174115 (2013)], we calculate the magnetic exchange couplings in a series of triply bridged ferromagnetic dinuclear Cu(II) complexes that have been recently synthesized [Phys. Chem. Chem. Phys. 15, 1966 (2013)]. We find that for any basis-set the couplings from our noncollinear coupled-perturbed methodology are practically identical to those of spin-projected energy-differences when a hybrid density functional approximation is employed. This demonstrates that our methodology properly recovers a Heisenberg description for these systems, and is robust in its predictive power of magnetic couplings. Furthermore, this indicates that the failure of density functional theory to capture the subtle variation of the exchange couplings in these complexes is not simply an artifact of broken-symmetry methods, but rather a fundamental weakness of current approximate density functionals for the description of magnetic couplings.

Spin-exchange-induced spin-orbit coupling in a superfluid mixture

NASA Astrophysics Data System (ADS)

Chen, Li; Zhu, Chuanzhou; Zhang, Yunbo; Pu, Han

2018-03-01

We investigate the ground-state properties of a dual-species spin-1/2 Bose-Einstein condensate. One of the species is subjected to a pair of Raman laser beams that induces spin-orbit (SO) coupling, whereas the other species is not coupled to the Raman laser. In certain limits, analytical results can be obtained. It is clearly shown that, through the interspecies spin-exchange interaction, the second species also exhibits SO coupling. This mixture system displays a very rich phase diagram, with many of the phases not present in an SO-coupled single-species condensate. Our work provides a way of creating SO coupling in atomic quantum gases, and opens up an avenue of research in SO-coupled superfluid mixtures. From a practical point of view, the spin-exchange-induced SO coupling may overcome the heating issue for certain atomic species when subjected to Raman beams.

Zero-field-cooled/field-cooled magnetization study of Dendrimer model

NASA Astrophysics Data System (ADS)

Arejdal, M.; Bahmad, L.; Benyoussef, A.

2017-01-01

Being motivated by Dendrimer model with mixed spins σ=3 and S=7/2, we investigated the magnetic nanoparticle system in this study. We analyzed and discussed the ground-state phase diagrams and the stable phases. Then, we elaborated and explained the magnetic properties of the system by using Monte Carlo Simulations (MCS) in the framework of the Ising model. In this way, we determined the blocking temperature, which is deduced through studying the partial-total magnetization and susceptibility as a function of the temperature, and we established the effects of both the exchange coupling interaction and the crystal field on the hysteresis loop.

Pulse propagation, dispersion, and energy in magnetic materials.

PubMed

Scalora, Michael; D'Aguanno, Giuseppe; Mattiucci, Nadia; Akozbek, Neset; Bloemer, Mark J; Centini, Marco; Sibilia, Concita; Bertolotti, Mario

2005-12-01

We discuss pulse propagation effects in generic, electrically and magnetically dispersive media that may display large material discontinuities, such as a surface boundary. Using the known basic constitutive relations between the fields, and an explicit Taylor expansion to describe the dielectric susceptibility and magnetic permeability, we derive expressions for energy density and energy dissipation rates, and equations of motion for the coupled electric and magnetic fields. We then solve the equations of motion in the presence of a single interface, and find that in addition to the now-established negative refraction process an energy exchange occurs between the electric and magnetic fields as the pulse traverses the boundary.

Interacting quantum dot coupled to a kondo spin: a universal Hamiltonian study.

PubMed

Rotter, Stefan; Türeci, Hakan E; Alhassid, Y; Stone, A Douglas

2008-04-25

We study a Kondo spin coupled to a mesoscopic interacting quantum dot that is described by the "universal Hamiltonian." The problem is solved numerically by diagonalizing the system Hamiltonian in a good-spin basis and analytically in the weak and strong Kondo coupling limits. The ferromagnetic exchange interaction within the dot leads to a stepwise increase of the ground-state spin (Stoner staircase), which is modified nontrivially by the Kondo interaction. We find that the spin-transition steps move to lower values of the exchange coupling for weak Kondo interaction, but shift back up for sufficiently strong Kondo coupling. The interplay between Kondo and ferromagnetic exchange correlations can be probed with experimentally tunable parameters.

Magnetic Exchange Coupling in Ferromagnetic/Superconducting/Ferromagnetic Multilayers

NASA Astrophysics Data System (ADS)

de Melo, C. A. R. Sa

2001-03-01

The possibility of magnetic exchange coupling between ferromagnets (F) separated by superconductor (S) spacers in F/S/F multilayers is analysed theoretically [1,2]. Ideal systems for the observation of magnetic coupling through superconductors are complex oxide multilayers consisting of Colossal Magneto-Resistance (CMR) Ferromagnets and High Critical Temperature Cuprate Superconductors. For this coupling to occur, three "prima facie" conditions need to be satisfied. First, an indirect exchange coupling between the ferromagnets must exist when the superconductor is in its normal state. Second, superconductivity must not be destroyed due to the proximity of ferromagnetic boundaries. Third, roughness of the F/S interfaces must be small. Under these conditions, when the superconductor is cooled below its critical temperature T_c, the magnetic coupling changes. The appearance of the superconducting gap introduces a new length scale (the coherence length of the superconductor) and modifies the temperature dependence of the indirect exchange coupling existent in the normal state. The magnetic coupling is oscillatory both above and below T_c, as well as strongly temperature-dependent. However at low temperatures the indirect exchange coupling decay length is controlled by the coherence length of the superconductor, while at temperatures close to and above Tc the magnetic coupling decay length is controlled by the thermal length. [I would like to thank the Georgia Institute of Technology, NSF (Grant No. DMR-9803111) and NATO (Grant No. CRG-972261) for financial support.] [1] C. A. R. Sa de Melo, Phys. Rev. Lett. 79, 1933 (1997). [2] C. A. R. Sa de Melo, Phys. Rev. B 62, 12303 (2000).

Measuring carbon and oxygen isotope signals of photosynthesis and respiration: first field results from a chamber system coupled to tunable diode laser spectrometers

NASA Astrophysics Data System (ADS)

Wingate, L.; Burlett, R.; Bosc, A.; Cross, A.; Devaux, M.; Grace, J.; Loustau, D.; Seibt, U.; Ogée, J.

2007-12-01

Studying the carbon and oxygen stable isotope signals from plants and soils can help us gain insight to mechanistic processes responsible for the net exchange of CO2 and water cycled between terrestrial ecosystems and the atmosphere. Chamber field measurements of component fluxes and their isotopic composition have been reported for a few ecosystems. These observations have revealed that isotopic signals for carbon and oxygen are dynamic over relatively short time scales (hrs and days) for both branches and soils (Seibt et al., 2006a; 2006b; Wingate et al., 2007), and not fully explained by currently available models (Seibt et al., 2006b; Wingate et al., 2007). Ecosystem isotope studies have been limited by flask sampling requirements in the past. To evaluate and refine our models of isotopic fractionation by plants and soil, we need high resolution continuous isotopic measurements over the growing season for different ecosystems. In this study, we coupled chambers with tunable diode laser spectroscopy techniques in the field to continuously capture the isotopic signals from the most important component fluxes contributing to the net ecosystem exchange of CO2 in a Pinus pinaster forest in south-west France. We obtained profiles of the carbon and oxygen isotope content of CO2 within and above the forest canopy. In addition, we measured branch photosynthetic 13C and 18O discrimination alongside the 13C and 18O isotopic composition of the branch, stem and soil respiration during a 6-month period in 2007. In this talk, we will present the first results from this field campaign. References Seibt, U., Wingate, L., Berry, J.A. and Lloyd, J. (2006a) Non steady state effects in diurnal 18O discrimination by Picea sitchensis branches in the field. Plant, Cell and Environment Vol 29, 928-939. Seibt, U., Wingate, L., Lloyd, J. and Berry, J.A. (2006b) Diurnally variable δ18O signatures of soil CO2 fluxes indicate carbonic anhydrase activity in a forest soil. JGR-Biogeosciences, Vol. 111, G04005, doi:10.1029/2006JG000177. Seibt, U., Wingate, L. and Berry, J.A. (2007) Nocturnal stomatal conductance effects on the δ18O of foliage gas exchange observed in two forest ecosystems. Tree Physiology, Vol. 27, 585-595. Wingate, L., Seibt, U., Moncrieff, J.B., Jarvis, P.G. and Lloyd, J. (2007) Variations in 13C discrimination during CO2 exchange by Picea sitchensis branches in the field. Plant, Cell and Environment doi: 10.1111/j.1365-3040.2007.01647.

ESTIMATING GASEOUS EXCHANGES BETWEEN THE ATMOSPHERE AND PLANTS USING A COUPLED BIOCHEMICAL DRY DEPOSITION MODEL

EPA Science Inventory

To study gaseous exchanges between the soil, biosphere and atmosphere, a biochemical model was coupled with the latest version of Meyers Multi-Layer Deposition Model. The biochemical model describes photosynthesis and respiration and their coupling with stomatal resistance for...

Influence of the Coulomb interaction on the exchange coupling in granular magnets.

PubMed

Udalov, O G; Beloborodov, I S

2017-04-20

We develop a theory of the exchange interaction between ferromagnetic (FM) metallic grains embedded into insulating matrix by taking into account the Coulomb blockade effects. For bulk ferromagnets separated by the insulating layer the exchange interaction strongly depends on the height and thickness of the tunneling barrier created by the insulator. We show that for FM grains embedded into insulating matrix the exchange coupling additionally depends on the dielectric properties of this matrix due to the Coulomb blockade effects. In particular, the FM coupling decreases with decreasing the dielectric permittivity of insulating matrix. We find that the change in the exchange interaction due to the Coulomb blockade effects can be a few tens of percent. Also, we study dependence of the intergrain exchange interaction on the grain size and other parameters of the system.

Search for Length Dependent Stable Structures of Polyglutamaine Proteins with Replica Exchange Molecular Dynamic

NASA Astrophysics Data System (ADS)

Kluber, Alexander; Hayre, Robert; Cox, Daniel

2012-02-01

Motivated by the need to find beta-structure aggregation nuclei for the polyQ diseases such as Huntington's, we have undertaken a search for length dependent structure in model polyglutamine proteins. We use the Onufriev-Bashford-Case (OBC) generalized Born implicit solvent GPU based AMBER11 molecular dynamics with the parm96 force field coupled with a replica exchange method to characterize monomeric strands of polyglutamine as a function of chain length and temperature. This force field and solvation method has been shown among other methods to accurately reproduce folded metastability in certain small peptides, and to yield accurately de novo folded structures in a millisecond time-scale protein. Using GPU molecular dynamics we can sample out into the microsecond range. Additionally, explicit solvent runs will be used to verify results from the implicit solvent runs. We will assess order using measures of secondary structure and hydrogen bond content.

Self-consistent hybrid functionals for solids: a fully-automated implementation

NASA Astrophysics Data System (ADS)

Erba, A.

2017-08-01

A fully-automated algorithm for the determination of the system-specific optimal fraction of exact exchange in self-consistent hybrid functionals of the density-functional-theory is illustrated, as implemented into the public Crystal program. The exchange fraction of this new class of functionals is self-consistently updated proportionally to the inverse of the dielectric response of the system within an iterative procedure (Skone et al 2014 Phys. Rev. B 89, 195112). Each iteration of the present scheme, in turn, implies convergence of a self-consistent-field (SCF) and a coupled-perturbed-Hartree-Fock/Kohn-Sham (CPHF/KS) procedure. The present implementation, beside improving the user-friendliness of self-consistent hybrids, exploits the unperturbed and electric-field perturbed density matrices from previous iterations as guesses for subsequent SCF and CPHF/KS iterations, which is documented to reduce the overall computational cost of the whole process by a factor of 2.

Determination of long-range scalar (1)H-(1)H coupling constants responsible for polarization transfer in SABRE.

PubMed

Eshuis, Nan; Aspers, Ruud L E G; van Weerdenburg, Bram J A; Feiters, Martin C; Rutjes, Floris P J T; Wijmenga, Sybren S; Tessari, Marco

2016-04-01

SABRE (Signal Amplification By Reversible Exchange) nuclear spin hyperpolarization method can provide strongly enhanced NMR signals as a result of the reversible association of small molecules with para-hydrogen (p-H2) at an iridium metal complex. The conversion of p-H2 singlet order to enhanced substrate proton magnetization within such complex is driven by the scalar coupling interactions between the p-H2 derived hydrides and substrate nuclear spins. In the present study these long-range homonuclear couplings are experimentally determined for several SABRE substrates using an NMR pulse sequence for coherent hyperpolarization transfer at high magnetic field. Pyridine and pyrazine derivatives appear to have a similar ∼1.2 Hz (4)J coupling to p-H2 derived hydrides for their ortho protons, and a much lower (5)J coupling for their meta protons. Interestingly, the (4)J hydride-substrate coupling for five-membered N-heterocyclic substrates is well below 1 Hz. Copyright © 2016 Elsevier Inc. All rights reserved.

Determination of long-range scalar 1H-1H coupling constants responsible for polarization transfer in SABRE

NASA Astrophysics Data System (ADS)

Eshuis, Nan; Aspers, Ruud L. E. G.; van Weerdenburg, Bram J. A.; Feiters, Martin C.; Rutjes, Floris P. J. T.; Wijmenga, Sybren S.; Tessari, Marco

2016-04-01

SABRE (Signal Amplification By Reversible Exchange) nuclear spin hyperpolarization method can provide strongly enhanced NMR signals as a result of the reversible association of small molecules with para-hydrogen (p-H2) at an iridium metal complex. The conversion of p-H2 singlet order to enhanced substrate proton magnetization within such complex is driven by the scalar coupling interactions between the p-H2 derived hydrides and substrate nuclear spins. In the present study these long-range homonuclear couplings are experimentally determined for several SABRE substrates using an NMR pulse sequence for coherent hyperpolarization transfer at high magnetic field. Pyridine and pyrazine derivatives appear to have a similar ∼1.2 Hz 4J coupling to p-H2 derived hydrides for their ortho protons, and a much lower 5J coupling for their meta protons. Interestingly, the 4J hydride-substrate coupling for five-membered N-heterocyclic substrates is well below 1 Hz.

Geodynamo Modeling of Core-Mantle Interactions

NASA Technical Reports Server (NTRS)

Kuang, Wei-Jia; Chao, Benjamin F.; Smith, David E. (Technical Monitor)

2001-01-01

Angular momentum exchange between the Earth's mantle and core influences the Earth's rotation on time scales of decades and longer, in particular in the length of day (LOD) which have been measured with progressively increasing accuracy for the last two centuries. There are four possible coupling mechanisms for transferring the axial angular momentum across the core-mantle boundary (CMB): viscous, magnetic, topography, and gravitational torques. Here we use our scalable, modularized, fully dynamic geodynamo model for the core to assess the importance of these torques. This numerical model, as an extension of the Kuang-Bloxham model that has successfully simulated the generation of the Earth's magnetic field, is used to obtain numerical results in various physical conditions in terms of specific parameterization consistent with the dynamical processes in the fluid outer core. The results show that depending on the electrical conductivity of the lower mantle and the amplitude of the boundary topography at CMB, both magnetic and topographic couplings can contribute significantly to the angular momentum exchange. This implies that the core-mantle interactions are far more complex than has been assumed and that there is unlikely a single dominant coupling mechanism for the observed decadal LOD variation.

Critical exponents and universal magnetic behavior of noncentrosymmetric Fe0.6Co0.4Si

NASA Astrophysics Data System (ADS)

Shanmukharao Samatham, S.; Suresh, K. G.

2018-05-01

The critical magnetic properties of a non-centrosymmetric B20 cubic helimagnet Fe0.6Co0.4Si are investigated using magnetization isotherms. It belongs to the 3D-Heisenberg universality class with short range magnetic coupling as inferred from the self-consistent critical exponents , , and in combination with exchange interaction . Itinerant magnetic nature of the compound is realized by the Rhodes–Wholfarth analysis. Field-induced weak first (parahelical) to second (parafield-polarized) order transition is reported to occur at low critical field due to the weak spin–orbit coupling arising from the weak Dzyaloshinksii–Moriya interactions. Our study suggests the distinct phenomenological magnetic structures for Fe-based cubic magnets (Fe1‑x Co x Si and FeGe) and MnSi which cause contrasting physical properties.

[Mo2(CN)11]:5- A detailed description of ligand-field spectra and magnetic properties by first-principles calculations.

PubMed

Hendrickx, Marc F A; Clima, S; Chibotaru, L F; Ceulemans, A

2005-10-06

An ab initio multiconfigurational approach has been used to calculate the ligand-field spectrum and magnetic properties of the title cyano-bridged dinuclear molybdenum complex. The rather large magnetic coupling parameter J for a single cyano bridge, as derived experimentally for this complex by susceptibility measurements, is confirmed to a high degree of accuracy by our CASPT2 calculations. Its electronic structure is rationalized in terms of spin-spin coupling between the two constituent hexacyano-monomolybdate complexes. An in-depth analysis on the basis of Anderson's kinetic exchange theory provides a qualitative picture of the calculated CASSCF antiferromagnetic ground-state eigenvector in the Mo dimer. Dynamic electron correlations as incorporated into our first-principles calculations by means of the CASPT2 method are essential to obtain quantitative agreement between theory and experiment.

Induced Ferromagnetism at BiFeO 3/YBa 2Cu 3O 7 Interfaces

DOE PAGES

Zhu, Jian-Xin; Wen, Xiao-Dong; Haraldsen, J. T.; ...

2014-06-20

We report that transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO 3/YBa 2Cu 3O 7 interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO 3, we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interactionmore » strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. Lastly, the emergence of interfacial ferromagnetism should have implications to electronic and transport properties.« less

Magnetization reversal in exchange biased Co/CoO probed with anisotropic magnetoresistance

NASA Astrophysics Data System (ADS)

Gredig, Thomas; Krivorotov, Ilya N.; Dahlberg, E. Dan

2002-05-01

The magnetization reversal in exchange coupled polycrystalline Co/CoO bilayers has been investigated as a function of CoO thickness using anisotropic magnetoresistance as a probe. The anisotropic magnetoresistance (AMR) was measured during the magnetization reversal and it was used to determine the orientation of the magnetization. For thin CoO layers large training effects were present; ergo the first hysteresis loop after field cooling was not the same as the second. The magnitude of the observed training was found to decrease with increasing CoO thickness. In the samples where substantial training was observed, the first magnetization reversal was dominated by nucleation of reversed domains. For the reversal from the antiparallel state back to the parallel direction, the AMR is consistent with a rotation process. In thicker CoO films where the training was less, the asymmetry was drastically reduced. A simple model that couples the antiferromagnetic grains to the ferromagnetic layer simulates qualitatively the observed magnetoresistance.

Exchangers man the pumps: Functional interplay between proton pumps and proton-coupled Ca(2+) exchangers

USDA-ARS?s Scientific Manuscript database

Tonoplast-localised proton-coupled Ca(2+) transporters encoded by cation/H(+) exchanger (CAX) genes play a critical role in sequestering Ca(2+) into the vacuole. These transporters may function in coordination with Ca(2+) release channels, to shape stimulus-induced cytosolic Ca(2+) elevations. Recen...

Neutrinoless double-β decay in effective field theory: The light-Majorana neutrino-exchange mechanism

NASA Astrophysics Data System (ADS)

Cirigliano, Vincenzo; Dekens, Wouter; Mereghetti, Emanuele; Walker-Loud, André

2018-06-01

We present the first chiral effective theory derivation of the neutrinoless double-β decay n n →p p potential induced by light Majorana neutrino exchange. The effective-field-theory framework has allowed us to identify and parametrize short- and long-range contributions previously missed in the literature. These contributions cannot be absorbed into parametrizations of the single-nucleon form factors. Starting from the quark and gluon level, we perform the matching onto chiral effective field theory and subsequently onto the nuclear potential. To derive the nuclear potential mediating neutrinoless double-β decay, the hard, soft, and potential neutrino modes must be integrated out. This is performed through next-to-next-to-leading order in the chiral power counting, in both the Weinberg and pionless schemes. At next-to-next-to-leading order, the amplitude receives additional contributions from the exchange of ultrasoft neutrinos, which can be expressed in terms of nuclear matrix elements of the weak current and excitation energies of the intermediate nucleus. These quantities also control the two-neutrino double-β decay amplitude. Finally, we outline strategies to determine the low-energy constants that appear in the potentials, by relating them to electromagnetic couplings and/or by matching to lattice QCD calculations.

Exchange coupled CoPd/TbCo magneto-optic storage films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gambino, R.J.; Ruf, R.R.; Rishi, R.

1989-09-01

Films of CoPd with weak perpendicular anisotropy are shown to exchange couple to square loop TbCo films on both the Tb and Co sides of compensation. The exchange is sensitive to reactive impurities at the interface and is broken under conditions that produce as little as one monolayer of paramagnetic compound. Even when the coupling at the interface is strong, the authors show that only a limited thickness of the CoPd layer is spin oriented perpendicular.

Modeling and calculation of RKKY exchange coupling to explain Ti-vacancy-induced ferromagnetism in Ta-doped TiO2

NASA Astrophysics Data System (ADS)

Majidi, Muhammad Aziz; Bupu, Annamaria; Fauzi, Angga Dito

2017-12-01

We present a theoretical study on Ti-vacancy-induced ferromagnetism in anatase TiO2. A recent experimental study has revealed room temperature ferromagnetism in Ta-doped anatase TiO2thin films (Rusydi et al., 2012) [7]. Ta doping assists the formation of Ti vacancies which then induce the formation of localized magnetic moments around the Ti vacancies. As neighboring Ti vacancies are a few unit cells apart, the ferromagnetic order is suspected to be mediated by itinerant electrons. We propose that such an electron-mediated ferromagnetism is driven by Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction. To examine our hypothesis, we construct a tight-binding based model Hamiltonian for the anatase TiO2 system. We calculate the RKKY exchange coupling constant of TiO2 as a function of distance between local magnetic moments at various temperatures. We model the system by taking only the layer containing a unit of TiO2, at which the Ti vacancy is believed to form, as our effective two-dimensional unit cell. Our model incorporates the Hubbard repulsive interactions between electrons occupying Ti d orbitals treated within mean-field approximation. The density of states profile resulting from the model captures the relevant electronic properties of TiO2, such as the energy gap of 3.4 eV and the n-type character, which may be a measure of the adequacy of the model. The calculated RKKY coupling constant shows that the ferromagnetic coupling extends up to 3-4 unit cells and enhances slightly as temperature is increased from 0 to 400 K. These results support our hypothesis that the ferromagnetism of this system is driven by RKKY mechanism.

Competing exchange interactions in multiferroic and ferrimagnetic CaBaCo 4 O 7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fishman, Randy Scott; Bordacs, S.; Kocsis, Vilmos

Competing exchange interactions can produce complex magnetic states together with spin-induced electric polarizations. With competing interactions on alternating triangular and kagome layers, the swedenborgite CaBaCo 4O 7 may have one of the largest measured spin-induced polarizations of ~1700 nC/cm 2 below its ferrimagnetic transition temperature at 70 K. Upon rotating our sample about c = [0,0,1] while the magnetic field is fixed along [1,0,0], the threefold splitting of the spin-wave frequencies indicates that our sample is hexagonally twinned. In addition, magnetization measurements then suggest that roughly 20% of the sample is in a domain with the a axis along [1,0,0]more » and that 80% of the sample is in one of two other domains with the a axis along either [-1/2,√3/2, 0] or [-1/2, -√3/2, 0] . Powder neutron-diffraction data, magnetization measurements, and terahertz (THz) absorption spectroscopy reveal that the complex spin order in each domain can be described as a triangular array of bitetrahedral c-axis chains ferrimagnetically coupled to each other in the ab plane. In conclusion, the electric-field dependence of bonds coupling those chains produces the large spin-induced polarization of CaBaCo 4O 7 .« less

Competing exchange interactions in multiferroic and ferrimagnetic CaBaCo 4 O 7

DOE PAGES

Fishman, Randy Scott; Bordacs, S.; Kocsis, Vilmos; ...

2017-01-23

Competing exchange interactions can produce complex magnetic states together with spin-induced electric polarizations. With competing interactions on alternating triangular and kagome layers, the swedenborgite CaBaCo 4O 7 may have one of the largest measured spin-induced polarizations of ~1700 nC/cm 2 below its ferrimagnetic transition temperature at 70 K. Upon rotating our sample about c = [0,0,1] while the magnetic field is fixed along [1,0,0], the threefold splitting of the spin-wave frequencies indicates that our sample is hexagonally twinned. In addition, magnetization measurements then suggest that roughly 20% of the sample is in a domain with the a axis along [1,0,0]more » and that 80% of the sample is in one of two other domains with the a axis along either [-1/2,√3/2, 0] or [-1/2, -√3/2, 0] . Powder neutron-diffraction data, magnetization measurements, and terahertz (THz) absorption spectroscopy reveal that the complex spin order in each domain can be described as a triangular array of bitetrahedral c-axis chains ferrimagnetically coupled to each other in the ab plane. In conclusion, the electric-field dependence of bonds coupling those chains produces the large spin-induced polarization of CaBaCo 4O 7 .« less

High Pressure EPR for Probing the Magnetic Anisotropy in Single Molecule Magnets

NASA Astrophysics Data System (ADS)

Bhaskaran, Lakshmi; Trociewitz, Bianca; Dubroca, Thierry; Hill, Stephen

Single-molecule magnets (SMM) are potential candidates for nanoscale magnetic information storage, and a platform for studying classical and quantum behaviors at the mesoscopic scale. Varying the structures of these molecules by chemical modification can give rise to changes in their magnetic properties. However, this approach can be unpredictable, leaving very little control via chemical synthesis. An alternate approach is to exert physical pressure. This convenient tool can be used to vary crystal packing, local coordination geometries, as well as inter-ion and intermolecular interactions without changing the chemical composition of a SMM. Moreover, pressure in combination with Electron Paramagnetic Resonance (EPR), can be employed to better understand the factors that control magnetic anisotropy, both at the single-ion level and in exchange-coupled molecules. Here we present a microwave cavity integrated with a diamond anvil cell with a pressure range up to 1.5 GPa. As an example we show results from single crystal high field EPR experiments performed on an exchange coupled system, [Fe8O2(OH)12(tacn)6] Br8.9H2O, better known as Fe8 with a giant spin of S =10. The obtained pressure-dependent results will be discussed. National High Magnetic Field Laboratory.

Hyperfine coupling constants of the nitrogen and phosphorus atoms: A challenge for exact-exchange density-functional and post-Hartree-Fock methods

NASA Astrophysics Data System (ADS)

Kaupp, Martin; Arbuznikov, Alexei V.; Heßelmann, Andreas; Görling, Andreas

2010-05-01

The isotropic hyperfine coupling constants of the free N(S4) and P(S4) atoms have been evaluated with high-level post-Hartree-Fock and density-functional methods. The phosphorus hyperfine coupling presents a significant challenge to both types of methods. With large basis sets, MP2 and coupled-cluster singles and doubles calculations give much too small values for the phosphorus atom. Triple excitations are needed in coupled-cluster calculations to achieve reasonable agreement with experiment. None of the standard density functionals reproduce even the correct sign of this hyperfine coupling. Similarly, the computed hyperfine couplings depend crucially on the self-consistent treatment in exact-exchange density-functional theory within the optimized effective potential (OEP) method. Well-balanced auxiliary and orbital basis sets are needed for basis-expansion exact-exchange-only OEP approaches to come close to Hartree-Fock or numerical OEP data. Results from the localized Hartree-Fock and Krieger-Li-Iafrate approximations deviate notably from exact OEP data in spite of very similar total energies. Of the functionals tested, only full exact-exchange methods augmented by a correlation functional gave at least the correct sign of the P(S4) hyperfine coupling but with too low absolute values. The subtle interplay between the spin-polarization contributions of the different core shells has been analyzed, and the influence of even very small changes in the exchange-correlation potential could be identified.

Magnetic field controlled electronic state and electric field controlled magnetic state in α-Fe1.6Ga0.4O3 oxide

NASA Astrophysics Data System (ADS)

Lone, Abdul Gaffar; Bhowmik, R. N.

2018-04-01

We have prepared α-Fe1.6Ga0.4O3 (Ga doped α-Fe2O3) system in rhombohedral phase. The material has shown room temperature ferroelectric and ferromagnetic properties. The existence of magneto-electric coupling at room temperature has been confirmed by the experimental observation of magnetic field controlled electric properties and electric field controlled magnetization. The current-voltage characteristics were controlled by external magnetic field. The magnetic state switching and exchange bias effect are highly sensitive to the polarity and ON and OFF modes of external electric field. Such materials can find novel applications in magneto-electronic devices, especially in the field of electric field controlled spintronics devices and energy storage devices which need low power consumption.

Overview of the CHarring Ablator Response (CHAR) Code

NASA Technical Reports Server (NTRS)

Amar, Adam J.; Oliver, A. Brandon; Kirk, Benjamin S.; Salazar, Giovanni; Droba, Justin

2016-01-01

An overview of the capabilities of the CHarring Ablator Response (CHAR) code is presented. CHAR is a one-, two-, and three-dimensional unstructured continuous Galerkin finite-element heat conduction and ablation solver with both direct and inverse modes. Additionally, CHAR includes a coupled linear thermoelastic solver for determination of internal stresses induced from the temperature field and surface loading. Background on the development process, governing equations, material models, discretization techniques, and numerical methods is provided. Special focus is put on the available boundary conditions including thermochemical ablation, surface-to-surface radiation exchange, and flowfield coupling. Finally, a discussion of ongoing development efforts is presented.

Thermodynamic model of a solid with RKKY interaction and magnetoelastic coupling

NASA Astrophysics Data System (ADS)

Balcerzak, T.; Szałowski, K.; Jaščur, M.

2018-04-01

Thermodynamic description of a model system with magnetoelastic coupling is presented. The elastic, vibrational, electronic and magnetic energy contributions are taken into account. The long-range RKKY interaction is considered together with the nearest-neighbour direct exchange. The generalized Gibbs potential and the set of equations of state are derived, from which all thermodynamic functions are self-consistently obtained. Thermodynamic properties are calculated numerically for FCC structure for arbitrary external pressure, magnetic field and temperature, and widely discussed. In particular, for some parameters of interaction potential and electron concentration corresponding to antiferromagnetic phase, the existence of negative thermal expansion coefficient is predicted.

Small scale changes of geochemistry and flow field due to transient heat storage in aquifers

NASA Astrophysics Data System (ADS)

Bauer, S.; Boockmeyer, A.; Li, D.; Beyer, C.

2013-12-01

Heat exchangers in the subsurface are increasingly installed for transient heat storage due to the need of heating or cooling of buildings as well as the interim storage of heat to compensate for the temporally fluctuating energy production by wind or solar energy. For heat storage to be efficient, high temperatures must be achieved in the subsurface. Significant temporal changes of the soil and groundwater temperatures however effect both the local flow field by temperature dependent fluid parameters as well as reactive mass transport through temperature dependent diffusion coefficients, geochemical reaction rates and mineral equilibria. As the use of heat storage will be concentrated in urban areas, the use of the subsurface for (drinking) water supply and heat storage will typically coincide and a reliable prognosis of the processes occurring is needed. In the present work, the effects of a temporal variation of the groundwater temperature, as induced by a local heat exchanger introduced into a groundwater aquifer, are studied. For this purpose, the coupled non-isothermal groundwater flow, heat transport and reactive mass transport is simulated in the near filed of such a heat exchanger. By explicitly discretizing and incorporating the borehole, the borehole cementation and the heat exchanger tubes, a realistic geometrical and process representation is obtained. The numerical simulation code OpenGeoSys is used in this work, which incorporates the required processes of coupled groundwater flow, heat and mass transport as well as temperature dependent geochemistry. Due to the use of a Finite Element Method, a close representation of the geometric effects can be achieved. Synthetic scenario simulations for typical settings of salt water formations in northern Germany are used to investigate the geochemical effects arising from a high temperature heat storage by quantifying changes in groundwater chemistry and overall reaction rates. This work presents the simulation approach used and results obtained for the synthetic scenarios. The model simulations show that locally in the direct vicinity of the borehole heat exchanger the flow field is changed, causing a ground water convergence and thus a mixing of water in the case of high temperatures. Also, geochemical reactions are induced due to shifting of temperature dependent mineral equilibria. Due to the moving groundwater, the changes are not reversible, and small impacts remain downstream of the borehole heat exchanger. However, the changes depend strongly on the mineral composition of the formation and the formation water present.

Non-Markovianity in the optimal control of an open quantum system described by hierarchical equations of motion

NASA Astrophysics Data System (ADS)

Mangaud, E.; Puthumpally-Joseph, R.; Sugny, D.; Meier, C.; Atabek, O.; Desouter-Lecomte, M.

2018-04-01

Optimal control theory is implemented with fully converged hierarchical equations of motion (HEOM) describing the time evolution of an open system density matrix strongly coupled to the bath in a spin-boson model. The populations of the two-level sub-system are taken as control objectives; namely, their revivals or exchange when switching off the field. We, in parallel, analyze how the optimal electric field consequently modifies the information back flow from the environment through different non-Markovian witnesses. Although the control field has a dipole interaction with the central sub-system only, its indirect influence on the bath collective mode dynamics is probed through HEOM auxiliary matrices, revealing a strong correlation between control and dissipation during a non-Markovian process. A heterojunction is taken as an illustrative example for modeling in a realistic way the two-level sub-system parameters and its spectral density function leading to a non-perturbative strong coupling regime with the bath. Although, due to strong system-bath couplings, control performances remain rather modest, the most important result is a noticeable increase of the non-Markovian bath response induced by the optimally driven processes.

A microscopic solution to the magnetic detwinning mystery in EuFe2As2

NASA Astrophysics Data System (ADS)

Maiwald, J.; Mazin, I. I.; Nandi, S.; Xiao, Y.; Gegenwart, P.

One of the greatest recent advances in studying nematic phenomena in Fe-based superconductors was the mechanical detwinning of the 122-family compounds. Unfortunately, these techniques generate considerable stress in the investigated samples, which contaminates the results. Recently, we observed that a minuscule magnetic field of the order of 0.1 T irreversibly and persistently detwins EuFe2As2, opening an entirely new avenue for addressing nematicity. However, further development was hindered by the absence of a microscopic theory explaining this magnetic detwinning. In fact, Eu2+ has zero orbital moment and does not couple to the lattice, and its exchange coupling with the Fe sublattice cancels by symmetry. Moreover, further increase of the field to 1 T leads to a reorientation of Fe domains, while even larger fields 10 T reorient the domains once again. We will present a new microscopic model, based on a sizable biquadratic coupling between the Fe 3 d and Eu 4 f moments. This model quantitatively explains our old and new magnetization and neutron diffraction data, thus removing the veil of mystery and finally opening the door to full-scale research into magnetic detwinning and nematicity in Fe-based superconductors.

Phase transition between quantum and classical regimes for the escape rate of dimeric molecular nanomagnets in a staggered magnetic field

NASA Astrophysics Data System (ADS)

Owerre, S. A.; Paranjape, M. B.

2014-04-01

We study the phase transition of the escape rate of exchange-coupled dimer of single-molecule magnets which are coupled either ferromagnetically or antiferromagnetically in a staggered magnetic field and an easy z-axis anisotropy. The Hamiltonian for this system has been used to study dimeric molecular nanomagnet [Mn4]2 which is comprised of two single molecule magnets coupled antiferromagnetically. We generalize the method of mapping a single-molecule magnetic spin problem onto a quantum-mechanical particle to dimeric molecular nanomagnets. The problem is mapped to a single particle quantum-mechanical Hamiltonian in terms of the relative coordinate and a coordinate dependent reduced mass. It is shown that the presence of the external staggered magnetic field creates a phase boundary separating the first- from the second-order transition. With the set of parameters used by R. Tiron et al. (2003) [25] and S. Hill et al. (2003) [20] to fit experimental data for [Mn4]2 dimer we find that the critical temperature at the phase boundary is T0(c)=0.29K. Therefore, thermally activated transitions should occur for temperatures greater than T0(c).

Effect of ferromagnetic exchange field on band gap and spin polarisation of graphene on a TMD substrate

NASA Astrophysics Data System (ADS)

Goswami, Partha

2018-03-01

We calculate the electronic band dispersion of graphene monolayer on a two-dimensional transition metal dichalcogenide substrate (GrTMD) around K and K^' } points by taking into account the interplay of the ferromagnetic impurities and the substrate-induced interactions. The latter are (strongly enhanced) intrinsic spin-orbit interaction (SOI), the extrinsic Rashba spin-orbit interaction (RSOI) and the one related to the transfer of the electronic charge from graphene to substrate. We introduce exchange field ( M) in the Hamiltonian to take into account the deposition of magnetic impurities on the graphene surface. The cavalcade of the perturbations yield particle-hole symmetric band dispersion with an effective Zeeman field due to the interplay of the substrate-induced interactions with RSOI as the prime player. Our graphical analysis with extremely low-lying states strongly suggests the following: The GrTMDs, such as graphene on WY2, exhibit (direct) band-gap narrowing / widening (Moss-Burstein (MB) gap shift) including the increase in spin polarisation ( P) at low temperature due to the increase in the exchange field ( M) at the Dirac points. The polarisation is found to be electric field tunable as well. Finally, there is anticrossing of non-parabolic bands with opposite spins, the gap closing with same spins, etc. around the Dirac points. A direct electric field control of magnetism at the nanoscale is needed here. The magnetic multiferroics, like BiFeO3 (BFO), are useful for this purpose due to the coupling between the magnetic and electric order parameters.

Organic flash cycles for efficient power production

DOEpatents

Ho, Tony; Mao, Samuel S.; Greif, Ralph

2016-03-15

This disclosure provides systems, methods, and apparatus related to an Organic Flash Cycle (OFC). In one aspect, a modified OFC system includes a pump, a heat exchanger, a flash evaporator, a high pressure turbine, a throttling valve, a mixer, a low pressure turbine, and a condenser. The heat exchanger is coupled to an outlet of the pump. The flash evaporator is coupled to an outlet of the heat exchanger. The high pressure turbine is coupled to a vapor outlet of the flash evaporator. The throttling valve is coupled to a liquid outlet of the flash evaporator. The mixer is coupled to an outlet of the throttling valve and to an outlet of the high pressure turbine. The low pressure turbine is coupled to an outlet of the mixer. The condenser is coupled to an outlet of the low pressure turbine and to an inlet of the pump.

Structural and magnetic characterization of Mn/NiFe bilayers with ion-beam-assisted deposition

NASA Astrophysics Data System (ADS)

Wu, Chun-Hsien; Zheng, Chao; Chiu, Chun-Cheng; Manna, Palash Kumar; van Lierop, Johan; Lin, Ko-Wei; Pong, Philip W. T.

2018-01-01

The exchange bias effect in ferromagnetic (FM)/antiferromagnetic (AF) bilayer structures has been widely investigated because its underlying principle is critical for spintronic applications. In this work, the effect of Ar+ beam bombardment on the microstructural and magnetic properties of the Mn/NiFe thin films was investigated. The in-situ Ar+ bombardment nontrivially promoted the Mn/NiFe intermixing and facilitated the formation of the FeMn phase, accompanied by a remarkable reduction of Mn and NiFe layer thickness. The enhanced Mn/NiFe intermixing greatly disordered the interfacial spins, inhibiting the interfacial exchange coupling and giving rise to a significant decrease of the exchange bias field (H ex). The facilitated Mn/NiFe intermixing effect also dramatically degraded the magnetocrystalline anisotropy of the NiFe crystallites, leading to a notable suppression of the coercivity (H c). These results indicate that both the exchange bias and coercivity of the Mn/NiFe bilayers can be directly affected by the in-situ Ar+ bombardment, offering an effective way to modify the magnetism of the exchange-bias systems.

Indirect Coupling of Magnetic Layers via Domain Wall Fringing fields

NASA Astrophysics Data System (ADS)

Parkin, Stuart

2001-03-01

Ferromagnetic films separated by thin metallic spacer layers are usually coupled through an indirect exchange interaction which oscillates in sign between ferro and antiferromagnetic coupling as a function of the spacer layer thickness^1. For both such metallic systems, and for multilayered systems in which the ferromagnetic films are separated by thin insulating layers, correlated roughness of the magnetic layers gives rise to a weak ferromagnetic coupling via dipole fields. Another type of dipolar coupling mechanism, which has largely been ignored, is that arising from domain wall fringing fields. These fields can be locally very large^2 and can result in the demagnetization of ferromagnetic films which are nominally highly coercive ("hard") in sandwiches comprised of "hard" and "soft" ferromagnetic layers. When the moment of the soft layer is reversed back and forth in small magnetic fields, much too small to affect the moment of the hard layer, substantial local fringing fields from domain walls created in the soft film gradually result in the demagnetization of the hard film. In some cases the moment of the hard layer decays in an oscillatory manner as it is successively partially demagnetized and remagnetized. This process has been observed on both macroscopic and microscopic length scales using SQUID magnetometry and high resolution photoemission electron microscopy, respectively^3. Magnetic interactions from domain wall fringing fields may be very important for magnetic devices, especially, magnetoresistance sensors and memory elements. [1] S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64, 2304 (1990); S.S.P. Parkin, Phys. Rev. Lett., 67, 3598 (1991). [2] L. Thomas, M. Samant and S.S.P. Parkin, Phys. Rev. Lett. 84, 1816 (2000). [3] L. Thomas, J Lüning, A. Scholl, F. Nolting, S. Anders, J. Stöhr and S.S.P. Parkin, Phys. Rev. Lett. 84, 3462 (2000).

Controllable positive exchange bias via redox-driven oxygen migration

DOE PAGES

Gilbert, Dustin A.; Olamit, Justin; Dumas, Randy K.; ...

2016-03-21

We report that ionic transport in metal/oxide heterostructures offers a highly effective means to tailor material properties via modification of the interfacial characteristics. However, direct observation of ionic motion under buried interfaces and demonstration of its correlation with physical properties has been challenging. Using the strong oxygen affinity of gadolinium, we design a model system of Gd xFe 1-x/NiCoO bilayer films, where the oxygen migration is observed and manifested in a controlled positive exchange bias over a relatively small cooling field range. The exchange bias characteristics are shown to be the result of an interfacial layer of elemental nickel andmore » cobalt, a few nanometres in thickness, whose moments are larger than expected from uncompensated NiCoO moments. This interface layer is attributed to a redox-driven oxygen migration from NiCoO to the gadolinium, during growth or soon after. Ultimately, these results demonstrate an effective path to tailoring the interfacial characteristics and interlayer exchange coupling in metal/oxide heterostructures.« less

Understanding temperature and magnetic-field actuated magnetization polarity reversal in the Prussian blue analogue Cu 0.73 Mn 0.77 [Fe(CN) 6 ]. z H 2 O, using XMCD

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lahiri, Debdutta; Choi, Yongseong; Yusuf, S. M.

2016-02-23

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)(6)]center dot zH(2)O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for themore » observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.« less

Interface roughness induced asymmetric magnetic property in sputter-deposited Co/CoO/Co exchange coupled trilayers

NASA Astrophysics Data System (ADS)

Wang, J.; Sannomiya, T.; Shi, J.; Nakamura, Y.

2012-04-01

The effect of interface roughness on magnetic properties of exchange coupled polycrystalline Co/CoO(tAF)/Co trilayers has been investigated by varying antiferromagnetic layer (CoO) thickness. It has been found that the upper CoO/Co interface becomes rougher with increasing CoO layer thickness, resulting in stronger exchange bias of the upper interface than the lower one. The interfacial exchange coupling is strengthened by the increase of defect-generated uncompensated antiferromagnetic spins; such spins form coupling with spins in the Co layer at the interface. As a result, the CoO layer thickness dependence of exchange bias is much enhanced for the upper Co layer. The transition from anisotropic magnetoresistance to isotropic magnetoresistance for the top Co layer has also been found. This could be attributed to the defects, probably partial thin oxide layers, between Co grains in the top Co layer that leads a switch from spin-orbit scattering related magnetoresistance to spin-dependent electron scattering dominated magnetoresistance.

Antiferromagnetic coupling and magnetoresistance enhancement in Co-Re metallic superlattices (abstract)

NASA Astrophysics Data System (ADS)

Freitas, P. P.; From, M.; Melo, L. V.; Ferreira, J.; Trindade, I.; Monteiro, P.

1991-11-01

Co-Re metallic superlattices were prepared that show antiferromagnetic exchange coupling and enhanced saturation magnetoresistance for particular values of the Re spacer thickness. We report studies on films with the structure glass /150 Å Re/[13 ÅCo/tRe]16/50 Å Re, with tRe ranging from 3 to 40 Å. These structures were grown by magnetron sputtering in a system with a base pressure of 1×10-7 Torr with deposition rates of 0.3 and 0.4 Å/s for Co and Re, respectively. x-ray diffractograms indicate the structure to be highly textured with the c axis perpendicular to the sample plane. The superlattice structure was obtained from high-angle θ-2θ scans. First-, second-, and third-order satellites are observed on both sides of the central [002] peak. Periodicity and bilayer composition are obtained from comparison of the data with a theoretical calculation of the x-ray diffractogram. Thickness calibration was confirmed by Rutherford backscattering and profilometer data. In-plane magnetization and magnetoresistance data (Δρ/ρ) indicate that stronger antiferromagnetic coupling and highest Δρ/ρ occur for tRe≊6 Å. The saturation field (Hs) needed to align contiguous antiferromagnetically coupled Co layers is about 1 T. This corresponds to an exchange coupling between the Co layer J≊-1 erg/cm2. (Δρ/ρ) reaches 2% in samples deposited at 170 °C. This data confirms results obtained by Parkin et al.1 in Co-Ru and Co-Cr superlattices.

Spin properties of charged Mn-doped quantum dota)

NASA Astrophysics Data System (ADS)

Besombes, L.; Léger, Y.; Maingault, L.; Mariette, H.

2007-04-01

The optical properties of individual quantum dots doped with a single Mn atom and charged with a single carrier are analyzed. The emission of the neutral, negatively and positively charged excitons coupled with a single magnetic atom (Mn) are observed in the same individual quantum dot. The spectrum of the charged excitons in interaction with the Mn atom shows a rich pattern attributed to a strong anisotropy of the hole-Mn exchange interaction slightly perturbed by a small valence-band mixing. The anisotropy in the exchange interaction between a single magnetic atom and a single hole is revealed by comparing the emission of a charged Mn-doped quantum dot in longitudinal and transverse magnetic field.

Spin valves with spin-engineered domain-biasing scheme

NASA Astrophysics Data System (ADS)

Lu, Z. Q.; Pan, G.

2003-06-01

Synthetic spin-filter spin valves with spin-engineered biasing scheme "sub/Ta/NiFe/IrMn/NiFe/NOL/Cu1/CoFe/Cu2/CoFe/Ru/CoFe/IrMn/Ta" were developed. In the structure, the orthogonal magnetic configuration for biasing and pinning field was obtained by one-step magnetic annealing process by means of spin flop, which eliminated the need for two antiferromagnetic materials with distinctively different blocking temperatures and two-step magnetic annealing as in conventional exchange biasing scheme. The longitudinal domain biasing of spin valves was achieved by using interlayer coupling field through Cu1 spacer. By adjusting the thickness of the Cu1 layer, the interlayer coupling biasing field can provide domain stabilization and was sufficiently strong to constrain the magnetization in coherent rotation. This can prevent Barkhausen noises associated with magnetization reversal. We report here a proof of concept study of such a domain-biasing scheme, which has its important technological applications in nanoscale spin valve and magnetic tunneling junction read heads and other spintronic devices.

Hybrid genetic algorithm with an adaptive penalty function for fitting multimodal experimental data: application to exchange-coupled non-Kramers binuclear iron active sites.

PubMed

Beaser, Eric; Schwartz, Jennifer K; Bell, Caleb B; Solomon, Edward I

2011-09-26

A Genetic Algorithm (GA) is a stochastic optimization technique based on the mechanisms of biological evolution. These algorithms have been successfully applied in many fields to solve a variety of complex nonlinear problems. While they have been used with some success in chemical problems such as fitting spectroscopic and kinetic data, many have avoided their use due to the unconstrained nature of the fitting process. In engineering, this problem is now being addressed through incorporation of adaptive penalty functions, but their transfer to other fields has been slow. This study updates the Nanakorrn Adaptive Penalty function theory, expanding its validity beyond maximization problems to minimization as well. The expanded theory, using a hybrid genetic algorithm with an adaptive penalty function, was applied to analyze variable temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopic data collected on exchange coupled Fe(II)Fe(II) enzyme active sites. The data obtained are described by a complex nonlinear multimodal solution space with at least 6 to 13 interdependent variables and are costly to search efficiently. The use of the hybrid GA is shown to improve the probability of detecting the global optimum. It also provides large gains in computational and user efficiency. This method allows a full search of a multimodal solution space, greatly improving the quality and confidence in the final solution obtained, and can be applied to other complex systems such as fitting of other spectroscopic or kinetics data.

Optimizing information flow in small genetic networks. IV. Spatial coupling

NASA Astrophysics Data System (ADS)

Sokolowski, Thomas R.; Tkačik, Gašper

2015-06-01

We typically think of cells as responding to external signals independently by regulating their gene expression levels, yet they often locally exchange information and coordinate. Can such spatial coupling be of benefit for conveying signals subject to gene regulatory noise? Here we extend our information-theoretic framework for gene regulation to spatially extended systems. As an example, we consider a lattice of nuclei responding to a concentration field of a transcriptional regulator (the input) by expressing a single diffusible target gene. When input concentrations are low, diffusive coupling markedly improves information transmission; optimal gene activation functions also systematically change. A qualitatively different regulatory strategy emerges where individual cells respond to the input in a nearly steplike fashion that is subsequently averaged out by strong diffusion. While motivated by early patterning events in the Drosophila embryo, our framework is generically applicable to spatially coupled stochastic gene expression models.

A density functional theory study of the magnetic exchange coupling in dinuclear manganese(II) inverse crown structures.

PubMed

Vélez, Ederley; Alberola, Antonio; Polo, Víctor

2009-12-17

The magnetic exchange coupling constants between two Mn(II) centers for a set of five inverse crown structures have been investigated by means of a methodology based on broken-symmetry unrestricted density functional theory. These novel and highly unstable compounds present superexchange interactions between two Mn centers, each one with S = 5/2 through anionic "guests" such as oxygen, benzene, or hydrides or through the cationic ring formed by amide ligands and alkali metals (Na, Li). Magnetic exchange couplings calculated at B3LYP/6-31G(d,p) level yield strong antiferromagnetic couplings for compounds linked via an oxygen atom or hydride and very small antiferromagnetic couplings for those linked via a benzene molecule, deprotonated in either 1,4- or 1,3- positions. Analysis of the magnetic orbitals and spin polarization maps provide an understanding of the exchange mechanism between the Mn centers. The dependence of J with respect to 10 different density functional theory potentials employed and the basis set has been analyzed.

Effects of Force Fields on Interface Dynamics, in view of Two-Phase Heat Transfer Enhancement and Phase Management for Space Applications

NASA Astrophysics Data System (ADS)

Di Marco, P.; Saccone, G.

2017-11-01

On earth, gravity barely influences the dynamics of interfaces. For what concerns bubbles, buoyancy governs the dynamics of boiling mechanism and thus affects boiling heat transfer capacity. While, for droplets, the coupled effects of wettability and gravity affects interface exchanges. In space, in the lack of gravity, rules are changed and new phenomena come into play. The present work is aimed to study the effects of electric field on the shape and behaviour of bubbles and droplets in order to understand how to handle microgravity applications; in particular, the replacement of gravity with electric field and their coupled effects are evaluated. The experiments spread over different setups, gravity conditions, working fluids, interface conditions. Droplets and bubbles have been analysed with and without electric field, with and without (adiabatic) heat and mass transfer across the interface. Furthermore, the results of the 4 ESA Parabolic Flight Campaigns (PFC 58, 60, 64 & 66), for adiabatic bubbles, adiabatic droplets and evaporating droplets, will be summarized, discussed, and compared with the ground tests.

Preliminary SP-100/Stirling Heat Exchanger Designs

NASA Astrophysics Data System (ADS)

Schmitz, Paul; Tower, Leonard; Dawson, Ronald; Blue, Brain; Dunn, Pat

1994-07-01

Analytic modeling of several heat exchanger concepts to couple the SP-100 nuclear reactor primary lithium loop and the Space Stirling Power Convertor(SSPC)was performed. Four 25 kWe SSPC's are used to produce the required 100 kW of electrical power. This design work focused on the interface between a single SSPC and the primary lithium loop. Manifolding to separate and collect the four channel flow was not modeled. This work modeled two separate types of heat exchanger interfaces (conductive coupling and radiative coupling) to explore their relative advantages and disadvantages. The minimum mass design of the conductively coupled concepts was 18 kg or 0.73 kg/kWe for a single 25 kWe convertor. The minimum mass radiatively coupled concept was 41 kg or 1.64 kg/kWe. The direct conduction heat exchanger provides a lighter weight system because of its ability to operate the Stirling convertor evaporator at higher heat fluxes than those attainable by the radiatively coupled systems. Additionally the conductively coupled concepts had relatively small volumes and provide potentially simpler assembly. Their disadvantages were the tight tolerances and material joining problems associated with this refractory to superalloy interface. The advantages of the radiatively coupled designs were the minimal material interface problems.

Preliminary SP-100/Stirling heat exchanger designs

NASA Astrophysics Data System (ADS)

Schmitz, Paul; Tower, Leonard; Dawson, Ronald; Blue, Brian; Dunn, Pat

1993-12-01

Analytic modeling of several heat exchanger concepts to couple the SP-100 nuclear reactor primary lithium loop and the Space Stirling Power Convertor (SSPC) was performed. Four 25 kWe SSPC's are used to produce the required 100 kW of electrical power. This design work focused on the interface between a single SSPC and the primary lithium loop. Manifolding to separate and collect the four channel flow was not modeled. This work modeled two separate types of heat exchanger interfaces (conductive coupling and radiative coupling) to explore their relative advantages and disadvantages. The minimum mass design of the conductively coupled concepts was 18 kg or 0.73 kg/kWe for a single 25 kWe convertor. The minimum mass radiatively coupled concept was 41 kg or 1.64 kg/kWe. The direct conduction heat exchanger provides a lighter weight system because of its ability to operate the Stirling convertor evaporator at higher heat fluxes than those attainable by the radiatively coupled systems. Additionally the conductively coupled concepts had relatively small volumes and provide potentially simpler assembly. Their disadvantages were the tight tolerances and material joining problems associated with this refractory to superalloy interface. The advantages of the radiatively coupled designs were the minimal material interface problems.

Preliminary SP-100/Stirling heat exchanger designs

NASA Technical Reports Server (NTRS)

Schmitz, Paul; Tower, Leonard; Dawson, Ronald; Blue, Brian; Dunn, Pat

1993-01-01

Analytic modeling of several heat exchanger concepts to couple the SP-100 nuclear reactor primary lithium loop and the Space Stirling Power Convertor (SSPC) was performed. Four 25 kWe SSPC's are used to produce the required 100 kW of electrical power. This design work focused on the interface between a single SSPC and the primary lithium loop. Manifolding to separate and collect the four channel flow was not modeled. This work modeled two separate types of heat exchanger interfaces (conductive coupling and radiative coupling) to explore their relative advantages and disadvantages. The minimum mass design of the conductively coupled concepts was 18 kg or 0.73 kg/kWe for a single 25 kWe convertor. The minimum mass radiatively coupled concept was 41 kg or 1.64 kg/kWe. The direct conduction heat exchanger provides a lighter weight system because of its ability to operate the Stirling convertor evaporator at higher heat fluxes than those attainable by the radiatively coupled systems. Additionally the conductively coupled concepts had relatively small volumes and provide potentially simpler assembly. Their disadvantages were the tight tolerances and material joining problems associated with this refractory to superalloy interface. The advantages of the radiatively coupled designs were the minimal material interface problems.

Validations of Coupled CSD/CFD and Particle Vortex Transport Method for Rotorcraft Applications: Hover, Transition, and High Speed Flights

NASA Technical Reports Server (NTRS)

Anusonti-Inthra, Phuriwat

2010-01-01

This paper presents validations of a novel rotorcraft analysis that coupled Computational Fluid Dynamics (CFD), Computational Structural Dynamics (CSD), and Particle Vortex Transport Method (PVTM) methodologies. The CSD with associated vehicle trim analysis is used to calculate blade deformations and trim parameters. The near body CFD analysis is employed to provide detailed near body flow field information which is used to obtain high-fidelity blade aerodynamic loadings. The far field wake dominated region is simulated using the PVTM analysis which provides accurate prediction of the evolution of the rotor wake released from the near body CFD domains. A loose coupling methodology between the CSD and CFD/PVTM modules are used with appropriate information exchange amongst the CSD/CFD/PVTM modules. The coupled CSD/CFD/PVTM methodology is used to simulate various rotorcraft flight conditions (i.e. hover, transition, and high speed flights), and the results are compared with several sets of experimental data. For the hover condition, the results are compared with hover data for the HART II rotor tested at DLR Institute of Flight Systems, Germany. For the forward flight conditions, the results are validated with the UH-60A flight test data.

CFD and Neutron codes coupling on a computational platform

NASA Astrophysics Data System (ADS)

Cerroni, D.; Da Vià, R.; Manservisi, S.; Menghini, F.; Scardovelli, R.

2017-01-01

In this work we investigate the thermal-hydraulics behavior of a PWR nuclear reactor core, evaluating the power generation distribution taking into account the local temperature field. The temperature field, evaluated using a self-developed CFD module, is exchanged with a neutron code, DONJON-DRAGON, which updates the macroscopic cross sections and evaluates the new neutron flux. From the updated neutron flux the new peak factor is evaluated and the new temperature field is computed. The exchange of data between the two codes is obtained thanks to their inclusion into the computational platform SALOME, an open-source tools developed by the collaborative project NURESAFE. The numerical libraries MEDmem, included into the SALOME platform, are used in this work, for the projection of computational fields from one problem to another. The two problems are driven by a common supervisor that can access to the computational fields of both systems, in every time step, the temperature field, is extracted from the CFD problem and set into the neutron problem. After this iteration the new power peak factor is projected back into the CFD problem and the new time step can be computed. Several computational examples, where both neutron and thermal-hydraulics quantities are parametrized, are finally reported in this work.

Training effect in specular spin valves

NASA Astrophysics Data System (ADS)

Ventura, J.; Araujo, J. P.; Sousa, J. B.; Veloso, A.; Freitas, P. P.

2008-05-01

Specular spin valves show an enhanced giant magnetoresistive (GMR) ratio due to specular reflection in nano-oxide layers (NOLs) formed by the partial oxidation of CoFe pinned and free layers. The oxides that form the (pinned layer) NOL were recently shown to antiferromagnetically order at Ttilde 175K . Here, we study the training effect (TE) in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves in the 300-15 K temperature range. The exchange bias direction between the MnIr and CoFe layers impressed during annealing is taken as the positive direction. The training effect is observed in antiferromagnetic (AFM)/ferromagnetic (FM) exchange systems and related to the rearrangement of interfacial AFM spin structure with the number of hysteretic cycles performed (n) , resulting in the decrease of the exchange field (Hexch) . Here, in the studied specular spin valve, TE was only observed for T<175K and is thus related to the pinned layer NOL-AFM ordering and to the evolution of the corresponding spin structure with n . We show that FM spins that are strongly coupled to AFM domains do not align with the applied positive magnetic field (H) , giving rise to a residual MR at H≫0 . Such nonsaturating MR will be related with a spin-glass-like behavior of the interfacial magnetism induced by the nano-oxide layer. The observed dependence of the training effect on the field cooling procedure is also likely associated with the existence of different spin configurations available in the magnetically disordered oxide. Furthermore, anomalous magnetoresistance cycles measured after cooling runs under -500Oe are here related to induced NOL exchange bias/applied magnetic field misalignment. The temperature dependence of the training effect was obtained and fitted by using a recent theoretical model.

Micromagnetic simulation of exchange coupled ferri-/ferromagnetic heterostructures

PubMed Central

Oezelt, Harald; Kovacs, Alexander; Reichel, Franz; Fischbacher, Johann; Bance, Simon; Gusenbauer, Markus; Schubert, Christian; Albrecht, Manfred; Schrefl, Thomas

2015-01-01

Exchange coupled ferri-/ferromagnetic heterostructures are a possible material composition for future magnetic storage and sensor applications. In order to understand the driving mechanisms in the demagnetization process, we perform micromagnetic simulations by employing the Landau–Lifshitz–Gilbert equation. The magnetization reversal is dominated by pinning events within the amorphous ferrimagnetic layer and at the interface between the ferrimagnetic and the ferromagnetic layer. The shape of the computed magnetization reversal loop corresponds well with experimental data, if a spatial variation of the exchange coupling across the ferri-/ferromagnetic interface is assumed. PMID:25937693

Experimental determination of exchange constants in antiferromagnetic Mn2Au

NASA Astrophysics Data System (ADS)

Sapozhnik, A. A.; Luo, C.; Ryll, H.; Radu, F.; Jourdan, M.; Zabel, H.; Elmers, Hans-Joachim

2018-05-01

Mn2Au is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high Néel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these parameters is further hampered in thin films by the unavoidable presence of uncompensated and quasiloose spins on antisites and at interfaces. Using x-ray magnetic circular dichroism (XMCD), we measured induced perpendicular spin and orbital moments for a Mn2Au (001) film in fields up to ±8 T. By performing these measurements at a low temperature of 7 K and at room temperature (RT), we were able to separate the loose spin contribution from the susceptibility of AF coupled spins. The value of the AF exchange constant obtained with this method for a 10-nm-thick Mn2Au (001) film is (22 ±5 )meV .

Spin polarization transfer by the radical pair mechanism

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zarea, Mehdi, E-mail: m-zarea@northwestern.edu; Ratner, Mark A.; Wasielewski, Michael R.

2015-08-07

In a three-site representation, we study a spin polarization transfer from radical pair spins to a nearby electron or nuclear spin. The quantum dynamics of the radical pair spins is governed by a constant exchange interaction between the radical pair spins which have different Zeeman frequencies. Radical pair spins can recombine to the singlet ground state or to lower energy triplet states. It is then shown that the coherent dynamics of the radical pair induces spin polarization on the nearby third spin in the presence of a magnetic field. The spin polarization transfer depends on the difference between Zeeman frequencies,more » the singlet and triplet recombination rates, and on the exchange and dipole-dipole interactions between the different spins. In particular, the sign of the polarization depends on the exchange coupling between radical pair spins and also on the difference between singlet and triplet recombination rate constants.« less

Effect of disorder on the magnetic and electronic structure of a prospective spin-gapless semiconductor MnCrVAl

DOE PAGES

Kharel, P.; Herran, J.; Lukashev, P.; ...

2016-12-19

Recent discovery of a new class of materials, spin-gapless semiconductors (SGS), has attracted considerable attention in the last few years, primarily due to potential applications in the emerging field of spin-based electronics (spintronics). Here, we investigate structural, electronic, and magnetic properties of one potential SGS compound, MnCrVAl, using various experimental and theoretical techniques. Our calculations show that this material exhibits ≈ 0.5 eV band gap for the majority-spin states, while for the minority-spin it is nearly gapless. The calculated magnetic moment for the completely ordered structure is 2.9 μB/f.u., which is different from our experimentally measured value of almost zero.more » Here, this discrepancy is explained by the structural disorder. In particular, A2 type disorder, where Mn or Cr atoms exchange their positions with Al atoms, results in induced antiferromagnetic exchange coupling, which, at a certain level of disorder, effectively reduces the total magnetic moment to zero. This is consistent with our x-ray diffraction measurements which indicate the presence of A2 disorder in all of our samples. In addition, we also show that B2 disorder does not result in antiferromagnetic exchange coupling and therefore does not significantly reduce the total magnetic moment.« less

Sequential magnetic switching in Fe/MgO(001) superlattices

NASA Astrophysics Data System (ADS)

Magnus, F.; Warnatz, T.; Palsson, G. K.; Devishvili, A.; Ukleev, V.; Palisaitis, J.; Persson, P. O. Å.; Hjörvarsson, B.

2018-05-01

Polarized neutron reflectometry is used to determine the sequence of magnetic switching in interlayer exchange coupled Fe/MgO(001) superlattices in an applied magnetic field. For 19.6 Å thick MgO layers we obtain a 90∘ periodic magnetic alignment between adjacent Fe layers at remanence. In an increasing applied field the top layer switches first followed by its second-nearest neighbor. For 16.4 Å MgO layers, a 180∘ periodic alignment is obtained at remanence and with increasing applied field the layer switching starts from the two outermost layers and proceeds inwards. This sequential tuneable switching opens up the possibility of designing three-dimensional magnetic structures with a predefined discrete switching sequence.

Strong Coupling and Entanglement of Quantum Emitters Embedded in a Nanoantenna-Enhanced Plasmonic Cavity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hensen, Matthias; Heilpern, Tal; Gray, Stephen K.

Establishing strong coupling between spatially separated and thus selectively addressable quantum emitters is a key ingredient to complex quantum optical schemes in future technologies. Insofar as many plasmonic nanostructures are concerned, however, the energy transfer and mutual interaction strength between distant quantum emitters can fail to provide strong coupling. Here, based on mode hybridization, the longevity and waveguide character of an elliptical plasmon cavity are combined with intense and highly localized field modes of suitably designed nanoantennas. Based on FDTD simulations a quantum emitter-plasmon coupling strength hg = 16.7 meV is reached while simultaneously keeping a small plasmon resonance linemore » width h gamma(s) = 33 meV. This facilitates strong coupling, and quantum dynamical simulations reveal an oscillatory exchange of excited state population arid a notable degree of entanglement between the quantum emitters spatially separated by 1.8 mu m, i.e., about twice the operating wavelength.« less

Ultrafast Study of Dynamic Exchange Coupling in Ferromagnet/Oxide/Semiconductor Heterostructures

NASA Astrophysics Data System (ADS)

Ou, Yu-Sheng

Spintronics is the area of research that aims at utilizing the quantum mechanical spin degree of freedom of electrons in solid-state materials for information processing and data storage application. Since the discovery of the giant magnetoresistance, the field of spintronics has attracted lots of attention for its numerous potential advantages over contemporary electronics, such as less power consumption, high integration density and non-volatility. The realization of a spin battery, defined by the ability to create spin current without associated charge current, has been a long-standing goal in the field of spintronics. The demonstration of pure spin current in ferromagnet/nonmagnetic material hybrid structures by ferromagnetic resonance spin pumping has defined a thrilling direction for this field. As such, this dissertation targets at exploring the spin and magnetization dynamics in ferromagnet/oxide/semiconductor heterostructures (Fe/MgO/GaAs) using time-resolved optical pump-probe spectroscopy with the long-range goal of understanding the fundamentals of FMR-driven spin pumping. Fe/GaAs heterostructures are complex systems that contain multiple spin species, including paramagnetic spins (GaAs electrons), nuclear spins (Ga and As nuclei) and ferromagnetic spins (Fe). Optical pump-probe studies on their interplay have revealed a number of novel phenomena that has not been explored before. As such they will be the major focus of this dissertation. First, I will discuss the effect of interfacial exchange coupling on the GaAs free-carrier spin relaxation. Temperature- and field-dependent spin-resolved pump-probe studies reveal a strong correlation of the electron spin relaxation with carrier freeze-out, in quantitative agreement with a theoretical interpretation that at low temperatures the free-carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. Second, we investigate the impact of tunnel barrier thickness on GaAs electron spin dynamics in Fe/MgO/GaAs heterostructures. Comparison of the Larmor frequency between samples with thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the spin lifetimes argues that inhomogeneity in the local hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs. Third, we investigated the origin and dynamics of an emergent spin population by pump power and magnetic field dependent spin-resolved pump-probe studies. Power dependent study confirms its origin to be filling of electronic states in GaAs, and further field dependent studies reveal the impact of contact hyperfine coupling on the dynamics of electron spins occupying distinct electronic states. Beyond above works, we also pursue optical detection of dynamic spin pumping in Fe/MgO/GaAs heterostructures in parallel. I will discuss the development and progress that we have made toward this goal. This project can be simply divided into two phases. In the first phase, we focused on microwave excitation and optical detection of spin pumping. In the second phase, we focused on all-optical excitation and detection of spin pumping. A number of measurement strategies have been developed and executed in both stages to hunt for a spin pumping signal. I will discuss the preliminary data based upon them.

Influence of super-horizon modes on correlation functions during inflation

NASA Astrophysics Data System (ADS)

Deutsch, Anne-Sylvie

2018-05-01

Coupling between sub- and super-Hubble modes can affect the locally observed statistics of our universe. In the context of Quasi-Single Field Inflation, we can compute correlation functions and derive the influence of those unobservable modes on observed correlation functions as well as on the inferred cosmological parameters. We study how different classes of diagrams affect the bispectrum in the squeezed limit; in particular, while contact-like diagrams leave the scaling between the long and short modes unchanged, exchange-like diagrams do modify the shape of the bispectrum. We show that the mass of the hidden sector field can hence be biased by an unavoidable cosmic variance that can reach a 1-σ uncertainty of Script O(10%) for a weakly non-Gaussian universe. Finally, we go beyond the bispectrum and show how couplings between unobservable and observable modes can affect generic correlation functions with arbitrary order non-derivative self-interactions.

Noncollinear magnetic ordering in a frustrated magnet: Metallic regime and the role of frustration

NASA Astrophysics Data System (ADS)

Shahzad, Munir; Sengupta, Pinaki

2017-12-01

We explore the magnetic phases in a Kondo lattice model on the geometrically frustrated Shastry-Sutherland lattice at metallic electron densities, searching for noncollinear and noncoplanar spin textures. Motivated by experimental observations in many rare-earth-based frustrated metallic magnets, we treat the local moments as classical spins and set the coupling between the itinerant electrons and local moments as the largest energy scale in the problem. Our results show that a noncollinear flux state is stabilized over an extended range of Hamiltonian parameters. These spin states can be quenched efficiently by external fields like temperature and magnetic field as well as by varying the degree of frustration in the electronic itinerancy and exchange coupling between local moments. Interestingly, unlike insulating electron densities that we discussed in paper I of this sequence, a Dzyaloshinskii-Moriya interaction between the local moments is not essential for the emergence of their noncollinear ordering.

Giant magnetoresistance in perpendicularly magnetized synthetic antiferromagnetic coupling with Ir spacer

NASA Astrophysics Data System (ADS)

Fukushima, A.; Taniguchi, T.; Sugihara, A.; Yakushiji, K.; Kubota, H.; Yuasa, S.

2018-05-01

Perpendicularly magnetized magnetic tunnel junction (p-MTJ) is a key element for developing high-density spin-transfer torque switching magnetoresistive random access memory. Recently, a large exchange coupling (IEC) in the synthetic antiferromagnetic reference layer with Ir interlayer was observed in p-MTJs. The evaluation of the IEC is, however, difficult due to the electrostatic breakdown of MTJs. This study demonstrates the evaluation of the IEC with Ir interlayer in giant magnetoresistive (GMR) nanopillar. We fabricated three kinds of perpendicularly magnetized GMR elements; bottom-free structures with Cu or Ir spacer, and top-free structure with Ir spacer. The magnetoresistance (RH) loops of all samples show sharp changes of the magnetoresistance at the magnetic fields over ±10 kOe, indicating the existence of the large IECs. In particular, a sharp change of the magnetoresistance at the field over ±20 kOe was found for the element with Cu of 2 nm thickness.

Magnetic field tunability of spin polarized excitations in a high temperature magnet

NASA Astrophysics Data System (ADS)

Holinsworth, Brian; Sims, Hunter; Cherian, Judy; Mazumdar, Dipanjan; Harms, Nathan; Chapman, Brandon; Gupta, Arun; McGill, Steve; Musfeldt, Janice

Magnetic semiconductors are at the heart of modern device physics because they naturally provide a non-zero magnetic moment below the ordering temperature, spin-dependent band gap, and spin polarization that originates from exchange-coupled magnetization or an applied field creating a spin-split band structure. Strongly correlated spinel ferrites are amongst the most noteworthy contenders for semiconductor spintronics. NiFe2O4, in particular, displays spin-filtering, linear magnetoresistance, and wide application in the microwave regime. To unravel the spin-charge interaction in NiFe2O4, we bring together magnetic circular dichroism, photoconductivity, and prior optical absorption with complementary first principles calculations. Analysis uncovers a metamagnetic transition modifying electronic structure in the minority channel below the majority channel gap, exchange splittings emerging from spin-split bands, anisotropy of excitons surrounding the indirect gap, and magnetic-field dependent photoconductivity. These findings open the door for the creation and control of spin-polarized excitations from minority channel charge charge transfer in NiFe2O4 and other members of the spinel ferrite family.

Antiferromagnetic layer thickness dependence of noncollinear uniaxial and unidirectional anisotropies in NiFe/FeMn/CoFe trilayers

NASA Astrophysics Data System (ADS)

Choi, Hyeok-Cheol; You, Chun-Yeol; Kim, Ki-Yeon; Lee, Jeong-Soo; Shim, Je-Ho; Kim, Dong-Hyun

2010-06-01

We have investigated the dependence of magnetic anisotropies of the exchange-biased NiFe/FeMn/CoFe trilayers on the antiferromagnetic (AF) layer thickness (tAF) by measuring in-plane angular-dependent ferromagnetic resonance fields. The resonance fields of NiFe and CoFe sublayers are shifted to lower and higher values compared to those of single unbiased ferromagnetic (F) layers, respectively, due to the interfacial exchange coupling when tAF≥2nm . In-plane angular dependence of resonance field reveals that uniaxial and unidirectional anisotropies coexist in the film plane, however, they are not collinear with each other. It is found that these peculiar noncollinear anisotropies significantly depend on tAF . The angle of misalignment displays a maximum around tAF=5nm and converges to zero when tAF is thicker than 10 nm. Contributions from thickness-dependent AF anisotropy and spin frustrations at both F/AF interfaces due to the structural imperfections should be accounted in order to understand the AF-layer thickness dependence of noncollinear magnetic anisotropies.

Magnetic field induced evolution of intertwined orders in the Kitaev magnet β -Li2IrO3

NASA Astrophysics Data System (ADS)

Rousochatzakis, Ioannis; Perkins, Natalia B.

2018-05-01

Recent scattering experiments in the 3D Kitaev magnet β -Li2IrO3 have shown that a relatively weak magnetic field along the crystallographic b axis drives the system from its incommensurate counter-rotating order to a correlated magnet, with a significant uniform `zigzag' component superimposing the magnetization along the field. Here it is shown that the zigzag order is not emerging from its linear coupling to the field (via a staggered, off-diagonal element of the g tensor) but from its intertwining with the incommensurate order and the longitudinal magnetization. The emerging picture explains all qualitative experimental findings at zero and finite fields, including the rapid decline of the incommensurate order with field and the so-called intensity sum rule. The latter are shown to be independent signatures of the smallness of the Heisenberg exchange J , compared to the Kitaev coupling K and the off-diagonal anisotropy Γ . Remarkably, in the regime of interest, the field H* at which the incommensurate component vanishes, depends essentially only on J , which allows us to extract an estimate of J ≃4 K from reported measurements of H*. We also comment on recent experiments in pressurized β -Li2IrO3 and conclude that J decreases with pressure.

Mo(V) co-ordination in the periplasmic nitrate reductase from Paracoccus pantotrophus probed by electron nuclear double resonance (ENDOR) spectroscopy.

PubMed Central

Butler, Clive S; Fairhurst, Shirley A; Ferguson, Stuart J; Thomson, Andrew J; Berks, Ben C; Richardson, David J; Lowe, David J

2002-01-01

The first electron nuclear double resonance (ENDOR) study of a member of the Mo-bis-molybdopterin guanine dinucleotide family of molybdoenzymes is presented, using the periplasmic nitrate reductase from Paracoccus pantotrophus. Rapid freeze-quenched time-resolved EPR revealed that during turnover the intensity of a Mo(V) EPR signal known as High-g [resting] increases. This signal is split by two interacting protons that are not solvent-exchangeable. X-band proton-ENDOR analysis resolved broad symmetrical resonance features that arose from four classes of protons weakly coupled to the Mo(V). Signals from two of these were lost upon exchange into deuterated buffer, suggesting that they may originate from OH(-) or H(2)O groups. One of these signals was also lost when the enzyme was redox-cycled in the presence of azide. Since these protons are very weakly coupled OH/H(2)O groups, they are not likely to be ligated directly to the Mo(V). This suggests that protonation of a Mo(VI)zO group does not occur on reduction to Mo(V), but most probably accompanies reduction of Mo(V) to Mo(IV). A resonance feature from a more strongly coupled proton, that was not lost following exchange into deuterated buffer, could also be resolved at 22-24 MHz. The anisotropy of this feature, determined from ENDOR spectra collected at a range of field positions, indicated a Mo-proton distance of approx. 3.2 A, consistent with this being one of the beta-methylene protons of a Mo-Cys ligand. PMID:11964184

Indirect NMR spin-spin coupling constants in diatomic alkali halides

NASA Astrophysics Data System (ADS)

Jaszuński, Michał; Antušek, Andrej; Demissie, Taye B.; Komorovsky, Stanislav; Repisky, Michal; Ruud, Kenneth

2016-12-01

We report the Nuclear Magnetic Resonance (NMR) spin-spin coupling constants for diatomic alkali halides MX, where M = Li, Na, K, Rb, or Cs and X = F, Cl, Br, or I. The coupling constants are determined by supplementing the non-relativistic coupled-cluster singles-and-doubles (CCSD) values with relativistic corrections evaluated at the four-component density-functional theory (DFT) level. These corrections are calculated as the differences between relativistic and non-relativistic values determined using the PBE0 functional with 50% exact-exchange admixture. The total coupling constants obtained in this approach are in much better agreement with experiment than the standard relativistic DFT values with 25% exact-exchange, and are also noticeably better than the relativistic PBE0 results obtained with 50% exact-exchange. Further improvement is achieved by adding rovibrational corrections, estimated using literature data.

Incorporating phosphorus cycling into global modeling efforts: a worthwhile, tractable endeavor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reed, Sasha C.; Yang, Xiaojuan; Thornton, Peter E.

2015-06-25

Myriad field, laboratory, and modeling studies show that nutrient availability plays a fundamental role in regulating CO 2 exchange between the Earth's biosphere and atmosphere, and in determining how carbon pools and fluxes respond to climatic change. Accordingly, global models that incorporate coupled climate-carbon cycle feedbacks made a significant advance with the introduction of a prognostic nitrogen cycle. Here we propose that incorporating phosphorus cycling represents an important next step in coupled climate-carbon cycling model development, particularly for lowland tropical forests where phosphorus availability is often presumed to limit primary production. We highlight challenges to including phosphorus in modeling effortsmore » and provide suggestions for how to move forward.« less

Incorporating phosphorus cycling into global modeling efforts: a worthwhile, tractable endeavor

USGS Publications Warehouse

Reed, Sasha C.; Yang, Xiaojuan; Thornton, Peter E.

2015-01-01

Myriad field, laboratory, and modeling studies show that nutrient availability plays a fundamental role in regulating CO2 exchange between the Earth's biosphere and atmosphere, and in determining how carbon pools and fluxes respond to climatic change. Accordingly, global models that incorporate coupled climate–carbon cycle feedbacks made a significant advance with the introduction of a prognostic nitrogen cycle. Here we propose that incorporating phosphorus cycling represents an important next step in coupled climate–carbon cycling model development, particularly for lowland tropical forests where phosphorus availability is often presumed to limit primary production. We highlight challenges to including phosphorus in modeling efforts and provide suggestions for how to move forward.

Two-Dimensional Jeff=1 /2 Antiferromagnetic Insulator Unraveled from Interlayer Exchange Coupling in Artificial Perovskite Iridate Superlattices

NASA Astrophysics Data System (ADS)

Hao, Lin; Meyers, D.; Frederick, Clayton; Fabbris, Gilberto; Yang, Junyi; Traynor, Nathan; Horak, Lukas; Kriegner, Dominik; Choi, Yongseong; Kim, Jong-Woo; Haskel, Daniel; Ryan, Phil J.; Dean, M. P. M.; Liu, Jian

2017-07-01

We report an experimental investigation of the two-dimensional Jeff=1 /2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO3)1 , (SrTiO3)m ] (m =1 , 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m =2 and m =3 samples than the m =1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Néel temperature from 150 K for m =1 to 40 K for m =2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO3 layer number m , for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the m =2 and the m =3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

Nuclear-driven electron spin rotations in a coupled silicon quantum dot and single donor system

NASA Astrophysics Data System (ADS)

Harvey-Collard, Patrick; Jacobson, Noah Tobias; Rudolph, Martin; Ten Eyck, Gregory A.; Wendt, Joel R.; Pluym, Tammy; Lilly, Michael P.; Pioro-Ladrière, Michel; Carroll, Malcolm S.

Single donors in silicon are very good qubits. However, a central challenge is to couple them to one another. To achieve this, many proposals rely on using a nearby quantum dot (QD) to mediate an interaction. In this work, we demonstrate the coherent coupling of electron spins between a single 31P donor and an enriched 28Si metal-oxide-semiconductor few-electron QD. We show that the electron-nuclear spin interaction can drive coherent rotations between singlet and triplet electron spin states. Moreover, we are able to tune electrically the exchange interaction between the QD and donor electrons. The combination of single-nucleus-driven rotations and voltage-tunable exchange provides all elements for future all-electrical control of a spin qubit, and requires only a single dot and no additional magnetic field gradients. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Engineering elliptical spin-excitations by complex anisotropy fields in Fe adatoms and dimers on Cu(111)

NASA Astrophysics Data System (ADS)

Guimarães, Filipe S. M.; dos Santos Dias, Manuel; Schweflinghaus, Benedikt; Lounis, Samir

2017-10-01

We investigate the dynamics of Fe adatoms and dimers deposited on the Cu(111) metallic surface in the presence of spin-orbit coupling, within time-dependent density functional theory. The ab initio results provide material-dependent parameters that can be used in semiclassical approaches, which are used for insightful interpretations of the excitation modes. By manipulating the surroundings of the magnetic elements, we show that elliptical precessional motion may be induced through the modification of the magnetic anisotropy energy. We also demonstrate how different kinds of spin precession are realized, considering the symmetry of the magnetic anisotropy energy, the ferro- or antiferromagnetic nature of the exchange coupling between the impurities, and the strength of the magnetic damping. In particular, the normal modes of a dimer depend on the initial magnetic configuration, changing drastically by going from a ferromagnetic metastable state to the antiferromagnetic ground state. By taking into account the effect of the damping into their resonant frequencies, we reveal that an important contribution arises for strongly biaxial systems and specially for the antiferromagnetic dimers with large exchange couplings. Counterintuitively, our results indicate that the magnetic damping influences the quantum fluctuations by decreasing the zero-point energy of the system.

Synthesis and controllable oxidation of monodisperse cobalt-doped wüstite nanoparticles and their core-shell stability and exchange-bias stabilization.

PubMed

Chen, Chih-Jung; Chiang, Ray-Kuang; Kamali, Saeed; Wang, Sue-Lein

2015-09-14

Cobalt-doped wüstite (CWT), Co0.33Fe0.67O, nanoparticles were prepared via the thermal decomposition of CoFe2-oleate complexes in organic solvents. A controllable oxidation process was then performed to obtain Co0.33Fe0.67O/CoFe2O4 core-shell structures with different core-to-shell volume ratios and exchange bias properties. The oxidized core-shell samples with a ∼4 nm CoFe2O4 shell showed good resistance to oxygen transmission. Thus, it is inferred that the cobalt ferrite shell provides a better oxidation barrier performance than magnetite in the un-doped case. The hysteresis loops of the oxidized 19 nm samples exhibited a high exchange bias field (H(E)), an enhanced coercivity field (H(C)), and a pronounced vertical shift, thus indicating the presence of a strong exchange bias coupling effect. More importantly, the onset temperature of H(E) was found to be higher than 200 K, which suggests that cobalt doping increases the Néel temperature (T(N)) of the CWT core. In general, the results show that the homogeneous dispersion of Co in iron precursors improves the stability of the final CWT nanoparticles. Moreover, the CoFe2O4 shells formed following oxidation increase the oxidation resistance of the CWT cores and enhance their anisotropy energy.

Proximity-induced mixed odd- and even-frequency pairing in monolayer NbSe2

NASA Astrophysics Data System (ADS)

Aliabad, Mojtaba Rahimi; Zare, Mohammad-Hossein

2018-06-01

Monolayer superconducting transition-metal dichalcogenide NbSe2 is a candidate for a nodal topological superconductor by magnetic field. Because of the so-called Ising spin-orbit coupling that strongly pins the electron spins to the out-of-plane direction, Cooper pairs in monolayer superconductor NbSe2 are protected against an applied in-plane magnetic field much larger than the Pauli limit. In monolayer NbSe2, in addition to the Fermi pockets at the corners of Brillouin zone with opposite crystal momentum similar to other semiconducting transition-metal dichalcogenids, there is an extra Fermi pocket around the Γ point with much smaller spin splitting, which could lead to an alternative strategy for pairing possibilities that are manipulable by a smaller magnetic field. By considering a monolayer NbSe2-ferromagnet substrate junction, we explore the modified pairing correlations on the pocket at Γ point in hole-doped monolayer NbSe2. The underlying physics is fascinating as there is a delicate interplay of the induced exchange field and the Ising spin-orbit coupling. We realize a mixed singlet-triplet superconductivity, s +f , due to the Ising spin-orbit coupling. Moreover, our results reveal the admixture state including both odd- and even-frequency components, associated with the ferromagnetic proximity effect. Different frequency symmetries of the induced pairing correlations can be realized by manipulating the magnitude and direction of the induced magnetization.

Light-matter interaction in the strong coupling regime: configurations, conditions, and applications.

PubMed

Dovzhenko, D S; Ryabchuk, S V; Rakovich, Yu P; Nabiev, I R

2018-02-22

Resonance interaction between a molecular transition and a confined electromagnetic field can reach the coupling regime where coherent exchange of energy between light and matter becomes reversible. In this case, two new hybrid states separated in energy are formed instead of independent eigenstates, which is known as Rabi splitting. This modification of the energy spectra of the system offers new possibilities for controlled impact on various fundamental properties of coupled matter (such as the rate of chemical reactions and the conductivity of organic semiconductors). To date, the strong coupling regime has been demonstrated in many configurations under different ambient conditions. However, there is still no comprehensive approach to determining parameters for achieving the strong coupling regime for a wide range of practical applications. In this review, a detailed analysis of various systems and corresponding conditions for reaching strong coupling is carried out and their advantages and disadvantages, as well as the prospects for application, are considered. The review also summarizes recent experiments in which the strong coupling regime has led to new interesting results, such as the possibility of collective strong coupling between X-rays and matter excitation in a periodic array of Fe isotopes, which extends the applications of quantum optics; a strong amplification of the Raman scattering signal from a coupled system, which can be used in surface-enhanced and tip-enhanced Raman spectroscopy; and more efficient second-harmonic generation from the low polaritonic state, which is promising for nonlinear optics. The results reviewed demonstrate great potential for further practical applications of strong coupling in the fields of photonics (low-threshold lasers), quantum communications (switches), and biophysics (molecular fingerprinting).

Complete magnetic field dependence of SABRE-derived polarization.

PubMed

Kiryutin, Alexey S; Yurkovskaya, Alexandra V; Zimmermann, Herbert; Vieth, Hans-Martin; Ivanov, Konstantin L

2018-07-01

Signal amplification by reversible exchange (SABRE) is a promising hyperpolarization technique, which makes use of spin-order transfer from parahydrogen (the H 2 molecule in its singlet spin state) to a to-be-polarized substrate in a transient organometallic complex, termed the SABRE complex. In this work, we present an experimental method for measuring the magnetic field dependence of the SABRE effect over an ultrawide field range, namely, from 10 nT to 10 T. This approach gives a way to determine the complete magnetic field dependence of SABRE-derived polarization. Here, we focus on SABRE polarization of spin-1/2 hetero-nuclei, such as 13 C and 15 N and measure their polarization in the entire accessible field range; experimental studies are supported by calculations of polarization. Features of the field dependence of polarization can be attributed to level anticrossings in the spin system of the SABRE complex. Features at magnetic fields of the order of 100 nT-1 μT correspond to "strong coupling" of protons and hetero-nuclei, whereas features found in the mT field range stem from "strong coupling" of the proton system. Our approach gives a way to measuring and analyzing the complete SABRE field dependence, to probing NMR parameters of SABRE complexes and to optimizing the polarization value. Copyright © 2017 John Wiley & Sons, Ltd.

The Influence of Different Mouthpieces on Impulse Oscillometry Results.

PubMed

Dionísio, Gustavo Henrique; Dos Santos, Daniele Oliveira; Perossi, Larissa; de Paula, Mayara Holtz; de Souza, Hugo Celso Dutra; Gastaldi, Ada Clarice

2018-05-01

Impulse oscillometry is a method of airway assessment and diagnosis that provides data on lung mechanics. In the literature, studies have used different types of mouthpieces or did not describe the model used for the tests. We sought to compare the 3 most commonly described mouthpieces in terms of test results, comfort, and subject preference. Thirty-nine healthy volunteers were evaluated with spirometry and impulse oscillometry, assessing the resistance at 5 Hz and 20 Hz (R5 and R20, respectively), reactance at 5 Hz (X5), reactance area, and resonant frequency. A filter heat exchanger with a circular mouthpiece (B1), a filter heat exchanger with an oval mouthpiece (B2), and a filter heat exchanger with a circular mouthpiece coupled with a free-flow piece (B3) were compared using an acceptability and tolerance scale, and subjects noted their preference. Statistical analysis showed differences between all the mouthpieces and the predicted values for R5, R20, and X5. The mouthpiece comparison showed differences in R5 between a filter heat exchanger with an oval mouthpiece (B2) and a circular mouthpiece coupled with a free-flow piece (B3) ( P = .007); resonant frequency between a filter heat exchanger with a circular mouthpiece (B1) and a filter heat exchanger with an oval mouthpiece (B2) ( P = .004) and between a filter heat exchanger with a circular mouthpiece (B1) and a circular mouthpiece coupled with a free-flow piece (B3) ( P = .003); and reactance area between a filter heat exchanger with a circular mouthpiece (B1) and a circular mouthpiece coupled with a free-flow piece (B3) ( P = .01). In the subjective evaluation, acceptability and tolerance differences were found in the ease of carrying out the evaluation, and no difference was found with regard to the degree of discomfort. Ten subjects preferred a filter heat exchanger with a circular mouthpiece (B1), 15 preferred a filter heat exchanger with an oval mouthpiece (B2), and 14 preferred a circular mouthpiece coupled with a free-flow piece (B3). A circular mouthpiece coupled with a free-flow piece (B3) appeared to be the most suitable mouthpiece for the impulse oscillometry tests. It assured smaller impedance values for the respiratory system, and subjects expressed the most confidence in using this mouthpiece. Copyright © 2018 by Daedalus Enterprises.

Blueprint for a coupled model of sedimentology, hydrology, and hydrogeology in streambeds

NASA Astrophysics Data System (ADS)

Partington, Daniel; Therrien, Rene; Simmons, Craig T.; Brunner, Philip

2017-06-01

The streambed constitutes the physical interface between the surface and the subsurface of a stream. Across all spatial scales, the physical properties of the streambed control surface water-groundwater interactions. Continuous alteration of streambed properties such as topography or hydraulic conductivity occurs through erosion and sedimentation processes. Recent studies from the fields of ecology, hydrogeology, and sedimentology provide field evidence that sedimentological processes themselves can be heavily influenced by surface water-groundwater interactions, giving rise to complex feedback mechanisms between sedimentology, hydrology, and hydrogeology. More explicitly, surface water-groundwater exchanges play a significant role in the deposition of fine sediments, which in turn modify the hydraulic properties of the streambed. We explore these feedback mechanisms and critically review the extent of current interaction between the different disciplines. We identify opportunities to improve current modeling practices. For example, hydrogeological models treat the streambed as a static rather than a dynamic entity, while sedimentological models do not account for critical catchment processes such as surface water-groundwater exchange. We propose a blueprint for a new modeling framework that bridges the conceptual gaps between sedimentology, hydrogeology, and hydrology. Specifically, this blueprint (1) fully integrates surface-subsurface flows with erosion, transport, and deposition of sediments and (2) accounts for the dynamic changes in surface elevation and hydraulic conductivity of the streambed. Finally, we discuss the opportunities for new research within the coupled framework.

Microscopic theory for coupled atomistic magnetization and lattice dynamics

NASA Astrophysics Data System (ADS)

Fransson, J.; Thonig, D.; Bessarab, P. F.; Bhattacharjee, S.; Hellsvik, J.; Nordström, L.

2017-12-01

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

3D Visualization of Global Ocean Circulation

NASA Astrophysics Data System (ADS)

Nelson, V. G.; Sharma, R.; Zhang, E.; Schmittner, A.; Jenny, B.

2015-12-01

Advanced 3D visualization techniques are seldom used to explore the dynamic behavior of ocean circulation. Streamlines are an effective method for visualization of flow, and they can be designed to clearly show the dynamic behavior of a fluidic system. We employ vector field editing and extraction software to examine the topology of velocity vector fields generated by a 3D global circulation model coupled to a one-layer atmosphere model simulating preindustrial and last glacial maximum (LGM) conditions. This results in a streamline-based visualization along multiple density isosurfaces on which we visualize points of vertical exchange and the distribution of properties such as temperature and biogeochemical tracers. Previous work involving this model examined the change in the energetics driving overturning circulation and mixing between simulations of LGM and preindustrial conditions. This visualization elucidates the relationship between locations of vertical exchange and mixing, as well as demonstrates the effects of circulation and mixing on the distribution of tracers such as carbon isotopes.

Infrared imaging - A validation technique for computational fluid dynamics codes used in STOVL applications

NASA Technical Reports Server (NTRS)

Hardman, R. R.; Mahan, J. R.; Smith, M. H.; Gelhausen, P. A.; Van Dalsem, W. R.

1991-01-01

The need for a validation technique for computational fluid dynamics (CFD) codes in STOVL applications has led to research efforts to apply infrared thermal imaging techniques to visualize gaseous flow fields. Specifically, a heated, free-jet test facility was constructed. The gaseous flow field of the jet exhaust was characterized using an infrared imaging technique in the 2 to 5.6 micron wavelength band as well as conventional pitot tube and thermocouple methods. These infrared images are compared to computer-generated images using the equations of radiative exchange based on the temperature distribution in the jet exhaust measured with the thermocouple traverses. Temperature and velocity measurement techniques, infrared imaging, and the computer model of the infrared imaging technique are presented and discussed. From the study, it is concluded that infrared imaging techniques coupled with the radiative exchange equations applied to CFD models are a valid method to qualitatively verify CFD codes used in STOVL applications.

Semiconducting double-dot exchange-only qubit dynamics in the presence of magnetic and charge noises

NASA Astrophysics Data System (ADS)

Ferraro, E.; Fanciulli, M.; De Michielis, M.

2018-06-01

The effects of magnetic and charge noises on the dynamical evolution of the double-dot exchange-only qubit (DEOQ) is theoretically investigated. The DEOQ consisting of three electrons arranged in an electrostatically defined double quantum dot deserves special interest in quantum computation applications. Its advantages are in terms of fabrication, control and manipulation in view of implementation of fast single and two-qubit operations through only electrical tuning. The presence of the environmental noise due to nuclear spins and charge traps, in addition to fluctuations in the applied magnetic field and charge fluctuations on the electrostatic gates adopted to confine the electrons, is taken into account including random magnetic field and random coupling terms in the Hamiltonian. The behavior of the return probability as a function of time for initial conditions of interest is presented. Moreover, through an envelope-fitting procedure on the return probabilities, coherence times are extracted when model parameters take values achievable experimentally in semiconducting devices.

Flexoelectro-optic effect and two-beam energy exchange in a hybrid photorefractive cholesteric cell with a short-pitch horizontal helix

NASA Astrophysics Data System (ADS)

Reshetnyak, V. Yu.; Pinkevych, I. P.; Evans, D. R.

2018-06-01

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive cholesteric cell with a short-pitch helix oriented parallel to the cell substrates (so-called uniformly lying helix configuration). Weak and strong light beams incident on the hybrid cell interfere and induce a periodic space-charge field in the photorefractive substrate of the cell, which penetrates into the cholesteric liquid crystal (LC). Due to the flexoelectro-optic effect an interaction of the photorefractive field with the LC flexopolarization causes the spatially periodic modulation of the helix axis in the plane parallel to the cell substrates. Coupling of a weak signal beam with a strong pump beam at the LC permittivity grating, induced by the periodically tilted helix axis, leads to the energy gain of the weak signal beam. Dependence of the signal beam gain coefficient on the parameters of the short-pitch cholesteric LC is studied.

Symmetric operation of the resonant exchange qubit

NASA Astrophysics Data System (ADS)

Malinowski, Filip K.; Martins, Frederico; Nissen, Peter D.; Fallahi, Saeed; Gardner, Geoffrey C.; Manfra, Michael J.; Marcus, Charles M.; Kuemmeth, Ferdinand

2017-07-01

We operate a resonant exchange qubit in a highly symmetric triple-dot configuration using IQ-modulated rf pulses. We find that the qubit splitting is an order of magnitude less sensitive to all relevant control voltages, compared to the conventional operating point, but we observe no significant improvement in the quality of Rabi oscillations. For weak driving this is consistent with Overhauser field fluctuations modulating the qubit splitting. For strong driving we infer that effective voltage noise modulates the coupling strength between rf drive and the qubit, thereby quickening Rabi decay. Application of CPMG dynamical decoupling sequences consisting of up to 32 π pulses significantly prolongs qubit coherence, leading to marginally longer dephasing times in the symmetric configuration. This is consistent with dynamical decoupling from low frequency noise, but quantitatively cannot be explained by effective gate voltage noise and Overhauser field fluctuations alone. Our results inform recent strategies for the utilization of symmetric configurations in the operation of triple-dot qubits.

Origin of the enhanced exchange bias in polycrystalline-BiFeO3/Co bilayers by X-ray absorption Spectroscopy

NASA Astrophysics Data System (ADS)

Shen, J. D.; Yang, W. B.; Kumar, A.; Zhao, H. H.; Lai, Y. J.; Feng, L. S.; Xu, Q. Y.; Zhang, Y. Q.; Du, J.; Li, Q.

2018-04-01

Polycrystalline-BiFeO3(BFO)/Co bilayers were grown by pulsed laser deposition (PLD) and magnetron sputtering, with fast laser annealing under magnetic field. The enhanced exchange bias (EB) had been found in the BFO/Co bilayers (Appl. Surf. Sci. 367 (2016) 418). In order to reveal the origin of the enhanced EB in the samples, X-ray absorption Spectroscopy (XAS) of Fe 2p, Co 2p and O 1s were performed. The Co 2p XAS indicated the increase of Co oxidation state and the Fe 2p XAS of sample A and B under laser annealing processes showed that crystal field splitting energy decreased and led to the weakening of spin-orbit coupling with the increasing of the laser fluence. It was considered that the appearance of the oxidation state of Co and Fe2+ ions and the existence of the unidirectional anisotropy due to the laser fluence was responsible for the results and also for the enhanced EB.

Exchange bias of Ni nanoparticles embedded in an antiferromagnetic IrMn matrix.

PubMed

Kuerbanjiang, Balati; Wiedwald, Ulf; Haering, Felix; Biskupek, Johannes; Kaiser, Ute; Ziemann, Paul; Herr, Ulrich

2013-11-15

The magnetic properties of Ni nanoparticles (Ni-NPs) embedded in an antiferromagnetic IrMn matrix were investigated. The Ni-NPs of 8.4 nm mean diameter were synthesized by inert gas aggregation. In a second processing step, the Ni-NPs were in situ embedded in IrMn films or SiOx films under ultrahigh vacuum (UHV) conditions. Findings showed that Ni-NPs embedded in IrMn have an exchange bias field HEB = 821 Oe at 10 K, and 50 Oe at 300 K. The extracted value of the exchange energy density is 0.06 mJ m(-2) at 10 K, which is in good accordance with the results from multilayered thin film systems. The Ni-NPs embedded in SiOx did not show exchange bias. As expected for this particle size, they are superparamagnetic at T = 300 K. A direct comparison of the Ni-NPs embedded in IrMn or SiOx reveals an increase of the blocking temperature from 210 K to around 400 K. The coercivity of the Ni-NPs exchange coupled to the IrMn matrix at 10 K is 8 times larger than the value for Ni-NPs embedded in SiOx. We studied time-dependent remanent magnetization at different temperatures. The relaxation behavior is described by a magnetic viscosity model which reflects a rather flat distribution of energy barriers. Furthermore, we investigated the effects of different field cooling processes on the magnetic properties of the embedded Ni-NPs. Exchange bias values fit to model calculations which correlate the contribution of the antiferromagnetic IrMn matrix to its grain size.

Interfacial Ferromagnetism and Exchange Bias in CaRuO3/CaMnO3 Superlattices

NASA Astrophysics Data System (ADS)

He, C.; Grutter, A. J.; Gu, M.; Browning, N. D.; Takamura, Y.; Kirby, B. J.; Borchers, J. A.; Kim, J. W.; Fitzsimmons, M. R.; Zhai, X.; Mehta, V. V.; Wong, F. J.; Suzuki, Y.

2012-11-01

We have found ferromagnetism in epitaxially grown superlattices of CaRuO3/CaMnO3 that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the CaMnO3 layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the CaMnO3 thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of CaMnO3 at each interface. The interfacial moment alternates between the 1μB/interface Mn ion for even CaMnO3 layers and the 0.5μB/interface Mn ion for odd CaMnO3 layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic CaMnO3 layers.

Effects of head field and AC field on magnetization reversal for microwave assisted magnetic recording

NASA Astrophysics Data System (ADS)

Kase, Aina; Akagi, Fumiko; Yoshida, Kazuetsu

2018-05-01

Microwave assisted magnetic recording (MAMR) is a promising recording method for achieving high recording densities in hard disk drives. In MAMR, the AC field from a spin-torque oscillator (STO) assists the head field with magnetization reversal in a medium. Therefore, the relationship between the head field and the AC field is very important. In this study, the effects of the head field and the AC field on magnetization reversal were analyzed using a micromagnetic simulator that takes the magnetic interactions between a single-pole type (SPT) write-head, an exchange coupled composite (ECC) medium, and the STO into account. As a result, the magnetization reversal was assisted not just by the y-component of the AC field (Hstoy) but also by the y-component of the head field (Hhy) in the medium. The Hhy over 100 kA/m with a frequency of about 15.5 GHz induced the magnetic resonance. The large Hhy was produced by the field from the STO to the SPT head.

Transition from an optical precursor in coupled-resonator-induced transparency to coherent energy exchange in Autler-Townes splitting

NASA Astrophysics Data System (ADS)

Oishi, Tohru; Suzuki, Ryuta; Talukder, Aminul I.; Tomita, Makoto

2013-08-01

We investigated the transient responses of coupled optical resonators, after they were injected with square modulated temporal pulses. A sharp spike, attributed to the optical precursor in coupled-resonator-induced transparency, appeared when the coupling between the resonators was weak. As the coupling strength increased, the resonance spectrum developed clearly separated double dips of Autler-Townes splitting, and the precursor spike transformed into an oscillatory structure. These temporal oscillations were attributed to the coherent energy exchange between two resonators. Theoretical calculations were in good agreement with the experimental observations.

Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on the CuN/Cu(100) surface

NASA Astrophysics Data System (ADS)

Žitko, R.; Pruschke, Th

2010-06-01

We study the effects of the exchange interaction between an adsorbed magnetic atom with easy-axis magnetic anisotropy and the conduction-band electrons from the substrate. We model the system using an anisotropic Kondo model and we compute the impurity spectral function, which is related to the differential conductance (dI/dV) spectra measured using a scanning tunneling microscope. To make contact with the known experimental results for iron atoms on the CuN/Cu(100) surface (Hirjibehedin et al 2007 Science 317 1199), we calculated the spectral functions in the presence of an external magnetic field of varying strength applied along all three spatial directions. It is possible to establish an upper bound on the coupling constant J: in the range of the magnetic fields for which the experimental results are currently known (up to 7 T), the low-energy features in the calculated spectra agree well with the measured dI/dV spectra if the exchange coupling constant J is at most half as large as that for cobalt atoms on the same surface. We show that for an even higher magnetic field (between 8 and 9 T) applied along the 'hollow direction', the impurity energy states cross, giving rise to a Kondo effect which takes the form of a zero-bias resonance. The coupling strength J could be determined experimentally by performing tunneling spectroscopy in this range of magnetic fields. On the technical side, the paper introduces an approach for calculating the expectation values of global spin operators and all the components of the impurity magnetic susceptibility tensor (including the out-of-diagonal ones) in numerical renormalization group (NRG) calculations with no spin symmetry. An appendix contains a density functional theory (DFT) study of the Co and Fe adsorbates on the CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies, occupancies, centers and spreads, by calculating the maximally localized Wannier orbitals of the adsorbates.

Effect of gauge-field interaction on fermion transport in two dimensions: Hartree conductivity correction and dephasing

NASA Astrophysics Data System (ADS)

Ludwig, T.; Gornyi, I. V.; Mirlin, A. D.; Wölfle, P.

2008-06-01

We consider the quantum corrections to the conductivity of fermions interacting via a Chern Simons gauge field and concentrate on the Hartree-type contributions. The first-order Hartree approximation is only valid in the limit of weak coupling λ≪g-1/2 to the gauge field ( g≫1 is the dimensionless conductance) and results in an antilocalizing conductivity correction ˜λ2gln2T . In the case of strong coupling, an infinite summation of higher-order terms is necessary, which includes both the virtual (renormalization of the frequency) and real (dephasing) processes. At intermediate temperatures, T0≪T≪gT0 , where T0˜1/g2τ and τ is the elastic scattering time, the T dependence of the conductivity is determined by the Hartree correction, δσH(T)-δσH(gT0)∝g1/2-(T/T0)1/2[1+ln(gT0/T)1/2] , so that σ(T) increases with lowering T . At low temperatures, T≪T0 , the temperature-dependent part of the Hartree correction assumes a logarithmic form with a coefficient of order unity, δσH∝ln(1/T) . As a result, the negative exchange contribution δσex∝-lngln(1/T) becomes dominant, which yields localization in the limit of T→0 . We further discuss dephasing at strong coupling and show that the dephasing rates are of the order of T , owing to the interplay of inelastic scattering and renormalization. On the other hand, the dephasing length is anomalously short, Lφ≪LT , where LT is the thermal length. For the case of composite fermions with long-range Coulomb interaction, the gauge-field propagator is less singular. The resulting Hartree correction has the usual sign and temperature dependence, δσH∝lngln(1/T) , and for realistic g is overcompensated by the negative exchange contribution due to the gauge-boson and scalar parts of the interaction. In this case, the dephasing length Lφ is of the order of LT for not too low temperatures and exceeds LT for T≲gT0 .

Tilt engineering of exchange coupling at G-type SrMnO3/(La,Sr)MnO3 interfaces

NASA Astrophysics Data System (ADS)

Li, F.; Song, C.; Wang, Y. Y.; Cui, B.; Mao, H. J.; Peng, J. J.; Li, S. N.; Wang, G. Y.; Pan, F.

2015-11-01

With the recent realization of hybrid improper ferroelectricity and room-temperature multiferroic by tilt engineering, “functional” octahedral tilting has become a novel concept in multifunctional perovskite oxides, showing great potential for property manipulation and device design. However, the control of magnetism by octahedral tilting has remained a challenging issue. Here a qualitative and quantitative tilt engineering of exchange coupling, one of the magnetic properties, is demonstrated at compensated G-type antiferromagnetic/ferromagnetic (SrMnO3/La2/3Sr1/3MnO3) interfaces. According to interfacial Hamiltonian, exchange bias (EB) in this system originates from an in-plane antiphase rotation (a-) in G-type antiferromagnetic layer. Based on first-principles calculation, tilt patterns in SrMnO3 are artificially designed in experiment with different epitaxial strain and a much stronger EB is attained in the tensile heterostructure than the compressive counterpart. By controlling the magnitude of octahedral tilting, the manipulation of exchange coupling is even performed in a quantitative manner, as expected in the theoretical estimation. This work realized the combination of tilt engineering and exchange coupling, which might be significant for the development of multifunctional materials and antiferromagnetic spintronics.

Unusual behavior of uranium dioxide at high magnetic fields. Part I

NASA Astrophysics Data System (ADS)

Gofryk, K.; Jaime, M.; Zapf, V.; Harrison, N.; Saul, A.; Radtke, G.; Lashley, J. C.; Salamon, M.; Andersson, A. D.; Stanek, C.; Durakiewicz, T.; Smith, J. L.

UO2 is a Mott-Hubbard insulator with well-localized 5 f-electrons and its crystal structure is the face-centered-cubic fluorite. It experiences a first-order antiferromagnetic phase transition at 30.8 K to a non-collinear antiferromagnetic structure that remains a topic of debate. It is believed that the first order nature of the transition results from the competition between the exchange interaction and the Jahn-Teller distortion of oxygen atoms. Despite extensive experimental and theoretical efforts the nature of the competing degrees of freedom and their couplings (such as spin-phonon coupling) are still unclear. Here we present results of our extensive thermodynamic investigations, on well-characterized and oriented single crystals of UO2, focusing on magnetization M(T,H) measurements in DC and pulsed magnetic fields to up 65 T at the NHMFL. Work supported by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences, and Engineering Division. The NHMFL Pulsed Field Facility is supported by the NSF, the U.S. D.O.E., and the State of Florida through NSF cooperative Grant DMR.

High-Nuclearity Magnetic Clusters: Generalized Spin Hamiltonian and Its Use for the Calculation of the Energy Levels, Bulk Magnetic Properties, and Inelastic Neutron Scattering Spectra.

PubMed

Borrás-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.; Tsukerblat, B. S.

1999-12-27

A general solution of the exchange problem in the high-nuclearity spin clusters (HNSC) containing arbitrary number of exchange-coupled centers and topology is developed. All constituent magnetic centers are supposed to possess well-isolated orbitally non-degenerate ground states so that the isotropic Heisenberg-Dirac-Van Vleck (HDVV) term is the leading part of the exchange spin Hamiltonian. Along with the HDVV term, we consider higher-order isotropic exchange terms (biquadratic exchange), as well as the anisotropic terms (anisotropic and antisymmetric exchange interactions and local single-ion anisotropies). All these terms are expressed as irreducible tensor operators (ITO). This allows us to take full advantage of the spin symmetry of the system. At the same time, we have also benefitted by taking into account the point group symmetry of the cluster, which allows us to work with symmetrized spin functions. This results in an additional reduction of the matrices to diagonalize. The approach developed here is accompanied by an efficient computational procedure that allows us to calculate the bulk magnetic properties (magnetic susceptibility, magnetization, and magnetic specific heat) as well as the spectroscopic properties of HNSC. Special attention is paid to calculate the magnetic excitations observed by inelastic neutron scattering (INS), their intensities, and their Q and temperature dependencies. This spectroscopic technique provides direct access to the energies and wave functions of the different spin states of the cluster; thus, it can be applied to spin clusters in order to obtain deep and detailed information on the nature of the magnetic exchange phenomenon. The general expression for the INS cross-section of spin clusters interacting by all kinds of exchange interactions, including also the single-ion zero-field splitting term, is derived for the first time. A closed-form expression is also derived for the particular case in which only the isotropic exchange interactions are involved. Finally this approach has been used to model the magnetic properties as well as the INS spectra of the polyoxometalate anion [Ni(9)(OH)(3)(H(2)O)(6)(HPO(4))(2)(PW(9)O(34))(3)](16)(-), which contains a central magnetic cluster formed by nine exchange-coupled Ni(II) ions surrounded by diamagnetic phosphotungstate ligands (PW(9)O(34))(9)(-).

Spin-orbit torque in a thin film of the topological insulator Bi2Se3: Crossover from the ballistic to diffusive regime

NASA Astrophysics Data System (ADS)

Ren, Y. J.; Deng, W. Y.; Geng, H.; Shen, R.; Shao, L. B.; Sheng, L.; Xing, D. Y.

2017-12-01

The spin-orbit torque provides an efficient method for switching the direction of a magnetization by using an electric field. Owing to the spin-orbit coupling, when an electric field is applied, a nonequilibrium spin density is generated, which exerts a torque on the local magnetization. Here, we investigate the spin-orbit torque in a thin film of topological insulator \\text{Bi}2\\text{Se}3 based upon a Boltzmann equation, with proper boundary conditions, which is applicable from the ballistic regime to the diffusive regime. It is shown that due to the spin-momentum interlocking of the electron surface states, the magnitude of the field-like torque is simply in linear proportion to the longitudinal electrical current. For a fixed electric field, the spin-orbit torque is proportional to the sample length in the ballistic limit, and saturates to a constant in the diffusive limit. The dependence of the torque on the magnetization direction and exchange coupling strength is also studied. Our theory may offer useful guidance for experimental investigations of the spin-orbit torque in finite-size systems.

Structural and magnetic investigations of single-crystalline neodymium zirconate pyrochlore Nd2Zr2O7

NASA Astrophysics Data System (ADS)

Hatnean, M. Ciomaga; Lees, M. R.; Petrenko, O. A.; Keeble, D. S.; Balakrishnan, G.; Gutmann, M. J.; Klekovkina, V. V.; Malkin, B. Z.

2015-05-01

We report structural and magnetic properties studies of large high-quality single crystals of the frustrated magnet Nd2Zr2O7 . Powder x-ray diffraction analysis confirms that Nd2Zr2O7 adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron-scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the <111 > axes of the Nd3 + ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below T ˜7 K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.

Electric Field Control of the Ferromagnetic CaRuO3 /CaMnO3 Interface

NASA Astrophysics Data System (ADS)

Grutter, Alexander; Kirby, Brian; Gray, Matthew; Flint, Charles; Suzuki, Yuri; Borchers, Julie

2015-03-01

Electric field control of magnetism has been recognized as one of the most important goals in nanoscale magnetics research. The most popular routes towards achieving magnetoelectric (ME) coupling have focused on heterostructures incorporating multiferroics or ferroelectrics. Such studies often rely on voltage induced distortion to induce strain in the magnetic film and alter the magnetic properties. However, successful attempts to induce ME coupling without multiferroicity or magnetoelasticity remain relatively rare. The ferromagnetic interface between the antiferromagnetic insulator CaMnO3 and the paramagnetic metal CaRuO3 is a promising candidate for direct magnetization control. This interfacial ferroagnetism is stabilized through the competition between interfacial double exchange and antiferromagnetic superexchange between adjacent Mn4+ so that the system is expected to be very sensitive to small changes in interfacial carrier density. Using polarized neutron reflectometry, we have probed the electric field dependence of the interfacial magnetization of CaRuO3/CaMnO3 bilayers deposited on SrTiO3. We find that electric fields of +/-8 kV/m are sufficient to switch the interfaces from largely ferromagnetic to completely antiferromagnetic.

The influence of a magnetic field on the heat transfer of a magnetic nanofluid in a sinusoidal channel

NASA Astrophysics Data System (ADS)

Valiallah Mousavi, S.; Barzegar Gerdroodbary, M.; Sheikholeslami, Mohsen; Ganji, D. D.

2016-09-01

In this study, two dimensional numerical simulations are performed to investigate the influence of the magnetic field on the nanofluid flow inside a sinusoidal channel. This work reveals the influence of variable magnetic field in the heat transfer of heat exchanger while the mixture is in a single phase. In this heat exchanger, the inner tube is sinusoidal and the outer tube is considered smooth. The magnetic field is applied orthogonal to the axis of the sinusoidal tube. In our study, the ferrofluid (water with 4 vol% nanoparticles (Fe3O4)) flows in a channel with sinusoidal bottom. The finite volume method with the SIMPLEC algorithm is used for handling the pressure-velocity coupling. The numerical results present validated data with experimentally measured data and show good agreement with measurement. The influence of different parameters, like the intensity of magnetic field and Reynolds number, on the heat transfer is investigated. According to the obtained results, the sinusoidal formation of the internal tube significantly increases the Nusselt number inside the channel. Our findings show that the magnetic field increases the probability of eddy formation inside the cavities and consequently enhances the heat transfer (more than 200%) in the vicinity of the magnetic field at low Reynolds number ( Re=50). In addition, the variation of the skin friction shows that the magnetic field increases the skin friction (more than 600%) inside the sinusoidal channel.

Magnetism of metallacrown single-molecule magnets: From a simplest model to realistic systems

NASA Astrophysics Data System (ADS)

Pavlyukh, Y.; Rentschler, E.; Elmers, H. J.; Hübner, W.; Lefkidis, G.

2018-06-01

Electronic and magnetic properties of molecular nanomagnets are determined by competing energy scales due to the crystal field splitting, the exchange interactions between transition metal atoms, and relativistic effects. We present a comprehensive theory embracing all these phenomena based on first-principles calculations. In order to achieve this goal, we start from the FeNi4 cluster as a paradigm. The system can be accurately described on the ab initio level yielding all expected electronic states in a range of multiplicities from 1 to 9, with a ferromagnetic ground state. By adding the spin-orbit coupling between them we obtain the zero-field splitting. This allows to introduce a spin Hamiltonian of a giant spin model, which operates on a smaller energy scale. We compare the computed parameters of this Hamiltonian with the experimental and theoretical magnetic anisotropy energies of the monolayer Ni/Cu(001). In line with them, we find that the anisotropy almost entirely originates from the second-order spin-orbit coupling, the spin-spin coupling constitutes only a small fraction. Finally, we include the ligand atoms in our consideration. This component has a decisive role for the stabilization of molecules in experimental synthesis and characterization, and also substantially complicates the theory by bringing the superexchange mechanisms into play. Since they are higher-order effects involving two hopping matrix elements, not every theory can describe them. Our generalization of the corresponding perturbation theory substantiates the use of complete active space methods for the description of superexchange. At the same time, our numerical results for the {CuFe4} system demonstrate that the Goodenough-Kanamori rules, which are often used to determine the sign of these exchange interactions, cannot deliver quantitative predictions due to the interplay of other mechanisms, e. g., involving multicenter Coulomb integrals. We conclude by comparing ab initio values of the exchange interaction constants for the {CuCu4} and {CuFe4} metallacrown magnetic molecules with experimental values determined by fitting of the magnetic susceptibility curves χMT (T ) , and attribute the remaining discrepancy between them to the role of virtual electron excitations into and out of the active space (dynamical correlations).

Two-Dimensional J eff = 1 / 2 Antiferromagnetic Insulator Unraveled from Interlayer Exchange Coupling in Artificial Perovskite Iridate Superlattices

DOE PAGES

Hao, Lin; Meyers, D.; Frederick, Clayton; ...

2017-07-14

We report an experimental investigation of the two-dimensional J eff=1/2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO 3) 1, (SrTiO 3) m] (m=1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m = 2 and m = 3 samples than the m = 1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO 3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. Themore » significant reduction of the Néel temperature from 150 K for m = 1 to 40 K for m = 2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO 3 layer number m, for maintaining canting-induced weak ferromagnetism. In conclusion, the nearly unaltered transition temperature between the m = 2 and the m = 3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.« less

Two-Dimensional J eff = 1 / 2 Antiferromagnetic Insulator Unraveled from Interlayer Exchange Coupling in Artificial Perovskite Iridate Superlattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hao, Lin; Meyers, D.; Frederick, Clayton

We report an experimental investigation of the two-dimensional J eff=1/2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO 3) 1, (SrTiO 3) m] (m=1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m = 2 and m = 3 samples than the m = 1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO 3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. Themore » significant reduction of the Néel temperature from 150 K for m = 1 to 40 K for m = 2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO 3 layer number m, for maintaining canting-induced weak ferromagnetism. In conclusion, the nearly unaltered transition temperature between the m = 2 and the m = 3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.« less

Magnetic self-assembly for the synthesis of magnetically exchange coupled MnBi/Fe–Co composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Xia; Hong, Yang-Ki, E-mail: ykhong@eng.ua.edu; Park, Jihoon

2015-11-15

Exchange coupled hard/soft MnBi/Fe–Co core/shell structured composites were synthesized using a magnetic self-assembly process. MnBi particles were prepared by arc-melting, and Fe–Co nanoparticles were synthesized by an oleic acid assisted chemical reduction method. Grinding a mixture of micron-sized MnBi and Fe–Co nanoparticles in hexane resulted in MnBi/Fe–Co core/shell structured composites. The MnBi/Fe–Co (95/5 wt%) composites showed smooth magnetic hysteresis loops, enhanced remanent magnetization, and positive values in the ΔM curve, indicating exchange coupling between MnBi and Fe–Co particles. - Graphical abstract: Both MnBi and Fe–Co particles were dispersed in hexane for grinding. Because of the oleic acid used during themore » Fe–Co nanoparticle synthesis, they could be well dispersed in hexane. During the grinding, the size of MnBi particles was decreased, hexane was evaporated, and the Fe–Co nanoparticles were concentrated in the solvent and magnetically attracted by MnBi particles, forming a core/shell structure. - Highlights: • Exchange coupled MnBi/Fe–Co composites are synthesized through magnetic selfassembly. • Magnetic exchange coupling is demonstrated by smooth magnetic hysteresis loops, enhanced remanent magnetization, and dominant positive peak in the ΔM curve. • The experimental results in magnetic properties are close to the theoretical calculation results.« less

Numerical Methodology for Coupled Time-Accurate Simulations of Primary and Secondary Flowpaths in Gas Turbines

NASA Technical Reports Server (NTRS)

Przekwas, A. J.; Athavale, M. M.; Hendricks, R. C.; Steinetz, B. M.

2006-01-01

Detailed information of the flow-fields in the secondary flowpaths and their interaction with the primary flows in gas turbine engines is necessary for successful designs with optimized secondary flow streams. Present work is focused on the development of a simulation methodology for coupled time-accurate solutions of the two flowpaths. The secondary flowstream is treated using SCISEAL, an unstructured adaptive Cartesian grid code developed for secondary flows and seals, while the mainpath flow is solved using TURBO, a density based code with capability of resolving rotor-stator interaction in multi-stage machines. An interface is being tested that links the two codes at the rim seal to allow data exchange between the two codes for parallel, coupled execution. A description of the coupling methodology and the current status of the interface development is presented. Representative steady-state solutions of the secondary flow in the UTRC HP Rig disc cavity are also presented.

Teleportation via thermally entangled states of a two-qubit Heisenberg XX chain

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yeo Ye

2002-12-01

Recently, entanglement teleportation has been investigated by Lee and Kim [Phys. Rev. Lett. 84, 4236 (2000)]. In this paper we study entanglement teleportation via two separate thermally entangled states of a two-qubit Heisenberg XX chain. We established the condition under which the parameters of the model have to satisfy in order to teleport entanglement. The necessary minimum amount of thermal entanglement for some fixed strength of exchange coupling is a function of the magnetic field and the temperature.

Biologically Assembled Quantum Electronic Arrays

DTIC Science & Technology

2013-06-07

characterizing the NP arrays. Theory of gate-tunable exchange coupling in the case of cobalt NP on graphene . Used Spin-density-functional theory and...polarization. We can estimate this field using the material parameters for Cobalt , which gives B neEo:N~ M;r; "󈧶 T zrv M M "’ m s s Here N1 is the...minority spin density of states at the Fermi surface for Cobalt , M5 is its saturation magnetization, while M:x is the x-component of the magnetization

Global analysis of the temperature and flow fields in samples heated in multizone resistance furnaces

NASA Astrophysics Data System (ADS)

Pérez-Grande, I.; Rivas, D.; de Pablo, V.

The temperature field in samples heated in multizone resistance furnaces will be analyzed, using a global model where the temperature fields in the sample, the furnace and the insulation are coupled; the input thermal data is the electric power supplied to the heaters. The radiation heat exchange between the sample and the furnace is formulated analytically, taking into account specular reflections at the sample; for the solid sample the reflectance is both diffuse and specular, and for the melt it is mostly specular. This behavior is modeled through the exchange view factors, which depend on whether the sample is solid or liquid, and, therefore, they are not known a priori. The effect of this specular behavior in the temperature field will be analyzed, by comparing with the case of diffuse samples. A parameter of great importance is the thermal conductivity of the insulation material; it will be shown that the temperature field depends strongly on it. A careful characterization of the insulation is therefore necessary, here it will be done with the aid of experimental results, which will also serve to validate the model. The heating process in the floating-zone technique in microgravity conditions will be simulated; parameters like the Marangoni number or the temperature gradient at the melt-crystal interface will be estimated. Application to the case of compound samples (graphite-silicon-graphite) will be made; the temperature distribution in the silicon part will be studied, especially the temperature difference between the two graphite rods that hold the silicon, since it drives the thermocapillary flow in the melt. This flow will be studied, after coupling the previous model with the convective effects. The possibility of suppresing this flow by the controlled vibration of the graphite rods will be also analyzed. Numerical results show that the thermocapillary flow can indeed be counterbalanced quite effectively.

Quantum impurity models for magnetic adsorbates on superconductor surfaces

NASA Astrophysics Data System (ADS)

Žitko, Rok

2018-05-01

Magnetic atoms adsorbed on surfaces have a quenched orbital moment while their ground-state spin multiplet is partially split as a consequence of the spin-orbit coupling which, even if intrinsically weak, has a large effect due to the abrupt change of the potential at the surface. Such metal adsorbates should be modelled using quantum impurity models that include the relevant internal degrees of freedom and the interaction terms, in particular the magnetic anisotropy and the Kondo exchange coupling. When adsorbed on superconducting surfaces, these impurities have complex spectra of sub-gap excitations due to magnetic anisotropy splitting and Kondo screening. Both anisotropy splitting and Zeeman splitting due to the external magnetic field are significantly renormalized by the coupling to the substrate electrons. In this work I discuss the quantum-to-classical crossover and the applicability of classical static-local-spin picture for discussing magnetic nanostructures on superconductors.

Universality from disorder in the random-bond Blume-Capel model

NASA Astrophysics Data System (ADS)

Fytas, N. G.; Zierenberg, J.; Theodorakis, P. E.; Weigel, M.; Janke, W.; Malakis, A.

2018-04-01

Using high-precision Monte Carlo simulations and finite-size scaling we study the effect of quenched disorder in the exchange couplings on the Blume-Capel model on the square lattice. The first-order transition for large crystal-field coupling is softened to become continuous, with a divergent correlation length. An analysis of the scaling of the correlation length as well as the susceptibility and specific heat reveals that it belongs to the universality class of the Ising model with additional logarithmic corrections which is also observed for the Ising model itself if coupled to weak disorder. While the leading scaling behavior of the disordered system is therefore identical between the second-order and first-order segments of the phase diagram of the pure model, the finite-size scaling in the ex-first-order regime is affected by strong transient effects with a crossover length scale L*≈32 for the chosen parameters.

PNR studies of spin-flop and spin-flip processes in magnetic multilayer, NiFeCo/Cu system

NASA Astrophysics Data System (ADS)

Ambaye, Hailemariam; Sato, Hideo; Mankey, Gary; Lauter, Valeria; Goyette, Richard

2010-03-01

Early GMR devices relied on antiferromagnetic interlayer coupling to work and it was shown that the interlayer coupling is in fact oscillatory, with both ferromagnetic and antiferromagnetic interlayer exchange depending on the thickness of the nonmagnetic layer [1,2]. Different competing interactions such as magnetic anisotropy and interlayer afm coupling occur in multilayer systems. Distinguishing the individual contributions is one of the major challenges in the study of multilayered systems. We used polarized neutron reflectivity with full polarization analysis to understand how the magnetization is distributed through the system and how deep the flipping process of the magnetization goes into the system. The easy axis field dependence of occurrence of spin-flop and spin-flip events in the system will be reported. [4pt] [1] S. S. P. Parkin, Phys. Rev. Lett. 71, 1641 (1993).[0pt] [2] D. Elefant, et al., Phys. Rev. B 77, 014426 (2008).

Classical-processing and quantum-processing signal separation methods for qubit uncoupling

NASA Astrophysics Data System (ADS)

Deville, Yannick; Deville, Alain

2012-12-01

The Blind Source Separation problem consists in estimating a set of unknown source signals from their measured combinations. It was only investigated in a non-quantum framework up to now. We propose its first quantum extensions. We thus introduce the Quantum Source Separation field, investigating both its blind and non-blind configurations. More precisely, we show how to retrieve individual quantum bits (qubits) only from the global state resulting from their undesired coupling. We consider cylindrical-symmetry Heisenberg coupling, which e.g. occurs when two electron spins interact through exchange. We first propose several qubit uncoupling methods which typically measure repeatedly the coupled quantum states resulting from individual qubits preparations, and which then statistically process the classical data provided by these measurements. Numerical tests prove the effectiveness of these methods. We then derive a combination of quantum gates for performing qubit uncoupling, thus avoiding repeated qubit preparations and irreversible measurements.

Engineering the Eigenstates of Coupled Spin-1/2 Atoms on a Surface.

PubMed

Yang, Kai; Bae, Yujeong; Paul, William; Natterer, Fabian D; Willke, Philip; Lado, Jose L; Ferrón, Alejandro; Choi, Taeyoung; Fernández-Rossier, Joaquín; Heinrich, Andreas J; Lutz, Christopher P

2017-12-01

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable, because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1/2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM) and subsequently perform electron spin resonance on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable the exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.

Engineering the Eigenstates of Coupled Spin-1 /2 Atoms on a Surface

NASA Astrophysics Data System (ADS)

Yang, Kai; Bae, Yujeong; Paul, William; Natterer, Fabian D.; Willke, Philip; Lado, Jose L.; Ferrón, Alejandro; Choi, Taeyoung; Fernández-Rossier, Joaquín; Heinrich, Andreas J.; Lutz, Christopher P.

2017-12-01

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1 /2 centers are particularly desirable, because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1 /2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM) and subsequently perform electron spin resonance on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable the exploration of quantum many-body systems based on networks of spin-1 /2 atoms on surfaces.

Coupled Kinetic-MHD Simulations of Divertor Heat Load with ELM Perturbations

NASA Astrophysics Data System (ADS)

Cummings, Julian; Chang, C. S.; Park, Gunyoung; Sugiyama, Linda; Pankin, Alexei; Klasky, Scott; Podhorszki, Norbert; Docan, Ciprian; Parashar, Manish

2010-11-01

The effect of Type-I ELM activity on divertor plate heat load is a key component of the DOE OFES Joint Research Target milestones for this year. In this talk, we present simulations of kinetic edge physics, ELM activity, and the associated divertor heat loads in which we couple the discrete guiding-center neoclassical transport code XGC0 with the nonlinear extended MHD code M3D using the End-to-end Framework for Fusion Integrated Simulations, or EFFIS. In these coupled simulations, the kinetic code and the MHD code run concurrently on the same massively parallel platform and periodic data exchanges are performed using a memory-to-memory coupling technology provided by EFFIS. The M3D code models the fast ELM event and sends frequent updates of the magnetic field perturbations and electrostatic potential to XGC0, which in turn tracks particle dynamics under the influence of these perturbations and collects divertor particle and energy flux statistics. We describe here how EFFIS technologies facilitate these coupled simulations and discuss results for DIII-D, NSTX and Alcator C-Mod tokamak discharges.

Fluorescence enhancement and strong-coupling in faceted plasmonic nanocavities

NASA Astrophysics Data System (ADS)

Kongsuwan, Nuttawut; Demetriadou, Angela; Chikkaraddy, Rohit; Baumberg, Jeremy J.; Hess, Ortwin

2018-06-01

Emission properties of a quantum emitter can be significantly modified inside nanometre-sized gaps between two plasmonic nanostructures. This forms a nanoscopic optical cavity which allows single-molecule detection and single-molecule strong-coupling at room temperature. However, plasmonic resonances of a plasmonic nanocavity are highly sensitive to the exact gap morphology. In this article, we shed light on the effect of gap morphology on the plasmonic resonances of a faceted nanoparticle-on-mirror (NPoM) nanocavity and their interaction with quantum emitters. We find that with increasing facet width the NPoM nanocavity provides weaker field enhancement and thus less coupling strength to a single quantum emitter since the effective mode volume increases with the facet width. However, if multiple emitters are present, a faceted NPoM nanocavity is capable of accommodating a larger number of emitters, and hence the overall coupling strength is larger due to the collective and coherent energy exchange from all the emitters. Our findings pave the way to more efficient designs of nanocavities for room-temperature light-matter strong-coupling, thus providing a big step forward to a non-cryogenic platform for quantum technologies.

Remote sensing of a coupled carbon-water-energy-radiation balances from the Globe to plot scales

NASA Astrophysics Data System (ADS)

Ryu, Y.; Jiang, C.; Huang, Y.; Kim, J.; Hwang, Y.; Kimm, H.; Kim, S.

2016-12-01

Advancements in near-surface and satellite remote sensing technologies have enabled us to monitor the global terrestrial ecosystems at multiple spatial and temporal scales. An emergent challenge is how to formulate a coupled water, carbon, energy, radiation, and nitrogen cycles from remote sensing. Here, we report Breathing Earth System Simulator (BESS), which coupled radiation (shortwave, longwave, PAR, diffuse PAR), carbon (gross primary productivity, ecosystem respiration, net ecosystem exchange), water (evaporation), and energy (latent and sensible heat) balances across the global land at 1 km resolution, 8 daily between 2000 and 2015 using multiple satellite remote sensing. The performance of BESS was tested against field observations (FLUXNET, BSRN) and other independent products (MPI-BGC, MODIS, GLASS). We found that the coupled model, BESS showed on par with, or better performance than the other products which computed land surface fluxes individually. Lastly, we show one plot-level study conducted in a paddy rice to demonstrate how to couple radiation, carbon, water, nitrogen balances with a series of near-surface spectral sensors.

One-electron versus electron-electron interaction contributions to the spin-spin coupling mechanism in nuclear magnetic resonance spectroscopy: Analysis of basic electronic effects

NASA Astrophysics Data System (ADS)

Gräfenstein, Jürgen; Cremer, Dieter

2004-12-01

For the first time, the nuclear magnetic resonance (NMR) spin-spin coupling mechanism is decomposed into one-electron and electron-electron interaction contributions to demonstrate that spin-information transport between different orbitals is not exclusively an electron-exchange phenomenon. This is done using coupled perturbed density-functional theory in conjunction with the recently developed J-OC-PSP [=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization)] method. One-orbital contributions comprise Ramsey response and self-exchange effects and the two-orbital contributions describe first-order delocalization and steric exchange. The two-orbital effects can be characterized as external orbital, echo, and spin transport contributions. A relationship of these electronic effects to zeroth-order orbital theory is demonstrated and their sign and magnitude predicted using simple models and graphical representations of first order orbitals. In the case of methane the two NMR spin-spin coupling constants result from totally different Fermi contact coupling mechanisms. 1J(C,H) is the result of the Ramsey response and the self-exchange of the bond orbital diminished by external first-order delocalization external one-orbital effects whereas 2J(H,H) spin-spin coupling is almost exclusively mitigated by a two-orbital steric exchange effect. From this analysis, a series of prediction can be made how geometrical deformations, electron lone pairs, and substituent effects lead to a change in the values of 1J(C,H) and 2J(H,H), respectively, for hydrocarbons.

Nonlinear beat excitation of low frequency wave in degenerate plasmas

NASA Astrophysics Data System (ADS)

Mir, Zahid; Shahid, M.; Jamil, M.; Rasheed, A.; Shahbaz, A.

2018-03-01

The beat phenomenon due to the coupling of two signals at slightly different frequencies that generates the low frequency signal is studied. The linear dispersive properties of the pump and sideband are analyzed. The modified nonlinear dispersion relation through the field coupling of linear modes against the beat frequency is derived in the homogeneous quantum dusty magnetoplasmas. The dispersion relation is used to derive the modified growth rate of three wave parametric instability. Moreover, significant quantum effects of electrons through the exchange-correlation potential, the Bohm potential, and the Fermi pressure evolved in macroscopic three wave interaction are presented. The analytical results are interpreted graphically describing the significance of the work. The applications of this study are pointed out at the end of introduction.

Synthesis and Characterization of Ferromagnetic/Antiferromagnetic Perovskite Oxide Superlattices

NASA Astrophysics Data System (ADS)

Jia, Yue

Perovskite oxides span a diverse range of functional properties such as ferromagnetism, superconductivity, and ferroelectricity, which makes them promising candidate materials for applications such as sensors, energy conversion and data storage devices. With recent advances in thin film deposition techniques, the precise manipulation of atomic layers on the unit cell level make it possible to synthesize epitaxial thin film heterostructures consisting of layers with different properties. The structural compatibility of perovskite oxides allows them to be epitaxially grown in complex heterostructures such as superlattices with a large density of interfaces where the interplay between spin, charge, orbital, and lattice degrees of freedom gives rise to new behaviors. The ferromagnetic (FM)/antiferromagnetic (AF) interface is particularly interesting due to exchange coupling which is not only of interest for fundamental research but also is of great significance for industrial applications. Unlike metallic systems that have been studied for decades with wide ranges of applications in devices such as hard disk drives, thin films of complex metal oxides is a relatively new field. Perovskite oxides show much more diverse functional properties than metals and open new pathways for tailoring propertiestowards specific device applications. Epitaxial La0.7Sr0.3MnO3 (LSMO)/La 0.7Sr0.3FeO3 (LSFO) superlattices serve as model systems to explore the magnetic structure and exchange coupling at perovskite oxide interfaces. Earlier work suggested that (001)-oriented LSMO/LSFO superlattices with compensated AF spins at the interface display spin-flop coupling characterized by perpendicular alignment between the AF spin axes and the FM moments at a sublayer thickness of 6 unit cells (u.c.). Changing the crystallographic orientation of the interface from (001) to (111) introduces changes to factors such as the charge density of each stacking layer, the magnetic iiistructure of the AF layer at the interface, the symmetry of the lattice, and the orbital degeneracy. Therefore, different properties and exchange coupling mechanisms are expected. (111)-oriented LSMO/LSFO superlattices with sublayer thicknesses ranging from 3 to 60 u.c. were synthesized and characterized. Detailed analysis of their structural, electronic, and magnetic properties were performed using synchrotron radiation based resonant x-ray reflectivity, soft x-ray magnetic spectroscopy, and photoemission electron microscopy to explore the effect of sublayer thickness on the magnetic structure and exchange coupling at (111)-oriented perovskite oxide interfaces. Interfacial effects and ultrathin superlattice sublayers can stabilize orientations of the LSFO AF spin axis which differ from that of LSFO films and LSMO/LSFO bilayers. In the ultrathin limit (3 to 6 u.c.), it was found that the AF properties of the LSFO sublayers are preserved with an out-of-plane canting of the AF spin axis, while the FM properties of the LSMO sublayers are significantly depressed. For thicker LSFO layers (> 9 u.c.), the out-of-plane canting of the AF spin axis is only present in superlattices with thick LSMO sublayers. As a result, exchange coupling in the form of spin-flop coupling exists only in superlattices which display both robust ferromagnetism and out-of-plane canting of the AF spin axis. A portion of the AF moments can be reoriented by a moderate external magnetic field through spin-flop coupling with the FM LSMO sublayers that have low magnetocrystalline anisotropy in the (111) plane. The AF order in the spin-flop coupled superlattices was studied using angle-dependent x-ray magnetic linear dichroism. The AF order can be categorized into two types: majority of the AF moments cant out-of-the-plane of the film along the or directions depending on the LSFO layer thickness, while a minority portion lies within the (111) plane in different AF domains. The energy difference between domains with their spin axes along the in-plane or out-of-plane directions is small, and the magnetic order of AF thin films is far ivmore complex than in bulk LSFO. The complex AF structure in these (111)-oriented LSMO/LSFO superlattices illustrates that complex metal oxide heterostructures can serve as fertile ground for discovery of new magnetic phases, which have potential applications in next generation information technology devices.

Realistic Modeling of Interaction of Quiet-Sun Magnetic Fields with the Chromosphere

NASA Technical Reports Server (NTRS)

Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray, A. A.

2017-01-01

High­-resolution observations and 3D MHD simulations reveal intense interaction between the convection zone dynamics and the solar atmosphere on subarcsecond scales. To investigate processes of the dynamical coupling and energy exchange between the subsurface layers and the chromosphere we perform 3D radiative MHD modeling for a computational domain that includes the upper convection zone and the chromosphere, and investigate the structure and dynamics for different intensity of the photospheric magnetic flux. For comparison with observations, the simulation models have been used to calculate synthetic Stokes profiles of various spectral lines. The results show intense energy exchange through small­-scale magnetized vortex tubes rooted below the photosphere, which provide extra heating of the chromosphere, initiate shock waves, and small­-scale eruptions.

Superconductivity in 2D and nearly 2D: A Conserving Description

NASA Astrophysics Data System (ADS)

Deisz, John; Hess, Daryl; Serene, Joe

1998-03-01

In a previous work,(J.J. Deisz, D.W. Hess, and J.W. Serene, Phys. Rev. Lett., to appear.) we used a 2D Hubbard model with an attractive interaction to explicitly show that a superconducting state in the fluctuation exchange approximation (FEA) could be detected from self-consistent calculations of the internal energy and free energy as a function of a threaded flux. The FEA is a conserving approximation beyond mean field theory that includes the exchange of Cooper pair, density, and spin fluctuations. Here, we present extensions of our previous calculations and show a phase diagram as a function of interaction strength and density. We discuss the nature of the FEA phase transition in 2D and focus on how it changes with increasing coupling between planes.

Unexpected magnetism, and transport properties in mixed lanthanide compound

NASA Astrophysics Data System (ADS)

Pathak, Arjun; Gschneidner, Karl, Jr.; Pecharsky, Vitalij; Ames Laboratory Team

For intelligent materials design it is desirable to have compounds which have multiple functionalities such as a large magnetoresistance, ferromagnetic and ferrimagnetic states, and field-induced first-order metamagnetic transitions. Here, we discuss one such example where we have combined two lanthanide elements Pr and Er in Pr0.6Er0.4Al2. This compound exhibits multiple functionalities in magnetic fields between 1 and 40 kOe. It undergoes only a trivial ferrimagnetism to paramagnetism transition in a zero magnetic field, but Pr0.6Er0.4Al2 exhibits a large positive magnetoresistance (MR) for H >=40 kOe, a small but non negligible negative MR for H <=30 kOe, and a clear Griffiths-like phase behavior at <1 kOe. The compound also exhibits an asymmetry of hysteresis loop, or exchange bias (EB) effect after field cooling from the paramagnetic state. These phenomena are attributed to the competition between single-ion anisotropies of Pr and Er ions coupled with the opposite nearest-neighbor and next-nearest-neighbor exchange interactions. This work was supported by the US Department of Energy, Office of Basic Energy Science, Division of Material Sciences and Engineering. The research was performed at the Ames Laboratory. The Ames Laboratory is operated by Iowa State University for the US D.

Cycloheptatrienyl trianion: an elusive bridge in the search of exchange coupled dinuclear organolanthanide single-molecule magnets† †Electronic supplementary information (ESI) available: Full synthetic details, crystallography, magnetic properties measurements, and ab initio details. CCDC 1454168–1454171. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc01224h Click here for additional data file. Click here for additional data file.

PubMed Central

Harriman, Katie L. M.; Le Roy, Jennifer J.; Holmberg, Rebecca J.; Korobkov, Ilia

2017-01-01

The preparation of η-cyclopentadienyl (η5-C5R5), η-arene (η6-C6R6), and η-cyclooctatetraenyl (η8-C8R8) bridging motifs are common in organometallic chemistry; however, the synthetic preparation of η-cycloheptatrienyl (η7-C7R7) bridging motifs has remained a synthetic challenge in 4f chemistry. To this end, we have developed a synthetic route towards a series of rare dinuclear organolanthanide inverse sandwich complexes containing the elusive η7-C7H7 bridge. Herein, we present the structures and magnetic properties of the lanthanide inverse sandwich complexes [KLn2(C7H7)(N(SiMe3)2)4] (Ln = GdIII (1), DyIII (2), ErIII (3)) and [K(THF)2Er2(C7H7)(N(SiMe3)2)4] (4). These compounds are the first single-molecule magnets (SMMs) to feature this type of bridging motif. Furthermore, η7-C7H7 was found to efficiently promote ferromagnetic exchange interactions between metal ions. Variable temperature dc magnetic susceptibility measurements and subsequent simulations give significant exchange constants of J = +1.384, +1.798, and +3.149 cm–1 and dipolar constants of J = –0.603, –0.601, and –0.475 cm–1 for compounds 2–4, respectively. Frequency dependent ac susceptibility measurements under an applied static field resulted in the observation of dual relaxation processes, and brought forth a greater understanding of the intermolecularly driven process at high frequency. In particular, this type of analysis of compound 3 under 800 Oe elicited an energy barrier of U eff = 58 K. Ab initio calculations were performed in order to understand the nature of magnetic coupling and the origin of slow relaxation of magnetisation. Through these studies, the effect of the amido ancillary ligands on the magnetic axiality of the lanthanide ions was found to be competitive with the crystal field of the η7-C7H7 π-electron cloud. Our findings suggest that the tunability of the dipolar and exchange components of the magnetic interactions lie within the dihedral angle imposed by the amido ligands, thus offering potential for the development of new exchange coupled lanthanide systems. PMID:28451170

Exchange and Correlation Effects in a Double Barrier Quantum Well

NASA Astrophysics Data System (ADS)

Hasbun, J. E.

2003-03-01

Studies of electronic properties in double barrier resonant tunneling systems usually involve the solution of the BenDaniel-Duke equation (BDK) in the presence of an applied voltage. Electron interactions are included by means of the Hartree term and the wavefunction is self-consistently obtained by coupling the BDK with the Poisson equation. In this work, we go beyond the Hartree approximation by including exchange and correlation effects (v_xc) through the local density functional theory. The analytically parametrized v_xc of Hedin and Lunqvist footnote[1]L. Hedin and B. I. Lundqvist, J. Phys C: Solid. St. Phys. 4, 2064(1971) is employed. While the calculation of v_xc involves extra self-consistency conditions, it is shown that the inclusion of exchange and correlation effects may be important in the electronic characteristics of these devices. For our earlier model footnote[2]J. E. Hasbun, APS Bull. 47, 914(2002) the v_xc potential is comparable to the Hartree electron term at low electric field values, but less so for much higher field. The v_xc employed here ^2 is not appreciably different from the more sophisticated one of Perdew and Zunger footnote[3]J. P. Perdew and A. Zunger, Phys. Rev. B 23, 5048(1981). JH wishes to thank Mei-Yin Chou for helpful discussions

Kitaev exchange and field-induced quantum spin-liquid states in honeycomb α-RuCl3

NASA Astrophysics Data System (ADS)

Yadav, Ravi; Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Nishimoto, Satoshi; van den Brink, Jeroen; Hozoi, Liviu

2016-11-01

Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3. From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d5 halides and oxides in general.

Kitaev exchange and field-induced quantum spin-liquid states in honeycomb α-RuCl3.

PubMed

Yadav, Ravi; Bogdanov, Nikolay A; Katukuri, Vamshi M; Nishimoto, Satoshi; van den Brink, Jeroen; Hozoi, Liviu

2016-11-30

Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d 5 honeycomb halide α-RuCl 3 . From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d 5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d 5 halides and oxides in general.

Spin-exchange relaxation-free magnetometer with nearly parallel pump and probe beams

DOE PAGES

Karaulanov, Todor; Savukov, Igor; Kim, Young Jin

2016-03-22

We constructed a spin-exchange relaxation-free (SERF) magnetometer with a small angle between the pump and probe beams facilitating a multi-channel design with a flat pancake cell. This configuration provides almost complete overlap of the beams in the cell, and prevents the pump beam from entering the probe detection channel. By coupling the lasers in multi-mode fibers, without an optical isolator or field modulation, we demonstrate a sensitivity of 10 fTmore » $$/\\sqrt{\\text{Hz}}$$ for frequencies between 10 Hz and 100 Hz. In addition to the experimental study of sensitivity, we present a theoretical analysis of SERF magnetometer response to magnetic fields for small-angle and parallel-beam configurations, and show that at optimal DC offset fields the magnetometer response is comparable to that in the orthogonal-beam configuration. Based on the analysis, we also derive fundamental and probe-limited sensitivities for the arbitrary non-orthogonal geometry. The expected practical and fundamental sensitivities are of the same order as those in the orthogonal geometry. As a result, we anticipate that our design will be useful for magnetoencephalography (MEG) and magnetocardiography (MCG) applications.« less

Kitaev exchange and field-induced quantum spin-liquid states in honeycomb α-RuCl3

PubMed Central

Yadav, Ravi; Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Nishimoto, Satoshi; van den Brink, Jeroen; Hozoi, Liviu

2016-01-01

Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3. From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d5 halides and oxides in general. PMID:27901091

Direct observation of the discrete energy spectrum of two lanthanide-based single-chain magnets by far-infrared spectroscopy

NASA Astrophysics Data System (ADS)

Haas, Sabrina; Heintze, Eric; Zapf, Sina; Gorshunov, Boris; Dressel, Martin; Bogani, Lapo

2014-05-01

The far-infrared optical transmission has been studied for two lanthanide-based single-chain magnets DyPhOPh and TbPhOPh in the frequency range between 3 and 80 cm-1. The spectra were acquired at temperatures between 2 and 80 K and magnetic fields up to 6 T. Based on their magnetic field dependence in DyPhOPh two of the observed absorption lines are identified as transitions inside the crystal field split Dy3+ ground multiplet 6H15/2, coupled to the neighboring spins. In TbPhOPh one transition was observed inside the crystal-field-split Tb3+ ground multiplet 7F6. The results allow a spectroscopic investigation of the role of single-ion anisotropy and exchange in Glauber dynamics.

Using Wavelets and Information Theory to Characterize the Direction, Strength, and Time Scale of Interaction between Environmental Drivers and Greenhouse Gas Exchange in Managed Wetlands of Northern California

NASA Astrophysics Data System (ADS)

Sturtevant, C. S.; Ruddell, B. L.; Knox, S. H.; Verfaillie, J. G.; Matthes, J. H.; Oikawa, P. Y.; Baldocchi, D. D.

2014-12-01

Restoring agricultural areas to wetlands in the Sacramento-San Joaquin River Delta of California can help reverse subsidence and reduce greenhouse gas (GHG) emissions. Predicting outcomes and developing best practices of wetland management therefore requires a robust understanding of the sensitivity of GHG exchange in these ecosystems to factors such as management and meteorology. However, wetlands can exhibit complex, overlapping, and asynchronous couplings between site characteristics, environmental drivers and GHG exchange. In this research we demonstrate the use of wavelets and information theory (process networks) as sophisticated tools to disentangle and characterize ecosystem couplings to CO2 and CH4 exchange (measured by eddy covariance) in two restored Delta wetlands. Using wavelets we isolated processes acting at different time scales, then used process networks to determine the direction, strength, and lag properties of ecosystem couplings. We found that despite differences in age, architecture and management, CO2 exchange at both wetlands was most sensitive to similar meteorological factors such as radiation and temperature up to a time scale of several days. At the monthly timescale, however, the effect of a more variable water table management in one wetland became dominant, revealing a reduction in net CO2 uptake during long term water table drawdowns. The analysis of CH4 exchange in this wetland revealed a more sensitive and complex coupling with water table. CH4 exchange was sensitive to relatively small, multi-day shifts in water table and displayed a lagged response to larger, longer shifts. With these methods we were able to disentangle the effects of management from meteorology and better understand the sensitivities of GHG exchange. Our results provide important insights for modeling efforts and management practices.

Role of temperature on static correlational properties in a spin-polarized electron gas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Arora, Priya; Moudgil, R. K., E-mail: rkmoudgil@kuk.ac.in; Kumar, Krishan

We have studied the effect of temperature on the static correlational properties of a spin-polarized three-dimensional electron gas (3DEG) over a wide coupling and temperature regime. This problem has been very recently studied by Brown et al. using the restricted path-integral Monte Carlo (RPIMC) technique in the warm-dense regime. To this endeavor, we have used the finite temperature version of the dynamical mean-field theory of Singwi et al, the so-called quantum STLS (qSTLS) approach. The static density structure factor and the static pair-correlation function are calculated, and compared with the RPIMC simulation data. We find an excellent agreement with themore » simulation at high temperature over a wide coupling range. However, the agreement is seen to somewhat deteriorate with decreasing temperature. The pair-correlation function is found to become small negative for small electron separation. This may be attributed to the inadequacy of the mean-field theory in dealing with the like spin electron correlations in the strong-coupling domain. A nice agreement with RPIMC data at high temperature seems to arise due to weakening of both the exchange and coulomb correlations with rising temperature.« less

Discovery of room-temperature spin-glass behaviors in two-dimensional oriented attached single crystals

NASA Astrophysics Data System (ADS)

Ma, Ji; Chen, Kezheng

2016-05-01

In this study, room-temperature spin-glass behaviors were observed in flake-like oriented attached hematite (α-Fe2O3) and iron phosphate hydroxide hydrate (Fe5(PO4)4(OH)3·2H2O) single crystals. Remarkably, their coercivity (HC) values were found to be almost invariable at various given temperatures from 5 to 300 K. The spin topographic map in these flakes was assumed as superparamagnetic (SPM) "islands" isolated by spin glass (SG)-like "bridges". A spin-glass model was then proposed to demonstrate the spin frustration within these "bridges", which were formed by the staggered atomic planes in the uneven surfaces belonging to different attached nanoparticles. Under the spatial limitation and coupling shield of these "bridges", the SPM "islands" were found to be collectively frozen to form a superspin glass (SSG) state below 80 K in weak applied magnetic fields; whereas, when strong magnetic fields were applied, the magnetic coupling of these "islands" would become superferromagnetic (SFM) through tunneling superexchange, so that, these SFM spins could antiferromagnetically couple with the SG-like "bridges" to yield pronounced exchange bias (EB) effect.

Controllable spin-charge transport in strained graphene nanoribbon devices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Diniz, Ginetom S., E-mail: ginetom@gmail.com; Guassi, Marcos R.; Qu, Fanyao

2014-09-21

We theoretically investigate the spin-charge transport in two-terminal device of graphene nanoribbons in the presence of a uniform uniaxial strain, spin-orbit coupling, exchange field, and smooth staggered potential. We show that the direction of applied strain can efficiently tune strain-strength induced oscillation of band-gap of armchair graphene nanoribbon (AGNR). It is also found that electronic conductance in both AGNR and zigzag graphene nanoribbon (ZGNR) oscillates with Rashba spin-orbit coupling akin to the Datta-Das field effect transistor. Two distinct strain response regimes of electronic conductance as function of spin-orbit couplings magnitude are found. In the regime of small strain, conductance ofmore » ZGNR presents stronger strain dependence along the longitudinal direction of strain. Whereas for high values of strain shows larger effect for the transversal direction. Furthermore, the local density of states shows that depending on the smoothness of the staggered potential, the edge states of AGNR can either emerge or be suppressed. These emerging states can be determined experimentally by either spatially scanning tunneling microscope or by scanning tunneling spectroscopy. Our findings open up new paradigms of manipulation and control of strained graphene based nanostructure for application on novel topological quantum devices.« less

Antiferromagnetic coupling between rare earth ions and semiquinones in a series of 1:1 complexes.

PubMed

Caneschi, Andrea; Dei, Andrea; Gatteschi, Dante; Poussereau, Sandrine; Sorace, Lorenzo

2004-04-07

We use the strategy of diamagnetic substitution for obtaining information on the crystal field effects in paramagnetic rare earth ions using the homologous series of compounds with the diamagnetic tropolonato ligand, Ln(Trp)(HBPz(3))(2), and the paramagnetic semiquinone ligand, Ln(DTBSQ)(HBPz(3))(2), (DTBSQ = 3,5-di-tert-butylsemiquinonato, Trp = tropolonate, HBPz(3)= hydrotrispyrazolylborate) for Ln = Sm(iii), Eu(iii), Gd(iii), Tb(iii), Dy(iii), Ho(iii), Er(iii) or Yb(iii). The X-ray crystal structure of a new form of tropolonate derivative is presented, which shows, as expected, a marked similarity with the structure of the semiquinonate derivative. The Ln(Trp)(HBPz(3))(2) derivatives were then used as a reference for the qualitative determination of crystal field effects in the exchange coupled semiquinone derivatives. Through magnetisation and susceptibility measurements this empirical diamagnetic substitution method evidenced for Er(iii), Tb(iii), Dy(iii) and Yb(iii) derivatives a dominating antiferromagnetic coupling. The increased antiferromagnetic contribution compared to other radical-rare earth metal complexes formed by nitronyl nitroxide ligands may be related to the increased donor strength of the semiquinone ligand.

Bidirectional private key exchange using delay-coupled semiconductor lasers.

PubMed

Porte, Xavier; Soriano, Miguel C; Brunner, Daniel; Fischer, Ingo

2016-06-15

We experimentally demonstrate a key exchange cryptosystem based on the phenomenon of identical chaos synchronization. In our protocol, the private key is symmetrically generated by the two communicating partners. It is built up from the synchronized bits occurring between two current-modulated bidirectionally coupled semiconductor lasers with additional self-feedback. We analyze the security of the exchanged key and discuss the amplification of its privacy. We demonstrate private key generation rates up to 11  Mbit/s over a public channel.

Effects of the nucleon radius on neutron stars in a quark mean field model

NASA Astrophysics Data System (ADS)

Zhu, Zhen-Yu; Li, Ang

2018-03-01

We study the effects of free space nucleon radius on nuclear matter and neutron stars within the framework of the quark mean field model. The nucleon radius is treated self-consistently with this model, where quark confinement is adjusted to fit different values of nucleon radius. Corrections due to center-of-mass motion, quark-pion coupling, and one gluon exchange are included to obtain the nucleon mass in vacuum. The meson coupling constants that describe the behavior of the many-body nucleonic system are constructed by reproducing the empirical saturation properties of nuclear matter, including the recent determinations of symmetry energy parameters. Our results show that the nucleon radius in free space has negligible effects on the nuclear matter equation of state and neutron star mass-radius relations, which is different from the conclusion drawn in previous studies. We further explore that the sensitivity of star radius on the nucleon radius found in earlier publications is actually from the symmetry energy and its slope.

DOE Office of Scientific and Technical Information (OSTI.GOV)

San Fabián, J.; Omar, S.; García de la Vega, J. M., E-mail: garcia.delavega@uam.es

The effect of a fraction of Hartree-Fock exchange on the calculated spin-spin coupling constants involving fluorine through a hydrogen bond is analyzed in detail. Coupling constants calculated using wavefunction methods are revisited in order to get high-level calculations using the same basis set. Accurate MCSCF results are obtained using an additive approach. These constants and their contributions are used as a reference for density functional calculations. Within the density functional theory, the Hartree-Fock exchange functional is split in short- and long-range using a modified version of the Coulomb-attenuating method with the SLYP functional as well as with the original B3LYP.more » Results support the difficulties for calculating hydrogen bond coupling constants using density functional methods when fluorine nuclei are involved. Coupling constants are very sensitive to the Hartree-Fock exchange and it seems that, contrary to other properties, it is important to include this exchange for short-range interactions. Best functionals are tested in two different groups of complexes: those related with anionic clusters of type [F(HF){sub n}]{sup −} and those formed by difluoroacetylene and either one or two hydrogen fluoride molecules.« less

Verification of antiferromagnetic exchange coupling at room temperature using polar magneto-optic Kerr effect in thin EuS/Co multilayers with perpendicular magnetic anisotropy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goschew, A., E-mail: alexander.goschew@fu-berlin.de; Scott, M.; Fumagalli, P.

2016-08-08

We report on magneto-optic Kerr measurements in polar geometry carried out on a series of thin Co/EuS multilayers on suitable Co/Pd-multilayer substrates. Thin Co/EuS multilayers of a few nanometers individual layer thickness usually have their magnetization in plane. Co/Pd multilayers introduce a perpendicular magnetic anisotropy in the Co/EuS layers deposited on top, thus making it possible to measure magneto-optic signals in the polar geometry in remanence in order to study exchange coupling. Magneto-optic Kerr-effect spectra and hysteresis loops were recorded in the visible and ultraviolet photon-energy range at room temperature. The EuS contribution to the magneto-optic signal is extracted atmore » 4.1 eV by combining hysteresis loops measured at different photon energies with polar magneto-optic Kerr-effect spectra recorded in remanence and in an applied magnetic field of 2.2 T. The extracted EuS signal shows clear signs of antiferromagnetic coupling of the Eu magnetic moments to the Co layers. This implies that the ordering temperature of at least a fraction of the EuS layers is above room temperature proving that magneto-optic Kerr-effect spectroscopy can be used here as a quasi-element-specific method.« less

Boundary Conditions for Diffusion-Mediated Processes within Linear Nanopores: Exact Treatment of Coupling to an Equilibrated External Fluid

DOE PAGES

Garcia, Andres; Evans, James W.

2017-04-03

In this paper, we consider a variety of diffusion-mediated processes occurring within linear nanopores, but which involve coupling to an equilibrated external fluid through adsorption and desorption. By determining adsorption and desorption rates through a set of tailored simulations, and by exploiting a spatial Markov property of the models, we develop a formulation for performing efficient pore-only simulations of these processes. Coupling to the external fluid is described exactly through appropriate nontrivial boundary conditions at the pore openings. This formalism is applied to analyze the following: (i) tracer counter permeation (TCP) where different labeled particles adsorb into opposite ends ofmore » the pore and establish a nonequilibrium steady state; (ii) tracer exchange (TE) with exchange of differently labeled particles within and outside the pore; (iii) catalytic conversion reactions where a reactant in the external fluid adsorbs into the pore and converts to a product which may desorb. The TCP analysis also generates a position-dependent generalized tracer diffusion coefficient, the form of which controls behavior in the TE and catalytic conversion processes. We focus on the regime of single-file diffusion within the pore which produces the strongest correlations and largest deviations from mean-field type behavior. Finally, behavior is quantified precisely via kinetic Monte Carlo simulations but is also captured with appropriate analytic treatments.« less

Thermal management systems and methods

DOEpatents

Gering, Kevin L.; Haefner, Daryl R.

2006-12-12

A thermal management system for a vehicle includes a heat exchanger having a thermal energy storage material provided therein, a first coolant loop thermally coupled to an electrochemical storage device located within the first coolant loop and to the heat exchanger, and a second coolant loop thermally coupled to the heat exchanger. The first and second coolant loops are configured to carry distinct thermal energy transfer media. The thermal management system also includes an interface configured to facilitate transfer of heat generated by an internal combustion engine to the heat exchanger via the second coolant loop in order to selectively deliver the heat to the electrochemical storage device. Thermal management methods are also provided.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Furlow, Julie Maupin-

Archaea, one of three major evolutionary lineages of life, are a fascinating and diverse group of microbes with deep roots overlapping those of eukaryotes. The focus of the 'Archaea: Ecology Metabolism & Molecular Biology' GRC conference expands on a number of emerging topics highlighting new paradigms in archaeal metabolism, genome function and systems biology; information processing; evolution and the tree of life; the ecology and diversity of archaea and their viruses; and industrial applications. The strength of this conference lies in its ability to couple a field with a rich history in high quality research with new scientific findings inmore » an atmosphere of stimulating exchange. This conference remains an excellent opportunity for younger scientists to interact with world experts in this field.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Keneth Stedman

Archaea, one of three major evolutionary lineages of life, are a fascinating and diverse group of microbes with deep roots overlapping those of eukaryotes. The focus of the 'Archaea: Ecology Metabolism & Molecular Biology' GRC conference expands on a number of emerging topics highlighting new paradigms in archaeal metabolism, genome function and systems biology; information processing; evolution and the tree of life; the ecology and diversity of archaea and their viruses. The strength of this conference lies in its ability to couple a field with a rich history in high quality research with new scientific findings in an atmosphere ofmore » stimulating exchange. This conference remains an excellent opportunity for younger scientists to interact with world experts in this field.« less

Effects of post-deposition magnetic field annealing on magnetic properties of NiO/Co90Fe10 bilayers

NASA Astrophysics Data System (ADS)

Zheng, Chao; Su, Shan; Chiu, Chun-Cheng; Skoropata, Elizabeth; Desautels, Ryan D.; van Lierop, Johan; Lin, Ko-Wei; Pong, Philip W. T.

2018-01-01

The ferromagnetic (FM)/antiferromagnetic (AF) bilayer structures have drawn intensive attention because of their wide applications in modern spintronic devices. While abundant published works have been reported on the interface effects of the FM/AF bilayers caused by the magnetic field annealing (MFA) process, the volume effects caused by the MFA treatment have been rarely considered. In this work, the microstructural and magnetic properties of the NiO/CoFe bilayers with various CoFe thicknesses were investigated under different annealing temperatures. At high annealing temperature, the interlayer mixing and exchange coupling between NiO and CoFe layers were promoted and consequently the interface effects were facilitated. The interfacial oxides acted as pinning centers and randomly pinned the FM domains, leading to an increase of coercivity and a considerable degradation of uniaxial anisotropy. The increase of coercivity was also contributed by the enhancement of the interfacial exchange coupling between the NiO and CoFe layers after MFA. As the CoFe thickness increased, the volume effects tended to dominate over the interface effects, resulting in the preservation the uniaxially anisotropic features of CoFe. These results indicate that both the coercivity and anisotropic features of the NiO/CoFe bilayers can be directly affected by the MFA process, opening up the possibility of modifying the magnetism in the NiO/CoFe bilayers and offering an effective way to improve the performance of modern spintronic devices.

Exact CNOT gates with a single nonlocal rotation for quantum-dot qubits

NASA Astrophysics Data System (ADS)

Pal, Arijeet; Rashba, Emmanuel I.; Halperin, Bertrand I.

2015-09-01

We investigate capacitively-coupled exchange-only two-qubit quantum gates based on quantum dots. For exchange-only coded qubits electron spin S and its projection Sz are exact quantum numbers. Capacitive coupling between qubits, as distinct from interqubit exchange, preserves these quantum numbers. We prove, both analytically and numerically, that conservation of the spins of individual qubits has a dramatic effect on the performance of two-qubit gates. By varying the level splittings of individual qubits, Ja and Jb, and the interqubit coupling time, t , we can find an infinite number of triples (Ja,Jb,t ) for which the two-qubit entanglement, in combination with appropriate single-qubit rotations, can produce an exact cnot gate. This statement is true for practically arbitrary magnitude and form of capacitive interqubit coupling. Our findings promise a large decrease in the number of nonlocal (two-qubit) operations in quantum circuits.

Angular dependence of spin-orbit spin-transfer torques

NASA Astrophysics Data System (ADS)

Lee, Ki-Seung; Go, Dongwook; Manchon, Aurélien; Haney, Paul M.; Stiles, M. D.; Lee, Hyun-Woo; Lee, Kyung-Jin

2015-04-01

In ferromagnet/heavy-metal bilayers, an in-plane current gives rise to spin-orbit spin-transfer torque, which is usually decomposed into fieldlike and dampinglike torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the fieldlike torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the fieldlike torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to the exchange interaction. On the other hand, the dampinglike torque acquires nontrivial angular dependence when the Rashba spin-orbit coupling is comparable to or stronger than the exchange interaction. It is related to the combined effects of the Fermi surface distortion and the Fermi sea contribution. The angular dependence is consistent with experimental observations and can be important to understand magnetization dynamics induced by spin-orbit spin-transfer torques.

Air-structure coupling features analysis of mining contra-rotating axial flow fan cascade

NASA Astrophysics Data System (ADS)

Chen, Q. G.; Sun, W.; Li, F.; Zhang, Y. J.

2013-12-01

The interaction between contra-rotating axial flow fan blade and working gas has been studied by means of establishing air-structure coupling control equation and combining Computational Fluid Dynamics (CFD) and Computational solid mechanics (CSM). Based on the single flow channel model, the Finite Volume Method was used to make the field discrete. Additionally, the SIMPLE algorithm, the Standard k-ε model and the Arbitrary Lagrangian-Eulerian dynamic grids technology were utilized to get the airflow motion by solving the discrete governing equations. At the same time, the Finite Element Method was used to make the field discrete to solve dynamic response characteristics of blade. Based on weak coupling method, data exchange from the fluid solver and the solid solver was processed on the coupling interface. Then interpolation was used to obtain the coupling characteristics. The results showed that the blade's maximum amplitude was on the tip of the last-stage blade and aerodynamic force signal could reflect the blade working conditions to some extent. By analyzing the flow regime in contra-rotating axial flow fan, it could be found that the vortex core region was mainly in the blade surface, the hub and the blade clearance. In those regions, the turbulence intensity was very high. The last-stage blade's operating life is shorter than that of the pre-stage blade due to the fatigue fracture occurs much more easily on the last-stage blade which bears more stress.

Magnetic Structure and Exchange Interactions in Quasi-One-Dimensional MnCl 2(urea) 2

DOE PAGES

Manson, Jamie L.; Huang, Qing-zhen; Brown, Craig M.; ...

2015-12-08

MnCl 2(urea) 2 is a new linear chain coordination polymer that exhibits slightly counter-rotated Mn 2Cl 2 rhomboids along the chain-axis. The material crystallizes in the noncentrosymmetric orthorhombic space group Iba2, with each Mn(II) ion equatorially surrounded by four Cl – that lead to bibridged ribbons. Additionally, urea ligands coordinate via O atoms in the axial positions. Hydrogen bonds of the Cl···H–N and O···H–N type link the chains into a quasi-3D network. Magnetic susceptibility data reveal a broad maximum at 9 K that is consistent with short-range magnetic order. Pulsed-field magnetization measurements conducted at 0.6 K show that a fullymore » polarized magnetic state is achieved at B sat = 19.6 T with another field-induced phase transition occurring at 2.8 T. Zero-field neutron diffraction studies made on a powdered sample of MnCl 2(urea) 2 reveal that long-range magnetic order occurs below T N = 3.2(1) K. Additional Bragg peaks due to antiferromagnetic (AFM) ordering can be indexed according to the Ib'a2' magnetic space group and propagation vector τ = [0, 0, 0]. Rietveld profile analysis of these data revealed a Néel-type collinear ordering of Mn(II) ions with an ordered magnetic moment of 4.06(6) μ B (5 μ B is expected for isotropic S = 5/2) oriented along the b-axis, i.e., perpendicular to the chain-axis that runs along the c-direction. Owing to the potential for spatial exchange anisotropy and the pitfalls in modeling bulk magnetic data, we analyzed inelastic neutron scattering data to retrieve the exchange constants: J c = 2.22 K (intrachain), J a = -0.10 K (interchain), and D = -0.14 K with J > 0 assigned to AFM coupling. Lastly, this J configuration is most unusual and contrasts the more commonly observed AFM interchain coupling of 1D chains.« less

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

PubMed Central

Shattock, Michael J; Ottolia, Michela; Bers, Donald M; Blaustein, Mordecai P; Boguslavskyi, Andrii; Bossuyt, Julie; Bridge, John H B; Chen-Izu, Ye; Clancy, Colleen E; Edwards, Andrew; Goldhaber, Joshua; Kaplan, Jack; Lingrel, Jerry B; Pavlovic, Davor; Philipson, Kenneth; Sipido, Karin R; Xie, Zi-Jian

2015-01-01

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na+ channel and Na+ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na+/Ca2+ exchange (NCX) and Na+/K+-ATPase (NKA). While the relevance of Ca2+ homeostasis in cardiac function has been extensively investigated, the role of Na+ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na+ content have multiple effects on the heart by influencing intracellular Ca2+ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na+ homeostasis. Among the proteins that accomplish this task are the Na+/Ca2+ exchanger (NCX) and the Na+/K+ pump (NKA). By transporting three Na+ ions into the cytoplasm in exchange for one Ca2+ moved out, NCX is one of the main Na+ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na+ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na+ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na+ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na+/Ca2+ exchanger (NCX1) and Na+/K+ pump and the controversies that still persist in the field. PMID:25772291

Strong electron-hole exchange in coherently coupled quantum dots.

PubMed

Fält, Stefan; Atatüre, Mete; Türeci, Hakan E; Zhao, Yong; Badolato, Antonio; Imamoglu, Atac

2008-03-14

We have investigated few-body states in vertically stacked quantum dots. Because of a small interdot tunneling rate, the coupling in our system is in a previously unexplored regime where electron-hole exchange plays a prominent role. By tuning the gate bias, we are able to turn this coupling off and study a complementary regime where total electron spin is a good quantum number. The use of differential transmission allows us to obtain unambiguous signatures of the interplay between electron and hole-spin interactions. Small tunnel coupling also enables us to demonstrate all-optical charge sensing, where a conditional exciton energy shift in one dot identifies the charging state of the coupled partner.

Modeling coupled interactions of carbon, water, and ozone exchange between terrestrial ecosystems and the atmosphere

Treesearch

Ned Nikolova; Karl F. Zeller

2003-01-01

A new biophysical model (FORFLUX) is presented to study the simultaneous exchange of ozone, carbon dioxide, and water vapor between terrestrial ecosystems and the atmosphere. The model mechanistically couples all major processes controlling ecosystem flows trace gases and water implementing recent concepts in plant eco-physiology, micrometeorology, and soil hydrology....

Induced Ti magnetization at La 0.7Sr 0.3MnO 3 and BaTiO 3 interfaces

DOE PAGES

Liu, Yaohua; Tornos, J.; te Velthuis, S. G. E.; ...

2016-04-01

In artificial multiferroics hybrids consisting of ferromagnetic La 0.7Sr 0.3MnO 3 (LSMO) and ferroelectric BaTiO 3 epitaxial layers, net Ti moments are found from polarized resonant soft x-ray reflectivity and absorption. The Ti dichroic reflectivity follows the Mn signal during the magnetization reversal, indicating exchange coupling between the Ti and Mn ions. However, the Ti dichroic reflectivity shows stronger temperature dependence than the Mn dichroic signal. Lastly, besides a reduced ferromagnetic exchange coupling in the interfacial LSMO layer, this may also be attributed to a weak Ti-Mn exchange coupling that is insufficient to overcome the thermal energy at elevated temperatures.

Calculation of exchange coupling constants in triply-bridged dinuclear Cu(II) compounds based on spin-flip constricted variational density functional theory.

PubMed

Seidu, Issaka; Zhekova, Hristina R; Seth, Michael; Ziegler, Tom

2012-03-08

The performance of the second-order spin-flip constricted variational density functional theory (SF-CV(2)-DFT) for the calculation of the exchange coupling constant (J) is assessed by application to a series of triply bridged Cu(II) dinuclear complexes. A comparison of the J values based on SF-CV(2)-DFT with those obtained by the broken symmetry (BS) DFT method and experiment is provided. It is demonstrated that our methodology constitutes a viable alternative to the BS-DFT method. The strong dependence of the calculated exchange coupling constants on the applied functionals is demonstrated. Both SF-CV(2)-DFT and BS-DFT affords the best agreement with experiment for hybrid functionals.

Pipe connector

DOEpatents

Sullivan, Thomas E.; Pardini, John A.

1978-01-01

A safety test facility for testing sodium-cooled nuclear reactor components includes a reactor vessel and a heat exchanger submerged in sodium in the tank. The reactor vessel and heat exchanger are connected by an expansion/deflection pipe coupling comprising a pair of coaxially and slidably engaged tubular elements having radially enlarged opposed end portions of which at least a part is of spherical contour adapted to engage conical sockets in the ends of pipes leading out of the reactor vessel and in to the heat exchanger. A spring surrounding the pipe coupling urges the end portions apart and into engagement with the spherical sockets. Since the pipe coupling is submerged in liquid a limited amount of leakage of sodium from the pipe can be tolerated.

Transition metal ions in ZnO: Effects of intrashell coulomb repulsion on electronic properties

NASA Astrophysics Data System (ADS)

Ciechan, A.; Bogusławski, P.

2018-05-01

Electronic structure of the transition metal (TM) dopants in ZnO is calculated by first principles approach. Analysis of the results is focused on the properties determined by the intrashell Coulomb coupling. The role of both direct and exchange interaction channel is analyzed. The coupling is manifested in the strong charge state dependence of the TM gap levels, which leads to the metastability of photoexcited Mn, and determines the accessible equilibrium charge states of TM ions. The varying magnitude of the exchange coupling is reflected in the dependence of the spin splitting energy on the chemical identity across the 3d series, as well as the charge state dependence of spin-up spin-down exchange splitting.

THE COUPLED EVOLUTION OF ELECTRONS AND IONS IN CORONAL MASS EJECTION-DRIVEN SHOCKS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Manchester IV, W. B.; Van der Holst, B.; Toth, G.

2012-09-01

We present simulations of coronal mass ejections (CMEs) performed with a new two-temperature coronal model developed at the University of Michigan, which is able to address the coupled thermodynamics of the electron and proton populations in the context of a single fluid. This model employs heat conduction for electrons, constant adiabatic index ({gamma} = 5/3), and includes Alfven wave pressure to accelerate the solar wind. The Wang-Sheeley-Arge empirical model is used to determine the Alfven wave pressure necessary to produce the observed bimodal solar wind speed. The Alfven waves are dissipated as they propagate from the Sun and heat protonsmore » on open magnetic field lines to temperatures above 2 MK. The model is driven by empirical boundary conditions that includes GONG magnetogram data to calculate the coronal field, and STEREO/EUVI observations to specify the density and temperature at the coronal boundary by the Differential Emission Measure Tomography method. With this model, we simulate the propagation of fast CMEs and study the thermodynamics of CME-driven shocks. Since the thermal speed of the electrons greatly exceeds the speed of the CME, only protons are directly heated by the shock. Coulomb collisions low in the corona couple the protons and electrons allowing heat exchange between the two species. However, the coupling is so brief that the electrons never achieve more than 10% of the maximum temperature of the protons. We find that heat is able to conduct on open magnetic field lines and rapidly propagates ahead of the CME to form a shock precursor of hot electrons.« less

Fascinating Magnetic Energy Storage Nanomaterials: A Brief Review.

PubMed

Sreenivasulu, Kummari V; Srikanth, Vadali V S S

2017-07-10

In this brief review, the importance of nanotechnology in developing novel magnetic energy storage materials is discussed. The discussion covers recent patents on permanent magnetic materials and especially covers processing of permanent magnets (rare-earth and rare-earth free magnets), importance of rare-earth permanent magnets and necessity of rare-earth free permanent magnets. Magnetic energy storage materials are those magnetic materials which exhibit very high energy product (BH)max (where B is the magnetic induction in Gauss (G) whereas H is the applied magnetic field in Oersted (Oe)). (BH)max is the direct measure of the ability of a magnetic material to store energy. In this context, processing of magnetic energy storage composite materials constituted by soft and hard magnetic materials played a predominant role in achieving high (BH)max values due to the exchange coupling phenomenon between the soft and hard magnetic phases within the composite. Magnetic energy storage composites are normally composed of rare-earth magnetic materials as well as rare-earth free magnetic materials. Nanotechnology's influence on the enhancement of energy product due to the exchange coupling phenomenon is of great prominence and therefore discussed in this review. Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.org.

Nuclear spin-isospin excitations from covariant quasiparticle-vibration coupling

NASA Astrophysics Data System (ADS)

Robin, Caroline; Litvinova, Elena

2016-09-01

Methods based on the relativistic Lagrangian of quantum hadrodynamics and nuclear field theory provide a consistent framework for the description of nuclear excitations, naturally connecting the high- and medium-energy scales of mesons to the low-energy domain of nucleonic collective motion. Applied in the neutral channel, this approach has been quite successful in describing the overall transition strength up to high excitation energies, as well as fine details of the low-lying distribution. Recently, this method has been extended to the description of spin-isospin excitations in open-shell nuclei. In the charge-exchange channel, the coupling between nucleons and collective vibrations generates a time-dependent proton-neutron effective interaction, in addition to the static pion and rho-meson exchange, and introduces complex configurations that induce fragmentation and spreading of the resonances. Such effects have a great impact on the quenching of the strength and on the computing of weak reaction rates that are needed for astrophysics modeling. Gamow-Teller transitions in medium-mass nuclei and associated beta-decay half-lives will be presented. Further developments aiming to include additional ground-state correlations will also be discussed. This work is supported by US-NSF Grants PHY-1404343 and PHY-1204486.

Dynamics of a Cr spin in a semiconductor quantum dot: Hole-Cr flip-flops and spin-phonon coupling

NASA Astrophysics Data System (ADS)

Lafuente-Sampietro, A.; Utsumi, H.; Sunaga, M.; Makita, K.; Boukari, H.; Kuroda, S.; Besombes, L.

2018-04-01

A detailed analysis of the photoluminescence (PL) intensity distribution in singly Cr-doped CdTe/ZnTe quantum dots (QDs) is performed. First of all, we demonstrate that hole-Cr flip-flops induced by an interplay of the hole-Cr exchange interaction and the coupling with acoustic phonons are the main source of spin relaxation within the exciton-Cr complex. This spin flip mechanism appears in the excitation power dependence of the PL of the exciton as well as in the intensity distribution of the resonant PL. The resonant optical pumping of the Cr spin which was recently demonstrated can also be explained by these hole-Cr flip-flops. Despite the fast exciton-Cr spin dynamics, an analysis of the PL intensity under magnetic field shows that the hole-Cr exchange interaction in CdTe/ZnTe QDs is antiferromagnetic. In addition to the Cr spin dynamics induced by the interaction with carriers' spin, we finally demonstrate using time resolved optical pumping measurements that a Cr spin interacts with nonequilibrium acoustic phonons generated during the optical excitation inside or near the QD.

Anisotropy effects in magnetic hyperthermia: A comparison between spherical and cubic exchange-coupled FeO/Fe{sub 3}O{sub 4} nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khurshid, H., E-mail: khurshid@usf.edu, E-mail: sharihar@usf.edu; Nemati, Z.; Phan, M. H.

2015-05-07

Spherical and cubic exchange-coupled FeO/Fe{sub 3}O{sub 4} nanoparticles, with different FeO:Fe{sub 3}O{sub 4} ratios, have been prepared by a thermal decomposition method to probe anisotropy effects on their heating efficiency. X-ray diffraction and transmission electron microscopy reveal that the nanoparticles are composed of FeO and Fe{sub 3}O{sub 4} phases, with an average size of ∼20 nm. Magnetometry and transverse susceptibility measurements show that the effective anisotropy field is 1.5 times larger for the cubes than for the spheres, while the saturation magnetization is 1.5 times larger for the spheres than for the cubes. Hyperthermia experiments evidence higher values of the specificmore » absorption rate (SAR) for the cubes as compared to the spheres (200 vs. 135 W/g at 600 Oe and 310 kHz). These observations point to an important fact that the saturation magnetization is not a sole factor in determining the SAR and the heating efficiency of the magnetic nanoparticles can be improved by tuning their effective anisotropy.« less

Dual Expander Cycle Rocket Engine with an Intermediate, Closed-cycle Heat Exchanger

NASA Technical Reports Server (NTRS)

Greene, William D. (Inventor)

2008-01-01

A dual expander cycle (DEC) rocket engine with an intermediate closed-cycle heat exchanger is provided. A conventional DEC rocket engine has a closed-cycle heat exchanger thermally coupled thereto. The heat exchanger utilizes heat extracted from the engine's fuel circuit to drive the engine's oxidizer turbomachinery.

The ground state of metallic nano-structures in heavily irradiated NaCl-KBF4

NASA Astrophysics Data System (ADS)

Cherkasov, F. G.; L'Vov, S. G.; Tikhonov, D. A.; den Hartog, H. W.; Vainshtein, D. I.

ESR, NMR and static magnetic susceptibility measurements of heavily irradiated NaCl-K and NaCl-KBF4 are reported. Up to 10% of the NaCl-molecules are transformed into metallic Na nanoparticles and Cl-2 precipitates. In addition, there are paramagnetic F- and F-aggregates, which are coupled by exchange interactions to the conduction electrons in the nanoparticles. Above 160 K the NMR and ESR signals of NaCl-K and NaCl-KBF4 show Pauli paramagnetism and the properties of the Na nanoparticles are similar to bulk sodium. A single ESR line is observed revealing exchange interaction between conduction electrons in the nano-particles and F-aggregates. The observed decrease of the ESR susceptibility with decreasing temperature is due to a metal-insulator transition. The conduction electrons are localized below 40 K and the above mentioned F-aggregate centers contribute significantly to the overall ESR signal. For NaCl-KBF4 we observed that with decreasing temperature the ESR line shifts towards lower fields due to antiferromagnetic ordering and internal magnetic fields.

Multi-excitonic (N=1,2 and 3) quantum dots in magnetic field: Analytical mapping of correlations (exchange) by multipole expansion

NASA Astrophysics Data System (ADS)

Singh, Sunny; Kaur, Harsimran; Sharma, Shivalika; Aggarwal, Priyanka; Hazra, Ram Kuntal

2017-04-01

The understanding of the physics of exciton, bi-exciton, tri-exciton and the subsequent insight into controlling the properties of mesoscopic systems holds the key to various exotic optical, electrical and magnetic phenomena like superconductivity, Mott insulation, Quantum Hall effect etc. Many of exciton properties are similar to atomic hydrogen that attracts researchers to explore electronic structure of exciton in quantum dots, but nontriviality arises due to coulombic interactions among electrons and holes. We propose an exact integral of coulomb (exchange) correlation in terms of finitely summed Lauricella functions to examine 3-D exciton of harmonic dots confined in zero and non-zero arbitrary magnetic field. The highlight of our work is the use of exact variational solution for coloumbic interaction between the hole and the electron and evaluation of the cross terms arising out of the coupling among centre-of-mass and relative coordinates. We also have extended the size of the system to generalized N-body problem with N=3,4 for tri-exciton (e-e-h/e-h-h)

Exchange-dominated eigenmodes in sub-100 nm permalloy dots: A micromagnetic study at finite temperature

NASA Astrophysics Data System (ADS)

Carlotti, G.; Gubbiotti, G.; Madami, M.; Tacchi, S.; Stamps, R. L.

2014-05-01

Micromagnetic simulations at room temperature (300 K) have been carried out in order to analyse the magnetic eigenmodes (frequency and spatial profile) in elliptical dots with sub-100 nm lateral size. Features are found that are qualitatively different from those typical of larger dots because of the dominant role played by the exchange-energy. These features can be understood most simply in terms of nodal planes defined relative to the orientation of the static magnetization. A new, generalized labeling scheme is proposed that simplifies discussion and comparison of modes from different geometries. It is shown that the lowest-frequency mode for small dots is characterized by an in-phase precession of spins, without nodal planes, but with a maximum amplitude at the edges. This mode softens at an applied switching field with magnitude comparable to the coercive field and determines specific aspects of magnetization reversal. This characteristic behavior can be relevant for optimization of microwave assisting switching as well as for maximizing interdot coupling in dense arrays of dots.

Superconducting state parameters of bulk amorphous alloys

NASA Astrophysics Data System (ADS)

Vora, A. M.

2012-12-01

Well recognized empty core pseudopotential of Ashcroft is used to investigate the superconducting state parameters viz; electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T C , isotope effect exponent α and effective interaction strength N O V of some (Ni33Zr67)1- x V x ( x = 0, 0.05, 0.1, 0.15) bulk amorphous alloys. We have incorporated five different types of local field correction functions, proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) to show the effect of exchange and correlation on the aforesaid properties. Very strong influence of the various exchange and correlation functions is concluded from the present study. The T C obtained from local field correction function proposed by Sarkar et al. (S) is in excellent agreement with available theoretical data. Quadratic T C equation has been proposed providing successfully the T C values of bulk amorphous alloys under consideration. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirm the superconducting phase in the s bulk amorphous alloys.

Theoretical study of the magnetic exchange coupling behavior substituting Cr(III) with Mo(III) in cyano-bridged transition metal complexes

NASA Astrophysics Data System (ADS)

Zhang, Yi-Quan; Luo, Cheng-Lin

Molecular magnetism in a series of cyano-bridged first and second transition metal complexes has been investigated using density functional theory (DFT) combined with the broken-symmetry (BS) approach. Several exchange-correlation (XC) functionals in the ADF package were used to investigate complexes I [-(Me3tacn)2(cyclam)NiMo2(CN)6]2+, II [-(Me3tacn)2(cyclam)Ni-Cr2(CN)6]2+, III [(Me3tacn)6MnMo6(CN)18]2+, and IV [(Me3tacn)6MnCr6(CN)18]2+ (Me3tacn = N,N?,N‴-trimethyl-1,4,7-triazacyclononane). For models A (the molded structure of complex I) and B (the modeled structure of complex II), all the XCs given qualitatively reasonable results and predict ferromagnetic coupling character between M (M = MoIII for A or CrIII for B) and NiII in coincidence with the experimental results (see Tables and ). The calculated using Operdew, OPBE, O3LYP, and B3LYP functionals and experimental J values show that substituting CrIII with MoIII will enhance the ferromagnetic exchange coupling interactions. But VWN, PW91, PBE, VSXC, and tau-HCTH functionals have no way to differentiate the relative strength of the intramolecular magnetic exchange coupling interactions of A and B correctly. For models C (the modeled structure of complex III) and D (the modeled structure of complex IV), all the XCs in ADF and B3LYP in Gaussian 03 with several basis sets show that substituting CrIII with MoIII will enhance the antiferromagnetic exchange coupling interactions. From the above calculations, the substitution of CrIII by MoIII will enhance the magnetic coupling interactions, whether the magnetic coupling interactions are ferro- or antiferromagnetic. Moreover, Kahn's model was applied to investigate the above facts.

Tunable Magnetic Resonance in Microwave Spintronics Devices

NASA Technical Reports Server (NTRS)

Chen, Yunpeng; Fan, Xin; Xie, Yunsong; Zhou, Yang; Wang, Tao; Wilson, Jeffrey D.; Simons, Rainee N.; Chui, Sui-Tat; Xiao, John Q.

2015-01-01

Magnetic resonance is one of the key properties of magnetic materials for the application of microwave spintronics devices. The conventional method for tuning magnetic resonance is to use an electromagnet, which provides very limited tuning range. Hence, the quest for enhancing the magnetic resonance tuning range without using an electromagnet has attracted tremendous attention. In this paper, we exploit the huge exchange coupling field between magnetic interlayers, which is on the order of 4000 Oe and also the high frequency modes of coupled oscillators to enhance the tuning range. Furthermore, we demonstrate a new scheme to control the magnetic resonance frequency. Moreover, we report a shift in the magnetic resonance frequency as high as 20 GHz in CoFe based tunable microwave spintronics devices, which is 10X higher than conventional methods.

Tunable Magnetic Resonance in Microwave Spintronics Devices

NASA Technical Reports Server (NTRS)

Chen, Yunpeng; Fan, Xin; Xie, Yungsong; Zhou, Yang; Wang, Tao; Wilson, Jeffrey D.; Simons, Rainee N.; Chui, Sui-Tat; Xiao, John Q.

2015-01-01

Magnetic resonance is one of the key properties of magnetic materials for the application of microwave spintronics devices. The conventional method for tuning magnetic resonance is to use an electromagnet, which provides very limited tuning range. Hence, the quest for enhancing the magnetic resonance tuning range without using an electromagnet has attracted tremendous attention. In this paper, we exploit the huge exchange coupling field between magnetic interlayers, which is on the order of 4000 Oe and also the high frequency modes of coupled oscillators to enhance the tuning range. Furthermore, we demonstrate a new scheme to control the magnetic resonance frequency. Moreover, we report a shift in the magnetic resonance frequency as high as 20 GHz in CoFe-based tunable microwave spintronics devices, which is 10X higher than conventional methods.

Relevance of 4f-3d exchange to finite-temperature magnetism of rare-earth permanent magnets: An ab-initio-based spin model approach for NdFe{sub 12}N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsumoto, Munehisa; Akai, Hisazumi; Doi, Shotaro

2016-06-07

A classical spin model derived ab initio for rare-earth-based permanent magnet compounds is presented. Our target compound, NdFe{sub 12}N, is a material that goes beyond today's champion magnet compound Nd{sub 2}Fe{sub 14}B in its intrinsic magnetic properties with a simpler crystal structure. Calculated temperature dependence of the magnetization and the anisotropy field agrees with the latest experimental results in the leading order. Having put the realistic observables under our numerical control, we propose that engineering 5d-electron-mediated indirect exchange coupling between 4f-electrons in Nd and 3d-electrons from Fe would most critically help enhance the material's utility over the operation-temperature range.

Micro-fluid exchange coupling apparatus

NASA Technical Reports Server (NTRS)

Johnson, J. E., Jr.; Swartz, P. F. (Inventor)

1980-01-01

In a macro-fluid exchange, a hollow needle, such as a syringe needle, is provided for penetrating the fluid conduit of the animal. The syringe needle is coupled to a plenum chamber having an inlet and outlet port. The plenum chamber is coupled to the syringe needle via the intermediary of a standard quick disconnect coupling fitting. The plenum chamber is carried at the end of a drive rod which is coupled to a micrometer drive head. The micrometer drive head is slidably and pivotably coupled to a pedestal for adjusting the height and angle of inclination of the needle relative to a reference base support. The needle is positioned adjacent to the incised trachea or a blood vessel of a small animal and the micrometer drive head is operated for penetrating the fluid conduit of the animal.

Unified TeV scale picture of baryogenesis and dark matter.

PubMed

Babu, K S; Mohapatra, R N; Nasri, Salah

2007-04-20

We present a simple extension of the minimal supersymmetric standard model which provides a unified picture of cosmological baryon asymmetry and dark matter. Our model introduces a gauge singlet field N and a color triplet field X which couple to the right-handed quark fields. The out-of-equilibrium decay of the Majorana fermion N mediated by the exchange of the scalar field X generates adequate baryon asymmetry for MN approximately 100 GeV and MX approximately TeV. The scalar partner of N (denoted N1) is naturally the lightest SUSY particle as it has no gauge interactions and plays the role of dark matter. The model is experimentally testable in (i) neutron-antineutron oscillations with a transition time estimated to be around 10(10)sec, (ii) discovery of colored particles X at LHC with mass of order TeV, and (iii) direct dark matter detection with a predicted cross section in the observable range.

Magnetization reversal mechanism for Co nanoparticles revealed by a magnetic hysteresis scaling technique

NASA Astrophysics Data System (ADS)

Kobayashi, Satoru; Sato, Takuma; Li, Zhang; Dong, Xing-Long; Murakami, Takeshi

2018-05-01

We report results of magnetic hysteresis scaling of minor loops for cobalt nanoparticles with variable mean particle size of 53 and 95 nm. A power-law scaling with an exponent of 1.40±0.05 was found to hold true between minor-loop remanence and hysteresis loss in the wide temperature range of 10 - 300 K, irrespective of particle size and cooling field. A coefficient deduced from the scaling law steeply increases with decreasing temperature and exhibits a cooling field dependence below T ˜ 150 K. The value obtained after field cooling at 5 T was lower than that after zero-field cooling, being opposite to a behavior of major-loop coercivity. These observations were explained from the viewpoint of the exchange coupling between ferromagnetic Co core and antiferromagnetic CoO shell, which becomes effective below T ˜ 150 K.

Heterometallic Zn3 Ln3 Ensembles Containing (μ6 -CO3 ) Ligand and Triangular Disposition of Ln3+ ions: Analysis of Single-Molecule Toroic (SMT) and Single-Molecule Magnet (SMM) Behavior.

PubMed

Goura, Joydeb; Colacio, Enrique; Herrera, Juan Manuel; Suturina, Elizaveta A; Kuprov, Ilya; Lan, Yanhua; Wernsdorfer, Wolfgang; Chandrasekhar, Vadapalli

2017-11-21

Two new heterometallic Zn 3 Ln 3 (Ln 3+ =Dy, Tb) complexes, with a double triangular topology of the metal ions, have been assembled from the polytopic Mannich base ligand 6,6'-{[2-(dimethylamino)ethylazanediyl]bis(methylene)}bis(2-methoxy-4-methylphenol) (H 2 L) with the aid of an in situ generated carbonate ligand from atmospheric CO 2 fixation. Theoretical calculations indicate axial ground states for the Ln 3+ ions in these complexes, with their local magnetic moments being almost coplanar and tangential to the Ln 3+ atoms that define the equilateral triangle. Therefore, they can be considered as single-molecule toroics (SMTs) with almost zero total magnetic moment. Micro-SQUID measurements on the Dy 3+ counterpart show hysteresis loops below 3 K that have an S-shape, with large coercive fields opening upon cooling. This behavior is typical of a single molecule magnet (SMM) with very slow zero-field relaxation. At around ±0.35 T, the loops have a broad step, which is due to a direct relaxation process and corresponds to an acceleration of the relaxation of the magnetization, also observed at this magnetic field from ac susceptibility measurements. Simulations suggest that the broad step corresponds to two level avoidance of crossing points where the spin chiral Kramers doublet meets excited states of the coupled manifold, whose position is defined by exchange and dipole interactions. The Tb 3+ counterpart does not exhibit SMM behavior, which is due to the fact that the degeneracy of the ground state of the exchange coupled system is lifted at zero field, thus favoring quantum tunneling of magnetization (QTM). © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rotation and magnetism in intermediate-mass stars

NASA Astrophysics Data System (ADS)

Quentin, Léo G.; Tout, Christopher A.

2018-06-01

Rotation and magnetism are increasingly recognized as important phenomena in stellar evolution. Surface magnetic fields from a few to 20 000 G have been observed and models have suggested that magnetohydrodynamic transport of angular momentum and chemical composition could explain the peculiar composition of some stars. Stellar remnants such as white dwarfs have been observed with fields from a few to more than 109 G. We investigate the origin of and the evolution, on thermal and nuclear rather than dynamical time-scales, of an averaged large-scale magnetic field throughout a star's life and its coupling to stellar rotation. Large-scale magnetic fields sustained until late stages of stellar evolution with conservation of magnetic flux could explain the very high fields observed in white dwarfs. We include these effects in the Cambridge stellar evolution code using three time-dependant advection-diffusion equations coupled to the structural and composition equations of stars to model the evolution of angular momentum and the two components of the magnetic field. We present the evolution in various cases for a 3 M_{⊙} star from the beginning to the late stages of its life. Our particular model assumes that turbulent motions, including convection, favour small-scale field at the expense of large-scale field. As a result, the large-scale field concentrates in radiative zones of the star and so is exchanged between the core and the envelope of the star as it evolves. The field is sustained until the end of the asymptotic giant branch, when it concentrates in the degenerate core.

Methods of forming thermal management systems and thermal management methods

DOEpatents

Gering, Kevin L.; Haefner, Daryl R.

2012-06-05

A thermal management system for a vehicle includes a heat exchanger having a thermal energy storage material provided therein, a first coolant loop thermally coupled to an electrochemical storage device located within the first coolant loop and to the heat exchanger, and a second coolant loop thermally coupled to the heat exchanger. The first and second coolant loops are configured to carry distinct thermal energy transfer media. The thermal management system also includes an interface configured to facilitate transfer of heat generated by an internal combustion engine to the heat exchanger via the second coolant loop in order to selectively deliver the heat to the electrochemical storage device. Thermal management methods are also provided.

Localized Magnetic Moments with Tunable Spin Exchange in a Gas of Ultracold Fermions

NASA Astrophysics Data System (ADS)

Riegger, L.; Darkwah Oppong, N.; Höfer, M.; Fernandes, D. R.; Bloch, I.; Fölling, S.

2018-04-01

We report on the experimental realization of a state-dependent lattice for a two-orbital fermionic quantum gas with strong interorbital spin exchange. In our state-dependent lattice, the ground and metastable excited electronic states of 173Yb take the roles of itinerant and localized magnetic moments, respectively. Repulsive on-site interactions in conjunction with the tunnel mobility lead to spin exchange between mobile and localized particles, modeling the coupling term in the well-known Kondo Hamiltonian. In addition, we find that this exchange process can be tuned resonantly by varying the on-site confinement. We attribute this to a resonant coupling to center-of-mass excited bound states of one interorbital scattering channel.

Electric-field-induced modification in Dzyaloshinskii-Moriya interaction of Co monolayer on Pt(111)

NASA Astrophysics Data System (ADS)

Nakamura, Kohji; Akiyama, Toru; Ito, Tomonori; Ono, Teruo; Weinert, Michael

Magnetism induced by an external electric field (E-field) has received much attention as a potential approach for controlling magnetism at the nano-scale with the promise of ultra-low energy power consumption. Here, the E-field-induced modification of the Dzyaloshinskii-Moriya interaction (DMI) for a prototypical transition-metal thin layer of a Co monolayer on Pt(111) is investigated by first-principles calculations by using the full-potential linearized augmented plane wave method that treats spin-spiral structures in an E-field. With inclusion of the spin-orbit coupling (SOC) by the second variational method for commensurate spin-spiral structures, the DMI constants were estimated from an asymmetric contribution in the total energy with respect to the spin-spiral wavevector. The results predicted that the DMI is modified by the E-field, but the change is found to be small compared to that in the exchange interaction (a symmetric contribution in the total energy) by a factor of ten.

Building energy simulation coupled with CFD for indoor environment: A critical review and recent applications

DOE PAGES

Tian, Wei; Han, Xu; Zuo, Wangda; ...

2018-01-31

This paper presents a comprehensive review of the open literature on motivations, methods and applications of linking stratified airflow simulation to building energy simulation (BES). First, we reviewed the motivations for coupling prediction models for building energy and indoor environment. This review classified various exchanged data in different applications as interface data and state data, and found that choosing different data sets may lead to varying performance of stability, convergence, and speed for the co-simulation. Second, our review shows that an external coupling scheme is substantially more popular in implementations of co-simulation than an internal coupling scheme. The external couplingmore » is shown to be generally faster in computational speed, as well as easier to implement, maintain and expand than the internal coupling. Third, the external coupling can be carried out in different data synchronization schemes, including static coupling and dynamic coupling. In comparison, the static coupling that performs data exchange only once is computationally faster and more stable than the dynamic coupling. However, concerning accuracy, the dynamic coupling that requires multiple times of data exchange is more accurate than the static coupling. Furthermore, the review identified that the implementation of the external coupling can be achieved through customized interfaces, middleware, and standard interfaces. The customized interface is straightforward but may be limited to a specific coupling application. The middleware is versatile and user-friendly but usually limited in data synchronization schemes. The standard interface is versatile and promising, but may be difficult to implement. Current applications of the co-simulation are mainly energy performance evaluation and control studies. Finally, we discussed the limitations of the current research and provided an overview for future research.« less

Building energy simulation coupled with CFD for indoor environment: A critical review and recent applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, Wei; Han, Xu; Zuo, Wangda

This paper presents a comprehensive review of the open literature on motivations, methods and applications of linking stratified airflow simulation to building energy simulation (BES). First, we reviewed the motivations for coupling prediction models for building energy and indoor environment. This review classified various exchanged data in different applications as interface data and state data, and found that choosing different data sets may lead to varying performance of stability, convergence, and speed for the co-simulation. Second, our review shows that an external coupling scheme is substantially more popular in implementations of co-simulation than an internal coupling scheme. The external couplingmore » is shown to be generally faster in computational speed, as well as easier to implement, maintain and expand than the internal coupling. Third, the external coupling can be carried out in different data synchronization schemes, including static coupling and dynamic coupling. In comparison, the static coupling that performs data exchange only once is computationally faster and more stable than the dynamic coupling. However, concerning accuracy, the dynamic coupling that requires multiple times of data exchange is more accurate than the static coupling. Furthermore, the review identified that the implementation of the external coupling can be achieved through customized interfaces, middleware, and standard interfaces. The customized interface is straightforward but may be limited to a specific coupling application. The middleware is versatile and user-friendly but usually limited in data synchronization schemes. The standard interface is versatile and promising, but may be difficult to implement. Current applications of the co-simulation are mainly energy performance evaluation and control studies. Finally, we discussed the limitations of the current research and provided an overview for future research.« less

Mediterranea Forecasting System: a focus on wave-current coupling

NASA Astrophysics Data System (ADS)

Clementi, Emanuela; Delrosso, Damiano; Pistoia, Jenny; Drudi, Massimiliano; Fratianni, Claudia; Grandi, Alessandro; Pinardi, Nadia; Oddo, Paolo; Tonani, Marina

2016-04-01

The Mediterranean Forecasting System (MFS) is a numerical ocean prediction system that produces analyses, reanalyses and short term forecasts for the entire Mediterranean Sea and its Atlantic Ocean adjacent areas. MFS became operational in the late 90's and has been developed and continuously improved in the framework of a series of EU and National funded programs and is now part of the Copernicus Marine Service. The MFS is composed by the hydrodynamic model NEMO (Nucleus for European Modelling of the Ocean) 2-way coupled with the third generation wave model WW3 (WaveWatchIII) implemented in the Mediterranean Sea with 1/16 horizontal resolution and forced by ECMWF atmospheric fields. The model solutions are corrected by the data assimilation system (3D variational scheme adapted to the oceanic assimilation problem) with a daily assimilation cycle, using a background error correlation matrix varying seasonally and in different sub-regions of the Mediterranean Sea. The focus of this work is to present the latest modelling system upgrades and the related achieved improvements. In order to evaluate the performance of the coupled system a set of experiments has been built by coupling the wave and circulation models that hourly exchange the following fields: the sea surface currents and air-sea temperature difference are transferred from NEMO model to WW3 model modifying respectively the mean momentum transfer of waves and the wind speed stability parameter; while the neutral drag coefficient computed by WW3 model is passed to NEMO that computes the turbulent component. In order to validate the modelling system, numerical results have been compared with in-situ and remote sensing data. This work suggests that a coupled model might be capable of a better description of wave-current interactions, in particular feedback from the ocean to the waves might assess an improvement on the prediction capability of wave characteristics, while suggests to proceed toward a fully coupled modelling system in order to achieve stronger enhancements of the hydrodynamic fields.

Measurement of the magnetic anisotropy energy constants for magneto-optical recording media

NASA Technical Reports Server (NTRS)

Hajjar, R. A.; Wu, T. H.; Mansuripur, M.

1992-01-01

Measurement of the magneto-optical polar Kerr effect is performed on rare earth-transition metal (RE-TM) amorphous films using in-plane fields. From this measurement and the measurement of the saturation magnetization using a vibrating sample magnetometer (VSM), the magnetic anisotropy constants are determined. The temperature dependence is presented of the magnetic anisotropy in the range of -175 to 175 C. The results show a dip in the anisotropy near magnetic compensation. This anomaly is explained based on the finite exchange coupling between the rare earth and transition metal subnetworks.

Strain-Induced Ferromagnetism in Antiferromagnetic LuMnO3 Thin Films

NASA Astrophysics Data System (ADS)

White, J. S.; Bator, M.; Hu, Y.; Luetkens, H.; Stahn, J.; Capelli, S.; Das, S.; Döbeli, M.; Lippert, Th.; Malik, V. K.; Martynczuk, J.; Wokaun, A.; Kenzelmann, M.; Niedermayer, Ch.; Schneider, C. W.

2013-07-01

Single phase and strained LuMnO3 thin films are discovered to display coexisting ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism (≈1μB), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth manganite thin films as promising candidate systems for new multifunctional devices.

Micromagnetic analysis of current-induced domain wall motion in a bilayer nanowire with synthetic antiferromagnetic coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Komine, Takashi, E-mail: komine@mx.ibaraki.ac.jp; Aono, Tomosuke

We demonstrate current-induced domain wall motion in bilayer nanowire with synthetic antiferromagnetic (SAF) coupling by modeling two body problems for motion equations of domain wall. The influence of interlayer exchange coupling and magnetostatic interactions on current-induced domain wall motion in SAF nanowires was also investigated. By assuming the rigid wall model for translational motion, the interlayer exchange coupling and the magnetostatic interaction between walls and domains in SAF nanowires enhances domain wall speed without any spin-orbit-torque. The enhancement of domain wall speed was discussed by energy distribution as a function of wall angle configuration in bilayer nanowires.

Concerted hydrogen atom exchange between three HF molecules

NASA Technical Reports Server (NTRS)

Komornicki, Andrew; Dixon, David A.; Taylor, Peter R.

1992-01-01

We have investigated the termolecular reaction involving concerted hydrogen exchange between three HF molecules, with particular emphasis on the effects of correlation at the various stationary points along the reaction. Using an extended basis, we have located the geometries of the stable hydrogen-bonded trimer, which is of C(sub 3h) symmetry, and the transition state for hydrogen exchange, which is of D(sub 3h) symmetry. The energies of the exchange reation were then evaluated at the correlated level, using a large atomic natural orbital basis and correlating all valence electrons. Several correlation treatments were used, namely, configration interaction with single and double excitations, coupled-pair functional, and coupled-cluster methods. We are thus able to measure the effect of accounting for size-extensivity. Zero-point corrections to the correlated level energetics were determined using analytic second derivative techniques at the SCF level. Our best calculations, which include the effects of connected triple excitations in the coupled-cluster procedure, indicate that the trimer is bound by 9 +/- 1 kcal/mol relative to three separate monomers, in excellent agreement with previous estimates. The barrier to concerted hydrogen exchange is 15 kcal/mol above the trimer, or only 4.7 kcal/mol above three separated monomers. Thus the barrier to hydrogen exchange between HF molecules via this termolecular process is very low.

Decoherence-Free Interaction between Giant Atoms in Waveguide Quantum Electrodynamics

NASA Astrophysics Data System (ADS)

Kockum, Anton Frisk; Johansson, Göran; Nori, Franco

2018-04-01

In quantum-optics experiments with both natural and artificial atoms, the atoms are usually small enough that they can be approximated as pointlike compared to the wavelength of the electromagnetic radiation with which they interact. However, superconducting qubits coupled to a meandering transmission line, or to surface acoustic waves, can realize "giant artificial atoms" that couple to a bosonic field at several points which are wavelengths apart. Here, we study setups with multiple giant atoms coupled at multiple points to a one-dimensional (1D) waveguide. We show that the giant atoms can be protected from decohering through the waveguide, but still have exchange interactions mediated by the waveguide. Unlike in decoherence-free subspaces, here the entire multiatom Hilbert space (2N states for N atoms) is protected from decoherence. This is not possible with "small" atoms. We further show how this decoherence-free interaction can be designed in setups with multiple atoms to implement, e.g., a 1D chain of atoms with nearest-neighbor couplings or a collection of atoms with all-to-all connectivity. This may have important applications in quantum simulation and quantum computing.

Decoherence-Free Interaction between Giant Atoms in Waveguide Quantum Electrodynamics.

PubMed

Kockum, Anton Frisk; Johansson, Göran; Nori, Franco

2018-04-06

In quantum-optics experiments with both natural and artificial atoms, the atoms are usually small enough that they can be approximated as pointlike compared to the wavelength of the electromagnetic radiation with which they interact. However, superconducting qubits coupled to a meandering transmission line, or to surface acoustic waves, can realize "giant artificial atoms" that couple to a bosonic field at several points which are wavelengths apart. Here, we study setups with multiple giant atoms coupled at multiple points to a one-dimensional (1D) waveguide. We show that the giant atoms can be protected from decohering through the waveguide, but still have exchange interactions mediated by the waveguide. Unlike in decoherence-free subspaces, here the entire multiatom Hilbert space (2^{N} states for N atoms) is protected from decoherence. This is not possible with "small" atoms. We further show how this decoherence-free interaction can be designed in setups with multiple atoms to implement, e.g., a 1D chain of atoms with nearest-neighbor couplings or a collection of atoms with all-to-all connectivity. This may have important applications in quantum simulation and quantum computing.

Thermodynamic properties of a layered S = 7/2 Heisenberg magnet Gd(OH)CO3

NASA Astrophysics Data System (ADS)

Orendac, Martin; Ulicny, Martin; Cizmar, Erik; Orendacova, Alzbeta; Chen, Yan-Cong; Meng, Zhao-Sha; Tong, Ming-Liang

2015-03-01

Thermodynamic quantities and ESR spectra of Gd(OH)CO3 (I) are reported. The material may be considered to consist of weakly coupled layers with potentially triangular arrangement of exchange paths within each layer. Different bridging groups and distances among Gd3+ ions may be responsible for spatial anisotropy of magnetic coupling. Preliminary analysis of magnetic susceptibility using Curie-Weiss law yielded θ = -1.05 K indicating weak antiferromagnetic coupling and consequently, spin frustration in (I). More detailed simultaneous analysis of specific heat, susceptibility and magnetization studied down to nominally 0.45 K revealed non-negligible role of single-ion anisotropy. Using the model of weakly interacting S =7/2 trimers, the gross features of measured data may be explained while assuming single-ion anisotropy D /kB ~ 0.6 K and effective intratrimer magnetic coupling | J /kB | ~0.3 K. The obtained D value reasonably reproduces the position and shape of ESR line. The performed analysis suggests that magnetism in (I) is governed predominantly by crystal field effects and frustration plays a minor role. Supported by ITMS26220120005 and VEGA 1/0143/13.

Modulation of spin dynamics via voltage control of spin-lattice coupling in multiferroics

DOE PAGES

Zhu, Mingmin; Zhou, Ziyao; Peng, Bin; ...

2017-02-03

Our work aims at magnonics manipulation by the magnetoelectric coupling effect and is motivated by the most recent progresses in both magnonics (spin dynamics) and multiferroics fields. Here, voltage control of magnonics, particularly the surface spin waves, is achieved in La 0.7Sr 0.3MnO 3/0.7Pb(Mg 1/3Nb 2/3)O 3-0.3PbTiO 3 multiferroic heterostructures. With the electron spin resonance method, a large 135 Oe shift of surface spin wave resonance (≈7 times greater than conventional voltage-induced ferromagnetic resonance shift of 20 Oe) is determined. A model of the spin-lattice coupling effect, i.e., varying exchange stiffness due to voltage-induced anisotropic lattice changes, has been establishedmore » to explain experiment results with good agreement. In addition, an “on” and “off” spin wave state switch near the critical angle upon applying a voltage is created. The modulation of spin dynamics by spin-lattice coupling effect provides a platform for realizing energy-efficient, tunable magnonics devices.« less

Performance of wave function and density functional methods for water hydrogen bond spin-spin coupling constants.

PubMed

García de la Vega, J M; Omar, S; San Fabián, J

2017-04-01

Spin-spin coupling constants in water monomer and dimer have been calculated using several wave function and density functional-based methods. CCSD, MCSCF, and SOPPA wave functions methods yield similar results, specially when an additive approach is used with the MCSCF. Several functionals have been used to analyze their performance with the Jacob's ladder and a set of functionals with different HF exchange were tested. Functionals with large HF exchange appropriately predict 1 J O H , 2 J H H and 2h J O O couplings, while 1h J O H is better calculated with functionals that include a reduced fraction of HF exchange. Accurate functionals for 1 J O H and 2 J H H have been tested in a tetramer water model. The hydrogen bond effects on these intramolecular couplings are additive when they are calculated by SOPPA(CCSD) wave function and DFT methods. Graphical Abstract Evaluation of the additive effect of the hydrogen bond on spin-spin coupling constants of water using WF and DFT methods.

Quantum Magnetism Applied to the Iron-Pnictides and Rare Earth Pyrochlores

NASA Astrophysics Data System (ADS)

Applegate, Ryan

This dissertation presents computational studies of two families of magnetic materials of significant current interest. The iron pnictides are new high temperature superconductors with interesting parent compound antiferromagnetism. The rare earth pyrochlore material Yb2Ti2O7 is a candidate quantum spin ice. The magnetic and structural phases of individual iron pnictides have both many common features and material specific differences. In an attempt to unify these behaviors as instances of a larger theoretical picture, we use Monte Carlo simulations of a two-dimensional Hamiltonian with coupled Heisenberg-spin and Ising-orbital degrees of freedom. We introduce spin-space and single-ion anisotropies and study the finite temperature transitions in our model. We develop a phase diagram and propose that the interplay of spin and orbital physics in the presence of anisotropy could explain how material details affect the transitions of the pnictide materials. Nuclear magnetic resonance (NMR) can study magnetic materials via the hyperfine interaction and the coupling between the nuclear moment and the field produced by the samples local moment environment. Recent measurements suggest that Zn doped BaFe2As2 may have quantum fluctuations about the striped phase that produce a distribution of fields at As nuclear sites. The non-magnetic ion Zn replaces Fe and can be treated as an impurity which can be studied by a zero-temperature Ising Series expansion method. We propose a Heisenberg-like J1a-J 1b-J2 model which has small ferromagnetic exchanges along the b axis and strong antiferromagnetic exchanges along the a axis. In our impurity model we find that the magnetic moments are everywhere reduced by quantum fluctuations, except on the nearest neighbor site in the AFM direction. We suggest that the presented impurity model may provide an explanation for the experimental measurements. Based on a recently proposed quantum spin ice model, we use numerical linked cluster (NLC) expansions to study thermodynamic properties of Yb 2Ti2O7. We show that high field fitting of inelastic neutron scattering experiments is an excellent method in determining the exchange constants of these materials. We calculate the heat capacity, entropy and magnetization as a function of temperature and field along a few high symmetry field directions. We compare our theoretical predictions to experiments and find remarkable agreement. These studies highlight the importance of localized model Hamiltonians in understanding magnetic properties of complex materials.

Excited State Atom-Ion Charge-Exchange

NASA Astrophysics Data System (ADS)

Li, Ming; Makrides, Constantinos; Petrov, Alexander; Kotochigova, Svetlana

2017-04-01

We theoretically investigate the exothermic charge-exchange reaction between an excited atom and a ground-state positive ion. In particular, we focus on MOT-excited Ca*(4s4p 1P) atoms colliding with ground-state Yb+ ions, which are under active study by the experimental group of E. Hudson at UCLA. Collisions between an excited atom and an ion are guided by two major contributions to the long-range interaction potentials, the induction C4 /R4 and charge-quadrupole C3 /R3 potentials, and their coupling by the electron-exchange interaction. Our model of these forces leads to close-coupling equations for multiple reaction channels. We find several avoided crossings between the potentials that couple to the nearby asymptotic limits of Yb*+Ca+, some of which can possibly provide large charge exchange rate coefficients above 10-10 cm3 / s. We acknowledge support from the US Army Research Office, MURI Grants W911NF-14-1-0378 and the US National Science Foundation, Grant PHY-1619788.

The notions of mass in gravitational and particle physics

NASA Astrophysics Data System (ADS)

Castellani, Gianluca

It is presently thought that the mass of all of the elementary particles is determined by the Higgs field. This scalar field couples directly into the trace of the energy momentum tensor of the elementary particles. The attraction between two or more masses arises from the exchange of gravitational quantum particles of spin 2, called gravitons. The gravitational field couples directly into the energy momentum tensor. Then there is a close connection between the Higgs field, that originates the mass, and the gravitational field that dictates how the masses interact. Our purpose in this thesis is to discuss this close connection in terms of fundamental definitions of inertial and gravitational masses. On a practical level we explore two properties of mass from the viewpoint of coupling into the Higgs field: (i) The coupling of the both the Higgs and gravity to the energy-pressure tensor allows for the decay of the Higgs particle into two gravitons. We use the self energy part of the Higgs propagator to calculate the electromagnetic, weak, fermionic and gravitational decay rate of the Higgs particle. We show that the former process appears to dominate the other decay modes. Since the gravitons are detectable with virtually zero probability, the number of Higgs particles with observable decay products will be much less than previously expected. (ii) Some new experimental results seem to indicate that the mass of the heavy elementary particles like the Z,W+,W- and especially the top quark, depends on the particle environment in which these particles are produced. The presence of a Higgs field due to neighboring particles could be responsible for induced mass shifts. Further measurements of mass shift effects might give an indirect proof of the Higgs particle. Such can be in principle done by re-analyzing some of the production data e +e- → ZZ (or W+W-) already collected at the LEP experiment. About the physical property of the top quark, it is too early to arrive at any conclusion. In the foreseeable future, there will be more extended top quark production statistics from the Tevatron accelerator so that the mass shift hypothesis can be experimentally probed.

Exchange-coupled hard magnetic Fe-Co/CoPt nanocomposite films fabricated by electro-infiltration

NASA Astrophysics Data System (ADS)

Wen, Xiao; Andrew, Jennifer S.; Arnold, David P.

2017-05-01

This paper introduces a potentially scalable electro-infiltration process to produce exchange-coupled hard magnetic nanocomposite thin films. Fe-Co/CoPt nanocomposite films are fabricated by deposition of CoFe2O4 nanoparticles onto Si substrate, followed by electroplating of CoPt. Samples are subsequently annealed under H2 to reduce the CoFe2O4 to magnetically soft Fe-Co and also induce L10 ordering in the CoPt. Resultant films exhibit 0.97 T saturation magnetization, 0.70 T remanent magnetization, 127 kA/m coercivity and 21.8 kJ/m3 maximum energy density. First order reversal curve (FORC) analysis and δM plot are used to prove the exchange coupling between soft and hard magnetic phases.

Cluster-Glass Phase in Pyrochlore X Y Antiferromagnets with Quenched Disorder

NASA Astrophysics Data System (ADS)

Andrade, Eric C.; Hoyos, José A.; Rachel, Stephan; Vojta, Matthias

2018-03-01

We study the impact of quenched disorder (random exchange couplings or site dilution) on easy-plane pyrochlore antiferromagnets. In the clean system, order by disorder selects a magnetically ordered state from a classically degenerate manifold. In the presence of randomness, however, different orders can be chosen locally depending on details of the disorder configuration. Using a combination of analytical considerations and classical Monte Carlo simulations, we argue that any long-range-ordered magnetic state is destroyed beyond a critical level of randomness where the system breaks into magnetic domains due to random exchange anisotropies, becoming, therefore, a glass of spin clusters, in accordance with the available experimental data. These random anisotropies originate from off-diagonal exchange couplings in the microscopic Hamiltonian, establishing their relevance to other magnets with strong spin-orbit coupling.

Spin relaxation in geometrically frustrated pyrochlores

NASA Astrophysics Data System (ADS)

Dunsiger, Sarah Ruth

This thesis describes muSR experiments which focus on systems where the magnetic ions occupy the vertices of edge or corner sharing triangular units, in particular the pyrochlores A2B2O7. The scientific interest in pyrochlores is based on the fact that they display novel magnetic behaviour at low temperatures due to geometrical frustration. The ground state of these systems is sensitively dependent on such factors as the range of the spin-spin interactions, disorder, anisotropy, thermal and quantum fluctuations. For example, Y2Mo2O7 shows many features reminiscent of a conventional spin glass, even though this material has nominally zero chemical disorder. It is found that the muon spin polarisation obeys a time-field scaling relation which indicates that the spin-spin autocorrelation function has a power law form in time, in stark contrast with the exponential form often assumed for conventional magnets above their transition temperature. Gd2Ti2O7 shows long range order, but only at a temperature much lower than its Curie-Weiss temperature, a signature of a frustrated system. In the paramagnetic regime, it is well described by an isotropic Heisenberg Hamiltonian with nearest neighbour couplings in the presence of a Zeeman interaction, from which the spin-spin autocorrelation function may be calculated as a power series in time. The muon spin relaxation rate decreases with magnetic field as the Zeeman energy becomes comparable with the exchange coupling between Gd spins. Thus, an independent measure of the exchange coupling or equivalently the Gd spin fluctuation rate is extracted. By contrast, Tb2Ti2O7 has been identified as a type of cooperative paramagnet. Short range correlations develop below 50 K. However, there is no long range ordering down to very low temperatures (0.075 K). The Tb3+ ion is subject to strong crystal electric field effects: point charge calculations indicate that this system is Ising like at low temperatures. Thus this system may be analogous to water ice, a system theoretically predicted to have finite entropy at zero temperature. It is possible to qualitatively explain the unusual changes in T1-1 as a function of applied magnetic field which are also observed using muSR.

Magnetic proximity control of spin currents and giant spin accumulation in graphene

NASA Astrophysics Data System (ADS)

Singh, Simranjeet

Two dimensional (2D) materials provide a unique platform to explore the full potential of magnetic proximity driven phenomena. We will present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to the ferromagnetic-insulator (FMI) magnetization in graphene/FMI heterostructures. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. We also discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the non-local spin signals in graphene, which is due to spin relaxation by thermally-induced transverse fluctuations of the FMI magnetization. Additionally, it has been a challenge to grow a smooth, robust and pin-hole free tunnel barriers on graphene, which can withstand large current densities for efficient electrical spin injection. We have experimentally demonstrated giant spin accumulation in graphene lateral spin valves employing SrO tunnel barriers. Nonlocal spin signals, as large as 2 mV, are observed in graphene lateral spin valves at room temperature. This high spin accumulations observed using SrO tunnel barriers puts graphene on the roadmap for exploring the possibility of achieving a non-local magnetization switching due to the spin torque from electrically injected spins. Financial support from ONR (No. N00014-14-1-0350), NSF (No. DMR-1310661), and C-SPIN, one of the six SRC STARnet Centers, sponsored by MARCO and DARPA.

Kinetics of the chiral phase transition in a linear σ model

NASA Astrophysics Data System (ADS)

Wesp, Christian; van Hees, Hendrik; Meistrenko, Alex; Greiner, Carsten

2018-02-01

We study the dynamics of the chiral phase transition in a linear quark-meson σ model using a novel approach based on semiclassical wave-particle duality. The quarks are treated as test particles in a Monte Carlo simulation of elastic collisions and the coupling to the σ meson, which is treated as a classical field, via a kinetic approach motivated by wave-particle duality. The exchange of energy and momentum between particles and fields is described in terms of appropriate Gaussian wave packets. It has been demonstrated that energy-momentum conservation and the principle of detailed balance are fulfilled, and that the dynamics leads to the correct equilibrium limit. First schematic studies of the dynamics of matter produced in heavy-ion collisions are presented.

Field-Free Programmable Spin Logics via Chirality-Reversible Spin-Orbit Torque Switching.

PubMed

Wang, Xiao; Wan, Caihua; Kong, Wenjie; Zhang, Xuan; Xing, Yaowen; Fang, Chi; Tao, Bingshan; Yang, Wenlong; Huang, Li; Wu, Hao; Irfan, Muhammad; Han, Xiufeng

2018-06-21

Spin-orbit torque (SOT)-induced magnetization switching exhibits chirality (clockwise or counterclockwise), which offers the prospect of programmable spin-logic devices integrating nonvolatile spintronic memory cells with logic functions. Chirality is usually fixed by an applied or effective magnetic field in reported studies. Herein, utilizing an in-plane magnetic layer that is also switchable by SOT, the chirality of a perpendicular magnetic layer that is exchange-coupled with the in-plane layer can be reversed in a purely electrical way. In a single Hall bar device designed from this multilayer structure, three logic gates including AND, NAND, and NOT are reconfigured, which opens a gateway toward practical programmable spin-logic devices. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Multiscale Kinetic Modeling Reveals an Ensemble of Cl–/H+ Exchange Pathways in ClC-ec1 Antiporter

PubMed Central

2018-01-01

Despite several years of research, the ion exchange mechanisms in chloride/proton antiporters and many other coupled transporters are not yet understood at the molecular level. Here, we present a novel approach to kinetic modeling and apply it to ion exchange in ClC-ec1. Our multiscale kinetic model is developed by (1) calculating the state-to-state rate coefficients with reactive and polarizable molecular dynamics simulations, (2) optimizing these rates in a global kinetic network, and (3) predicting new electrophysiological results. The model shows that the robust Cl:H exchange ratio (2.2:1) can indeed arise from kinetic coupling without large protein conformational changes, indicating a possible facile evolutionary connection to chloride channels. The E148 amino acid residue is shown to couple chloride and proton transport through protonation-dependent blockage of the central anion binding site and an anion-dependent pKa value, which influences proton transport. The results demonstrate how an ensemble of different exchange pathways, as opposed to a single series of transitions, culminates in the macroscopic observables of the antiporter, such as transport rates, chloride/proton stoichiometry, and pH dependence. PMID:29332400

Improved Limits for Higgs-Portal Dark Matter from LHC Searches.

PubMed

Hoferichter, Martin; Klos, Philipp; Menéndez, Javier; Schwenk, Achim

2017-11-03

Searches for invisible Higgs decays at the Large Hadron Collider constrain dark matter Higgs-portal models, where dark matter interacts with the standard model fields via the Higgs boson. While these searches complement dark matter direct-detection experiments, a comparison of the two limits depends on the coupling of the Higgs boson to the nucleons forming the direct-detection nuclear target, typically parametrized in a single quantity f_{N}. We evaluate f_{N} using recent phenomenological and lattice-QCD calculations, and include for the first time the coupling of the Higgs boson to two nucleons via pion-exchange currents. We observe a partial cancellation for Higgs-portal models that makes the two-nucleon contribution anomalously small. Our results, summarized as f_{N}=0.308(18), show that the uncertainty of the Higgs-nucleon coupling has been vastly overestimated in the past. The improved limits highlight that state-of-the-art nuclear physics input is key to fully exploiting experimental searches.

A future Chinese mission proposed to investigate the coupling of the Earth's magnetoshpere, ionosphere and thermoshpere

NASA Astrophysics Data System (ADS)

Liu, Y.; Wang, C.; Xu, J.

2013-12-01

Under the support of Chinese Strategy Pioneer Program for space science, we are developing a mission to investigate a key question for space weather. The Earth's magnetosphere, ionosphere and thermosphere (MIT) are crucial regions for the space and they are coupling together through exchange of energy, momentum and mass. The mission, named as MIT, focuses on the polar upflow ions which are vital for the understanding of the physical processes relating to the MIT coupling. The mission plans a constellation composed of four spacecrafts, each spacecraft has its own orbit and crosses the polar region at the nearly the same time but at different altitude. With particles detectors, field detectors and imagers onboard each spacecraft, we will be able to track the polar upflow ions and study the acceleration mechanism at different altitude. Currently we have determined the orbits, the payloads for each the spacecraft and the expected lunch time is 2019 to 2020.

Entanglement of Electron Spins in Two Coupled Quantum Dots

NASA Astrophysics Data System (ADS)

Chen, Yuanzhen; Webb, Richard

2004-03-01

We study the entanglement of electron spins in a coupled quantum dots system at 70 mK. Two quantum dots are fabricated in a GaAs/AlGaAs heterostructure containing a high mobility 2-D electron gas. The two dots can be tuned independently and the electron spins in the dots are coupled through an exchange interaction between them. An exchange gate is used to vary the height and width of a potential barrier between the two dots, thus controlling the strength of the exchange interaction. Electrons are injected to the coupled dots by two independent DC currents and the output of the dots is incident on a beam splitter, which introduces quantum interferences. Cross-correlations of the shot noise of currents from the two output channels are measured and compared with theory (1). *Work supported by LPS and ARDA under MDA90401C0903 and NSF under DMR 0103223. (1) Burkard, Loss, & Sukhorukov, Phys. Rev. B61, R16303 (2000).

Existence of diproton-like particles in 3+1 lattice QCD with two flavors and strong coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Faria da Veiga, Paulo A.; O'Carroll, Michael; Neto, A. Francisco

2011-02-01

Starting from quarks, gluons, and their dynamics, we consider the existence of two-baryon bound states of total isospin I=1 in an imaginary-time formulation of a strongly coupled 3+1-dimensional SU(3){sub c} lattice QCD with two flavors and 4x4 spin matrices, defined using the Wilson action. For a small hopping parameter {kappa}>0 and a much smaller gauge coupling 0<{beta}<<{kappa}<<1 (heavy quarks and large glueball mass), using a ladder approximation to a lattice Bethe-Salpeter equation, diproton-like bound states are found in the I=1 isospin sector, with asymptotic masses -6ln{kappa} and binding energies of order {kappa}{sup 2}. By isospin symmetry, for each diproton theremore » is also a dineutron bound state with the same mass and binding energy. The dominant two-baryon interaction is an energy-independent spatial range-one potential with an O({kappa}{sup 2}) strength. There is also an attraction arising from gauge field correlations associated with six overlapping bonds, but it is subdominant. The overall range-one potential results from a quark-antiquark exchange with no meson exchange interpretation (wrong spin indices). The repulsive or attractive nature of the interaction does depend on the isospin and spin of the two-baryon states. A novel representation in term of permanents is obtained for the spin, isospin interaction between the baryons, which is valid for any isospin sector.« less

Interplay between spin frustration and magnetism in the exactly solved two-leg mixed spin ladder

NASA Astrophysics Data System (ADS)

Qi, Yan; Lv, Song-Wei; Du, An; Yu, Nai-sen

2016-11-01

We study a mixed spin-(3/2, 1) ladder system with antiferromagnetic rung coupling and next-nearest-neighbor interaction. The exactly solved Ising-chain model is employed to investigate the ground-state properties and thermodynamics of the low-dimensional ladder system. Our results show that the competition between different exchange couplings brings in a large variety of ground states characterized by various values of normalized magnetization equal to 0, 1/5, 2/5, 3/5, 1. Moreover, an interesting double-peak structure is also detected in the thermal dependence of magnetic susceptibility and specific heat when the frustration comes into play. It is shown that the double-peak phenomenon at zero-field for the case of AF2 ground-state arises from the very strong antiferromagnetic rung coupling, while other cases are attributed to the excitations induced by temperature and external field around the phase boundary. Project supported by the National Natural Science Foundation of China (Grant No. 11547236), the General Project of the Education Department of Liaoning Province, China (Grant No. L2015130), the Fundamental Research Funds for the Central Universities, China (Grant Nos. DC201501065 and DCPY2016014), and the Doctoral Starting-up Foundation of Dalian Nationalities University, China.

Development of a coupled wave-flow-vegetation interaction model

USGS Publications Warehouse

Beudin, Alexis; Kalra, Tarandeep S.; Ganju, Neil K.; Warner, John C.

2017-01-01

Emergent and submerged vegetation can significantly affect coastal hydrodynamics. However, most deterministic numerical models do not take into account their influence on currents, waves, and turbulence. In this paper, we describe the implementation of a wave-flow-vegetation module into a Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system that includes a flow model (ROMS) and a wave model (SWAN), and illustrate various interacting processes using an idealized shallow basin application. The flow model has been modified to include plant posture-dependent three-dimensional drag, in-canopy wave-induced streaming, and production of turbulent kinetic energy and enstrophy to parameterize vertical mixing. The coupling framework has been updated to exchange vegetation-related variables between the flow model and the wave model to account for wave energy dissipation due to vegetation. This study i) demonstrates the validity of the plant posture-dependent drag parameterization against field measurements, ii) shows that the model is capable of reproducing the mean and turbulent flow field in the presence of vegetation as compared to various laboratory experiments, iii) provides insight into the flow-vegetation interaction through an analysis of the terms in the momentum balance, iv) describes the influence of a submerged vegetation patch on tidal currents and waves separately and combined, and v) proposes future directions for research and development.

Radiochemical Applications of Insoluble Sulfate Columns. Analytical Possibilities in the Field of the Fission Product Solutions; APLICACIONES RADIO-QUIMICAS DE LAS COLUMNAS DE PRECIPITADOS DE SULFATOS INSOLUBLES. CONTRIBUCION AL ESTUDIO DE LAS SOLUCIONES ENVEJECIDAS DE PRODUCTOS DE FISION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barrachina, M.; Sauvagnac, R.

1962-01-01

The heterogeneous ion-isotopic exchange column is used to determine the radiochemical composition of raw solutions used in the industrial recuperation of long-lived fission products, The separation of the radioelements is made by small columns, 1--3 cm height, of BaSO/sub 4/ or SrSO/sub 4/, under selected experimental conditions. These columns behave like inorganic exchangers, working by adsorption or ion-isotopic exchange depending on the cases, and they provide selective separation of fission products employing very small volumes of fixing and eluting solutions. By coupling the separative capabilities of these columns and the liquid--liquid extraction with the 2-thenoyltrifluoroacetone and the di-2 ethylexyl orthophosphoricmore » acid, a set of new radiochemical methods, for the determination of Sr/sup 90/, Y/sup 90/, Ce/sup 144/ - Pr/sup 144/, and Pm/sup 147/ in the fission product solutions of Marcoule, were developed. (auth)« less

Transregional Collaborative Research Centre 32: Patterns in Soil-Vegetation-Atmosphere-Systems

NASA Astrophysics Data System (ADS)

Masbou, M.; Simmer, C.; Kollet, S.; Boessenkool, K.; Crewell, S.; Diekkrüger, B.; Huber, K.; Klitzsch, N.; Koyama, C.; Vereecken, H.

2012-04-01

The soil-vegetation-atmosphere system is characterized by non-linear exchanges of mass, momentum and energy with complex patterns, structures and processes that act at different temporal and spatial scales. Under the TR32 framework, the characterisation of these structures and patterns will lead to a deeper qualitative and quantitative understanding of the SVA system, and ultimately to better predictions of the SVA state. Research in TR32 is based on three methodological pillars: Monitoring, Modelling and Data Assimilation. Focusing our research on the Rur Catchment (Germany), patterns are monitored since 2006 continuously using existing and novel geophysical and remote sensing techniques from the local to the catchment scale based on ground penetrating radar methods, induced polarization, radiomagnetotellurics, electrical resistivity tomography, boundary layer scintillometry, lidar techniques, cosmic-ray, microwave radiometry, and precipitation radars with polarization diversity. Modelling approaches involve development of scaled consistent coupled model platform: high resolution numerical weather prediction (NWP; 400m) and hydrological models (few meters). In the second phase (2011-2014), the focus is on the integration of models from the groundwater to the atmosphere for both the m- and km-scale and the extension of the experimental monitoring in respect to vegetation. The coupled modelling platform is based on the atmospheric model COSMO, the land surface model CLM and the hydrological model ParFlow. A scale consistent two-way coupling is performed using the external OASIS coupler. Example work includes the transfer of laboratory methods to the field; the measurements of patterns of soil-carbon, evapotranspiration and respiration measured in the field; catchment-scale modeling of exchange processes and the setup of an atmospheric boundary layer monitoring network. These modern and predominantly non-invasive measurement techniques are exploited in combination with advanced modelling systems by data assimilation to yield improved numerical models for the prediction of water-, energy and CO2-transfer by accounting for the patterns occurring at various scales.

Kinetic Monte Carlo simulations of thermally activated magnetization reversal in dual-layer Exchange Coupled Composite recording media

NASA Astrophysics Data System (ADS)

Plumer, M. L.; Almudallal, A. M.; Mercer, J. I.; Whitehead, J. P.; Fal, T. J.

The kinetic Monte Carlo (KMC) method developed for thermally activated magnetic reversal processes in single-layer recording media has been extended to study dual-layer Exchange Coupled Composition (ECC) media used in current and next generations of disc drives. The attempt frequency is derived from the Langer formalism with the saddle point determined using a variant of Bellman Ford algorithm. Complication (such as stagnation) arising from coupled grains having metastable states are addressed. MH-hysteresis loops are calculated over a wide range of anisotropy ratios, sweep rates and inter-layer coupling parameter. Results are compared with standard micromagnetics at fast sweep rates and experimental results at slow sweep rates.

Permanent-Magnet Free Biasing of MR Sensors with Tunable Sensitivity

NASA Astrophysics Data System (ADS)

Halloran, Sean; Dasilva, Fabio; Pappas, David

2007-03-01

Exchange coupling^1 has been previously observed in a trilayer structure of ferromagnet (FM)/non-magnetic/antiferromagnet (AFM) and the exchange bias was found to be a function of the thickness of the buffer layer.^2,3,4 This unique coupling is used as a stabilizing bias for the sense layer with the additional ability to tailor the magnetic gain of the sensor for various applications. The elimination of permanent magnet bias results in the elimination of one patterning and one deposition step. Ruthenium (Ru) is used as the buffer layer and is self aligned with the FM and AFM layers and the thickness is varied to change the slope of the transfer curve in the linear region. Sensor devices are fabricated with a bipolar output, a medium sensitivity, and a wide field range. The results show that this biasing scheme is well suited for barber pole and soft adjacent layer (SAL) anisotropic magnetoresistance (AMR) stripes used in magnetic field sensors with a FM layer of Permalloy (NiFe) and an AFM layer of Iridium-Manganese (IrMn). Applications include a 256 channel read head used for magnetic forensics. 1N.J. Gokemeijer, T. Ambrose, C.L. Chien, N. Wang and K.K. Fung, J. Appl. Phys. 81 (8), 4999, 15 April 1997. 2W.H. Meiklejohn and C.P. Bean, Phys. Rev. 102, 1413 1956; 105, 904, 1957. 3L. Thomas, A.J. Kellock and S.S.P. Parkin, J. Appl. Phys. 87 (9), 5061, 1 May 2000. 4D. Wang, J. Daughton, C. Nordman, P. Eames and J. Fink, J. Appl. Phys. 99, 2006.

Coupling molecular spin centers to microwave resonators: steps towards the implementation of molecular qubits for hybrid quantum circuits

NASA Astrophysics Data System (ADS)

Bonizzoni, Claudio; Ghirri, Alberto; Affronte, Marco

Hybrid spin-photons quantum bits can be obtained under strong coupling regime between microwave photons and a spin ensemble, where coherent exchange of photons is realized. Molecular spins systems, thanks to their tailorable magnetic properties, are retained promising candidates for hybrid qubits. We present an experimental study of the coupling regimes between a high critical temperature YBCO superconducting resonator and different molecular spin ensembles. Three mononuclear compounds, (PPh4)2[Cu(mnt)2], [ErPc2]-TBA+ , Dy(trensal) and two organic radicals, DPPH and PyBTM, are studied. Strong coupling is found in radicals thanks to exchange narrowing. Possible strategies to achieve strong coupling with mononuclear compounds are discussed, and several hints in the design of molecular spins are given.

Mutual interactions of redox couples via electron exchange in silicate melts - Models for geochemical melt systems

NASA Technical Reports Server (NTRS)

Schreiber, Henry D.; Merkel, Robert C., Jr.; Schreiber, V. Lea; Balazs, G. Bryan

1987-01-01

The mutual interactions via electron exchange of redox couples in glass-forming melts were investigated both theoretically and experimentally. A thermodynamic approach for considering the mutual interactions leads to conclusion that the degree of mutual interaction in the melt should be proportional in part to the difference in relative reduction potentials of the interacting redox couples. Experimental studies verify this conclusion for numerous redox couples in several composition/temperature/oxygen fugacity regimes. Geochemical systems simultaneously possess many potentially multivalent elements; the stabilized redox states in the resulting magmas can be explained in part by mutual interactions and by redox buffering through the central Fe(III)- Fe(II) couples in the melts. The significance of these results for basaltic magmas of the earth, moon, and meteorites is addressed.