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Sample records for substitutionally disordered antiferromagnets

  1. Concomitant antiferromagnetic transition and disorder-induced weak localization in an interacting electron system

    NASA Astrophysics Data System (ADS)

    Ghosh, Tanmoy; Fukuda, Takashi; Kakeshita, Tomoyuki; Kaul, S. N.; Mukhopadhyay, P. K.

    2017-04-01

    In this Rapid Communication we report a phenomenon in a disordered interacting electron system. The measurements of structural, magnetic, and transport properties of FeA l2 -xG ax (0 ≤x ≤0.5 ) show that antiferromagnetic transition in these intermetallic compounds occurs concomitantly with a disorder-induced weak localization of electrons; the temperatures TN and Tm, at which antiferromagnetic transition and the weak localization respectively occur, closely track each other as the Ga concentration is varied. The antiferromagnetic transition is confirmed from the magnetic and specific heat measurements, and the occurrence of weak localization is confirmed from the temperature variation of resistivity and magnetoresistance measurements. With increasing Ga concentration, substitutional disorder in the system increases, and the consequent disorder-enhanced magnetic exchange interaction and disorder-induced fluctuations simultaneously drive antiferromagnetic transition and weak localization, respectively, to higher temperatures.

  2. Quantum phase transitions in disordered antiferromagnets

    NASA Astrophysics Data System (ADS)

    Yu, Rong

    Recently quantum phase transitions have attracted the interest of both theorists and experimentalists in condensed matter physics. Quantum magnets provide a perfect playground for studying these phase transitions since they can be triggered by many control parameters such as frustration, lattice dimerization, and magnetic field. Most previous studies have focused on the magnetic properties in pure systems. In these systems, responses to the triggering parameters are found to be uniform, leading to homogeneous phases. However little progress has been made so far on the phase transitions and properties in disordered quantum magnets because they are more complicated systems, and few theoretical tools can be applied. In this thesis we use the stochastic series expansion quantum Monte Carlo method to study quantum phase transitions in disordered magnets. We find that disordered magnets can behave quite differently from pure systems. The system inhomogeneity can strongly affect phase transitions by changing their universality class. We also find order-disorder transitions are often accompanied by the appearance of novel quantum disordered phases, in which magnetic properties behave highly nontrivial, even singular. In this thesis two examples are studied in great detail. The first one is the phase diagram of an inhomogeneous, bond-diluted two-dimensional antiferromagnet near the percolation threshold. We show that the magnetic transition can be tuned by the inhomogeneity of the dilution from a classical percolation to a quantum phase transition. Interestingly the quantum transition still takes the nature of a renormalized percolative transition, with continuously varying critical exponents. A gapless quantum disordered phase with no magnetic long-range order but geometric percolation is found. The low-temperature uniform susceptibility diverges as a non-universal power-law of the temperature in this phase, indicating that this is a quantum Griffiths phase. In the second

  3. Spin diffusion and torques in disordered antiferromagnets

    NASA Astrophysics Data System (ADS)

    Manchon, Aurelien

    2017-03-01

    We have developed a drift-diffusion equation of spin transport in collinear bipartite metallic antiferromagnets. Starting from a model tight-binding Hamiltonian, we obtain the quantum kinetic equation within Keldysh formalism and expand it to the lowest order in spatial gradient using Wigner expansion method. In the diffusive limit, these equations track the spatio-temporal evolution of the spin accumulations and spin currents on each sublattice of the antiferromagnet. We use these equations to address the nature of the spin transfer torque in (i) a spin-valve composed of a ferromagnet and an antiferromagnet, (ii) a metallic bilayer consisting of an antiferromagnet adjacent to a heavy metal possessing spin Hall effect, and in (iii) a single antiferromagnet possessing spin Hall effect. We show that the latter can experience a self-torque thanks to the non-vanishing spin Hall effect in the antiferromagnet.

  4. Spin diffusion and torques in disordered antiferromagnets.

    PubMed

    Manchon, Aurelien

    2017-03-15

    We have developed a drift-diffusion equation of spin transport in collinear bipartite metallic antiferromagnets. Starting from a model tight-binding Hamiltonian, we obtain the quantum kinetic equation within Keldysh formalism and expand it to the lowest order in spatial gradient using Wigner expansion method. In the diffusive limit, these equations track the spatio-temporal evolution of the spin accumulations and spin currents on each sublattice of the antiferromagnet. We use these equations to address the nature of the spin transfer torque in (i) a spin-valve composed of a ferromagnet and an antiferromagnet, (ii) a metallic bilayer consisting of an antiferromagnet adjacent to a heavy metal possessing spin Hall effect, and in (iii) a single antiferromagnet possessing spin Hall effect. We show that the latter can experience a self-torque thanks to the non-vanishing spin Hall effect in the antiferromagnet.

  5. Quantum Spin Liquid Emerging from Antiferromagnetic Order by Introducing Disorder.

    PubMed

    Furukawa, T; Miyagawa, K; Itou, T; Ito, M; Taniguchi, H; Saito, M; Iguchi, S; Sasaki, T; Kanoda, K

    2015-08-14

    Quantum spin liquids, which are spin versions of quantum matter, have been sought after in systems with geometrical frustration. We show that disorder drives a classical magnet into a quantum spin liquid through conducting NMR experiments on an organic Mott insulator, κ-(ET)_{2}Cu[N(CN)_{2}]Cl. Antiferromagnetic ordering in the pristine crystal, when irradiated by x rays, disappears. Spin freezing, spin gap, and critical slowing down are not observed, but gapless spin excitations emerge, suggesting a novel role of disorder that brings forth a quantum spin liquid from a classical ordered state.

  6. Quantum Spin Liquid Emerging from Antiferromagnetic Order by Introducing Disorder

    NASA Astrophysics Data System (ADS)

    Furukawa, T.; Miyagawa, K.; Itou, T.; Ito, M.; Taniguchi, H.; Saito, M.; Iguchi, S.; Sasaki, T.; Kanoda, K.

    2015-08-01

    Quantum spin liquids, which are spin versions of quantum matter, have been sought after in systems with geometrical frustration. We show that disorder drives a classical magnet into a quantum spin liquid through conducting NMR experiments on an organic Mott insulator, κ -(ET) 2Cu [N (CN) 2]Cl . Antiferromagnetic ordering in the pristine crystal, when irradiated by x rays, disappears. Spin freezing, spin gap, and critical slowing down are not observed, but gapless spin excitations emerge, suggesting a novel role of disorder that brings forth a quantum spin liquid from a classical ordered state.

  7. Antiferromagnetic order induced by gadolinium substitution in Bi{sub 2}Se{sub 3} single crystals

    SciTech Connect

    Kim, S. W.; Jung, M. H.; Vrtnik, S.; Dolinšek, J.

    2015-06-22

    Magnetic topological insulators can serve as a fundamental platform for various spin-based device applications. We report the antiferromagnetic order induced by the magnetic impurity dopants of Gd in Gd{sub x}Bi{sub 2−x}Se{sub 3} and the systematic results with varying the Gd concentration x ( = 0.14, 0.20, 0.30, and 0.40). The antiferromagnetic order is demonstrated by the magnetic susceptibility, electrical resistivity, and specific heat measurements. The anomaly observed at T{sub N} = 6 K for x ≥ 0.30 shifts towards lower temperature with increasing the magnetic field, indicative of antiferromagnetic ground state. The Gd substitution into Bi{sub 2}Se{sub 3} enables not only tuning the magnetism from paramagnetic to antiferromagnetic for high x (≥ 0.30) but also giving a promising candidate for antiferromagnetic topological insulators.

  8. Antiferromagnetism and high- Tc superconductivity in F-substituted four-layered cuprates probed by Cu-NMR

    NASA Astrophysics Data System (ADS)

    Shimizu, S.; Mukuda, H.; Matoba, K.; Kitaoka, Y.; Tokiwa, K.; Watanabe, T.; Iyo, A.; Tanaka, Y.; Kito, H.

    2007-03-01

    We report on the onset of antiferromagnetism in F-substituted four-layered high- Tc compounds Ba2Ca3Cu4O8(O1-yFy)2 ( 2y=1.2,1.4,1.6 and 2.0) which are composed of two types of CuO2 planes in a unit cell; three inner planes (IPs) and two outer planes (OPs). The Cu-NMR study has revealed that the hole density at the OPs are slightly larger than that at the IPs, and a total carrier density decreases as F content increases. The observation of zero-field Cu-NMR spectra over a broad frequency range has demonstrated that antiferromagnetically ordered phases emerges at low temperatures for all compounds. This result is similar to the case for the five-layered cuprates HgBa2Ca4Cu5Oy where the optimally doped OP undergoes a superconducting (SC) transition, whereas the three underdoped IPs do an antiferromagnetic (AFM) transition [H. Kotegawa, et al., Phys. Rev. B 64 (2001) 064515; H. Mukuda, et al., Phys. Rev. Lett. 96 (2006) 087001]. The present result gives evidence for a coexistence of antiferromagnetism and superconductivity in four-layered high- Tc cuprates.

  9. Field-Tuned Order by Disorder in Frustrated Ising Magnets with Antiferromagnetic Interactions.

    PubMed

    Guruciaga, P C; Tarzia, M; Ferreyra, M V; Cugliandolo, L F; Grigera, S A; Borzi, R A

    2016-10-14

    We demonstrate the appearance of thermal order by disorder in Ising pyrochlores with staggered antiferromagnetic order frustrated by an applied magnetic field. We use a mean-field cluster variational method, a low-temperature expansion, and Monte Carlo simulations to characterize the order-by-disorder transition. By direct evaluation of the density of states, we quantitatively show how a symmetry-broken state is selected by thermal excitations. We discuss the relevance of our results to experiments in 2D and 3D samples and evaluate how anomalous finite-size effects could be exploited to detect this phenomenon experimentally in two-dimensional artificial systems, or in antiferromagnetic all-in-all-out pyrochlores like Nd_{2}Hf_{2}O_{7} or Nd_{2}Zr_{2}O_{7}, for the first time.

  10. Field-Tuned Order by Disorder in Frustrated Ising Magnets with Antiferromagnetic Interactions

    NASA Astrophysics Data System (ADS)

    Guruciaga, P. C.; Tarzia, M.; Ferreyra, M. V.; Cugliandolo, L. F.; Grigera, S. A.; Borzi, R. A.

    2016-10-01

    We demonstrate the appearance of thermal order by disorder in Ising pyrochlores with staggered antiferromagnetic order frustrated by an applied magnetic field. We use a mean-field cluster variational method, a low-temperature expansion, and Monte Carlo simulations to characterize the order-by-disorder transition. By direct evaluation of the density of states, we quantitatively show how a symmetry-broken state is selected by thermal excitations. We discuss the relevance of our results to experiments in 2D and 3D samples and evaluate how anomalous finite-size effects could be exploited to detect this phenomenon experimentally in two-dimensional artificial systems, or in antiferromagnetic all-in-all-out pyrochlores like Nd2 Hf2 O7 or Nd2 Zr2 O7 , for the first time.

  11. Velocity of excitations in ordered, disordered, and critical antiferromagnets

    NASA Astrophysics Data System (ADS)

    Sen, Arnab; Suwa, Hidemaro; Sandvik, Anders W.

    2015-11-01

    We test three different approaches, based on quantum Monte Carlo simulations, for computing the velocity c of triplet excitations in antiferromagnets. We consider the standard S =1 /2 one- and two-dimensional Heisenberg models, as well as a bilayer Heisenberg model at its critical point. Computing correlation functions in imaginary time and using their long-time behavior, we extract the lowest excitation energy versus momentum using improved fitting procedures and a generalized moment method. The velocity is then obtained from the dispersion relation. We also exploit winding numbers to define a cubic space-time geometry, where the velocity is obtained as the ratio of the spatial and temporal lengths of the system when all winding number fluctuations are equal. The two methods give consistent results for both ordered and critical systems, but the winding number estimator is more precise. For the Heisenberg chain, we accurately reproduce the exactly known velocity. For the two-dimensional Heisenberg model, our results are consistent with other recent calculations, but with an improved statistical precision, c =1.65847 (4 ) . We also use the hydrodynamic relation c2=ρs/χ⊥(q →0 ) between c , the spin stiffness ρs, and the transversal susceptibility χ⊥, using the smallest non-zero momentum q =2 π /L . This method also is well controlled in two dimensions, but the cubic criterion for winding numbers delivers better numerical precision. In one dimension, the hydrodynamic relation is affected by logarithmic corrections which make accurate extrapolations difficult. As an application of the winding number method, for the quantum-critical bilayer model our high-precision determination of the velocity enables us to quantitatively test, at an unprecedented level, the field-theoretic low-temperature scaling forms χ =(a /c2)T and C =(b /c2) T2 . We find agreement to within 3 % with the leading 1 /N results for the factors a and b in the O (N ) model, but the agreement

  12. Exotic versus conventional scaling and universality in a disordered bilayer quantum heisenberg antiferromagnet.

    PubMed

    Sknepnek, Rastko; Vojta, Thomas; Vojta, Matthias

    2004-08-27

    We present Monte Carlo simulations of a two-dimensional bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In contrast with exotic scaling scenarios found in other random quantum systems, the quantum phase transition in this system is characterized by a finite-disorder fixed point with power-law scaling. After accounting for corrections to scaling, with a leading irrelevant exponent of omega approximately 0.48, we find universal critical exponents z=1.310(6) and nu=1.16(3). We discuss the consequences of these findings and suggest new experiments.

  13. Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

    PubMed

    Langridge; Schmalian; Marrows; Dekadjevi; Hickey

    2000-12-04

    The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.

  14. Order by disorder in the antiferromagnetic Ising model on an elastic triangular lattice

    PubMed Central

    Shokef, Yair; Souslov, Anton; Lubensky, T. C.

    2011-01-01

    Geometrically frustrated materials have a ground-state degeneracy that may be lifted by subtle effects, such as higher-order interactions causing small energetic preferences for ordered structures. Alternatively, ordering may result from entropic differences between configurations in an effect termed order by disorder. Motivated by recent experiments in a frustrated colloidal system in which ordering is suspected to result from entropy, we consider in this paper the antiferromagnetic Ising model on a deformable triangular lattice. We calculate the displacements exactly at the microscopic level and, contrary to previous studies, find a partially disordered ground state of randomly zigzagging stripes. Each such configuration is deformed differently and thus has a unique phonon spectrum with distinct entropy, lifting the degeneracy at finite temperature. Nonetheless, due to the free-energy barriers between the ground-state configurations, the system falls into a disordered glassy state. PMID:21730164

  15. Effect of rare locally ordered regions on a disordered itinerant quantum antiferromagnet with cubic anisotropy

    NASA Astrophysics Data System (ADS)

    Narayanan, Rajesh; Vojta, Thomas

    2001-01-01

    We study the quantum phase transition of an itinerant antiferromagnet with cubic anisotropy in the presence of quenched disorder, paying particular attention to the locally ordered spatial regions that form in the Griffiths region. We derive an effective action where these rare regions are described in terms of static annealed disorder. A one-loop renormalization-group analysis of the effective action shows that for order-parameter dimensions p<4, the rare regions destroy the conventional critical behavior, and the renormalized disorder flows to infinity. For order-parameter dimensions p>4, the critical behavior is not influenced by the rare regions; it is described by the conventional dirty cubic fixed point. We also discuss the influence of the rare regions on the fluctuation-driven first-order transition in this system.

  16. Quantum Disordered State without Frustration in the Double Layer Heisenberg Antiferromagnet —Dimer Expansion and Projector Monte Carlo Study—

    NASA Astrophysics Data System (ADS)

    Hida, Kazuo

    1992-03-01

    The quantum disordered state (QDOS) of the spin 1/2 double layer square lattice Heisenberg antiferromagnet is studied. Using the dimer expansion from the limit of the large interlayer coupling J', the staggered susceptibility χ, the antiferromagnetic structure factor Sπ and the antiferromagnetic correlation length ξ are calculated up to the 6-th order in the intralayer coupling J. The ratio analysis shows that the QDOS becomes unstable against the Néel ordering at J'/J≃2.56. The critical exponents are not inconsistent with the universality class of the 3-dimensional classical Heisenberg model, suggesting that our QDOS corresponds to that expected in the 2-dimensional square lattice Heisenberg antiferromagnet with unphysically small spin (<0.276). The results of the projector Monte Carlo simulation also confirms the dimer expansion results.

  17. Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice

    NASA Astrophysics Data System (ADS)

    Chern, Gia-Wei

    2014-03-01

    The first experiments on the ``kagome bilayer'' SCGO triggered a wave of interest in kagome antiferromagnets in particular, and frustrated systems in general. A cluster of early seminal theoretical papers established kagome magnets as model systems for novel ordering phenomena, discussing in particular spin liquidity, partial order, disorder-free glassiness and order by disorder. Despite significant recent progress in understanding the ground state for the quantum S = 1 / 2 model, the nature of the low-temperature phase for the classical kagome Heisenberg antiferromagnet has remained a mystery: the non-linear nature of the fluctuations around the exponentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex energy landscape has precluded numerical simulations at low temperature. Here we present an efficient Monte Carlo algorithm which removes the latter obstacle. Our simulations detect a low-temperature regime in which correlations saturate at a remarkably small value. Feeding these results into an effective model and analyzing the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with a tripled unit cell.

  18. Effect of disorder in the frustrated Ising FCC antiferromagnet: phase diagram and stretched exponential relaxation

    NASA Astrophysics Data System (ADS)

    Ngo, V. Thanh; Hoang, D. Tien; Diep, H. T.; Campbell, I. A.

    2014-04-01

    In this paper, we study the phase transition in a face-centered-cubic antiferromagnet with Ising spins as a function of the concentration p of ferromagnetic bonds randomly introduced into the system. Such a model describes the spin-glass phase at strong bond disorder. Using the standard Monte Carlo simulation and the powerful Wang-Landau flat-histogram method, we carry out in this work intensive simulations over the whole range of p. We show that the first-order transition disappears with a tiny amount of ferromagnetic bonds, namely p 0.01, in agreement with theories and simulations on other 3D models. The antiferromagnetic long-range order is also destroyed with a very small p (≃5%). With increasing p, the system changes into a spin glass and then to a ferromagnetic phase when p > 0.65. The phase diagram in the space (Tc, p) shows an asymmetry, unlike the case of the ±J Ising spin glass on the simple cubic lattice. We calculate the relaxation time around the spin-glass transition temperature and we show that the spin autocorrelation follows a stretched exponential relaxation law where the factor b is equal to ≃1/3 at the transition as suggested by the percolation-based theory. This value is in agreement with experiments performed on various spin glasses and with Monte Carlo simulations on different SG models.

  19. Capacitive and magnetoresistive origin of magnetodielectric effects in Sm-substituted spiral antiferromagnet BiMnFe{sub 2}O{sub 6}

    SciTech Connect

    Ghara, Somnath; Sundaresan, A.; Yoo, Kyongjun; Kim, Kee Hoon

    2015-10-28

    BiMnFe{sub 2}O{sub 6} exhibits a spiral antiferromagnetic ordering below 212 K and a reentrant spin glass transition at 34 K. Further, magnetic and dielectric anomalies occur at the same temperature (T = 170 K) with a significant magnetodielectric effect. Upon substitution of Sm{sup 3+} for Bi{sup 3+} ions in Bi{sub 1−x}Sm{sub x}MnFe{sub 2}O{sub 6} (x = 0.1 and 0.2), the dielectric anomaly shifts to low temperatures (T = 135 and 72 K, respectively), whereas the magnetic anomaly develops into a weak ferromagnetism. For x = 0.2, the weak ferromagnetism occurs in a wide temperature range (90–201 K). Below 90 K, it undergoes a transition to an antiferromagnetic state. In contrast to the parent compound (x = 0), the magnetodielectric effect is observed both in the antiferromagnetic region (T < 90 K) with a maximum at the dielectric anomaly (72 K) and also in the weak ferromagnetic region. It has been shown that the magnetodielectric effect in the antiferromagnetic region has an intrinsic capacitive origin while that observed at the weak ferromagnetic region originates from magnetoresistance.

  20. Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr2Ir1-xRuxO4

    DOE PAGES

    Cao, Yue; Liu, X.; Xu, Wenhu; ...

    2017-03-06

    Here, we study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr2Ir1–xRuxO4. The maximum energy of the magnetic excitation remains robust up to x = 0.77, with a gap opening at low dopings and increasing to over 150 meV at x = 0.77. At these higher Ru concentrations, the dispersive magnetic excitations in Sr2IrO4 are rendered essentially momentum independent. Up to a Ru concentration of x = 0.77, both experiments and first-principles calculations show the Ir Jeff = 1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder.more » Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.« less

  1. Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr2Ir1 -xRuxO4

    NASA Astrophysics Data System (ADS)

    Cao, Yue; Liu, X.; Xu, Wenhu; Yin, Wei-Guo; Meyers, D.; Kim, Jungho; Casa, Diego; Upton, M. H.; Gog, Thomas; Berlijn, Tom; Alvarez, Gonzalo; Yuan, Shujuan; Terzic, Jasminka; Tranquada, J. M.; Hill, John P.; Cao, Gang; Konik, Robert M.; Dean, M. P. M.

    2017-03-01

    We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr2Ir1 -xRuxO4 . The maximum energy of the magnetic excitation remains robust up to x =0.77 , with a gap opening at low dopings and increasing to over 150 meV at x =0.77 . At these higher Ru concentrations, the dispersive magnetic excitations in Sr2IrO4 are rendered essentially momentum independent. Up to a Ru concentration of x =0.77 , both experiments and first-principles calculations show the Ir Jeff=1 /2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.

  2. Spin dynamics near a putative antiferromagnetic quantum critical point in Cu-substituted BaFe2As2 and its relation to high-temperature superconductivity

    DOE PAGES

    Kim, M. G.; Wang, M.; Tucker, G. S.; ...

    2015-12-02

    We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe0.957Cu0.043)2As2, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe2As2 and superconducting Ba(Fe1–xNix)2As2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe0.957Cu0.043)2As2, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. In addition, we show that themore » spin-spin correlation length ξ(T) increases rapidly as the temperature is lowered and find ω/T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.« less

  3. Position-Space Renormalization-Group Treatment of the Triangular Ising Antiferromagnet with Quenched Disorder

    NASA Astrophysics Data System (ADS)

    Mihalco, Michael E.; McKay, Susan R.

    2006-03-01

    We apply the Niemeijer-van Leeuwen cluster approximation [1] to frustrated Ising models on a triangular lattice. The homogeneous Ising antiferromagnet is fully frustrated and shows no ordered phase. Frustration can be relieved via the addition of quenched randomness through either dilution or the introduction of ferromagnetic bonds. The result is a rich phase diagram with different types of ordering depending upon the details of the quenched disorder. [2] Using a binning procedure to retain the full distribution of interactions under rescaling [3], we are able to calculate the phase diagram of this system, with each phase having its own characteristic attractor. This model system provides a two-dimensional example of the impacts of tunable frustration on short- and long-range order. 1. T. Niemeijer and J.M.J. van Leeuwen, Phys. Rev. Lett. 31, 1411 (1973); Physica (Utr.) 71, 17 (1974). 2. G. S. Grest and E.G. Gabl, Phys. Rev. Lett. 43, 1183 (1979); H. Kaya and A.N. Berker, Phys. Rev. E 62, 1469 (2000).; M. Robinson, M.S. Thesis, University of Maine (2003). 3. E. Hartford and S. McKay, J. Appl. Phys. 70, 6068 (1991); E. Hartford, Ph.D. Thesis, University of Maine (1994); A. Falicov, A.N. Berker, and S.R. McKay, Phys. Rev. B 51, 8266 (1995).

  4. Color ice states, weathervane modes, and order by disorder in the bilinear-biquadratic pyrochlore Heisenberg antiferromagnet

    NASA Astrophysics Data System (ADS)

    Wan, Yuan; Gingras, Michel J. P.

    2016-11-01

    We study the pyrochlore Heisenberg antiferromagnet with additional positive biquadratic interaction in the semiclassical limit. The classical ground-state manifold of the model contains an extensively large family of noncoplanar spin states known as "color ice states." Starting from a color ice state, a subset of spins may rotate collectively at no energy cost. Such excitation may be viewed in this three-dimensional system as a "membranelike" analog of the well-known weathervane modes in the classical kagome Heisenberg antiferromagnet. We investigate the weathervane modes in detail and elucidate their physical properties. Furthermore, we study the order by disorder phenomenon in this model, focusing on the role of harmonic fluctuations. Our computationally limited phase space search suggests that quantum fluctuations select three different states as the magnitude of the biquadratic interaction increases relative to the bilinear interaction, implying a sequence of phase transitions solely driven by fluctuations.

  5. Prediction of protein disorder on amino acid substitutions.

    PubMed

    Anoosha, P; Sakthivel, R; Gromiha, M Michael

    2015-12-15

    Intrinsically disordered regions of proteins are known to have many functional roles in cell signaling and regulatory pathways. The altered expression of these proteins due to mutations is associated with various diseases. Currently, most of the available methods focus on predicting the disordered proteins or the disordered regions in a protein. On the other hand, methods developed for predicting protein disorder on mutation showed a poor performance with a maximum accuracy of 70%. Hence, in this work, we have developed a novel method to classify the disorder-related amino acid substitutions using amino acid properties, substitution matrices, and the effect of neighboring residues that showed an accuracy of 90.0% with a sensitivity and specificity of 94.9 and 80.6%, respectively, in 10-fold cross-validation. The method was evaluated with a test set of 20% data using 10 iterations, which showed an average accuracy of 88.9%. Furthermore, we systematically analyzed the features responsible for the better performance of our method and observed that neighboring residues play an important role in defining the disorder of a given residue in a protein sequence. We have developed a prediction server to identify disorder-related mutations, and it is available at http://www.iitm.ac.in/bioinfo/DIM_Pred/. Copyright © 2015 Elsevier Inc. All rights reserved.

  6. Effect of magnetoelastic coupling on spin-glass behavior in Heisenberg pyrochlore antiferromagnets with bond disorder

    NASA Astrophysics Data System (ADS)

    Shinaoka, Hiroshi; Tomita, Yusuke; Motome, Yukitoshi

    2014-10-01

    Motivated by puzzling aspects of spin-glass behavior reported in frustrated magnetic materials, we theoretically investigate effects of magnetoelastic coupling in geometrically frustrated classical spin models. In particular, we consider bond-disordered Heisenberg antiferromagnets on a pyrochlore lattice coupled to local lattice distortions. By integrating out the lattice degree of freedom, we derive an effective spin-only model, the bilinear-biquadratic model with bond disorder. The effective model is analyzed by classical Monte Carlo simulations using an extended loop algorithm. First, we discuss the phase diagrams in detail by showing the comprehensive Monte Carlo data for thermodynamic and magnetic properties. We show that the spin-glass transition temperature Tf is largely enhanced by the spin-lattice coupling b in the weakly disordered regime. By considering the limit of strong spin-lattice coupling, this enhancement is ascribed to the suppression of thermal fluctuations in semidiscrete degenerate manifold formed in the presence of the spin-lattice coupling. We also find that, by increasing the strength of disorder Δ, the system shows a concomitant transition of the nematic order and spin glass at a temperature determined by b, being almost independent of Δ. This is due to the fact that the spin-glass transition is triggered by the spin collinearity developed by the nematic order. Although further-neighbor exchange interactions originating in the cooperative lattice distortions result in spin-lattice order in the weakly disordered regime, the concomitant transition remains robust with Tf almost independent of Δ. We find that the magnetic susceptibility shows hysteresis between the field-cooled and zero-field-cooled data below Tf, and that the nonlinear susceptibility shows a negative divergence at the transition. These features are common to conventional spin-glass systems. Meanwhile, we find that the specific heat exhibits a broad peak at Tf, and that the

  7. Antiferromagnetic spintronics.

    PubMed

    Jungwirth, T; Marti, X; Wadley, P; Wunderlich, J

    2016-03-01

    Antiferromagnetic materials are internally magnetic, but the direction of their ordered microscopic moments alternates between individual atomic sites. The resulting zero net magnetic moment makes magnetism in antiferromagnets externally invisible. This implies that information stored in antiferromagnetic moments would be invisible to common magnetic probes, insensitive to disturbing magnetic fields, and the antiferromagnetic element would not magnetically affect its neighbours, regardless of how densely the elements are arranged in the device. The intrinsic high frequencies of antiferromagnetic dynamics represent another property that makes antiferromagnets distinct from ferromagnets. Among the outstanding questions is how to manipulate and detect the magnetic state of an antiferromagnet efficiently. In this Review we focus on recent works that have addressed this question. The field of antiferromagnetic spintronics can also be viewed from the general perspectives of spin transport, magnetic textures and dynamics, and materials research. We briefly mention this broader context, together with an outlook of future research and applications of antiferromagnetic spintronics.

  8. Order by Quenched Disorder in the Model Triangular Antiferromagnet RbFe (MoO4 )2

    NASA Astrophysics Data System (ADS)

    Smirnov, A. I.; Soldatov, T. A.; Petrenko, O. A.; Takata, A.; Kida, T.; Hagiwara, M.; Shapiro, A. Ya.; Zhitomirsky, M. E.

    2017-07-01

    We observe a disappearance of the 1 /3 magnetization plateau and a striking change of the magnetic configuration under a moderate doping of the model triangular antiferromagnet RbFe (MoO4 )2 . The reason is an effective lifting of degeneracy of mean-field ground states by a random potential of impurities, which compensates, in the low-temperature limit, the fluctuation contribution to free energy. These results provide a direct experimental confirmation of the fluctuation origin of the ground state in a real frustrated system. The change of the ground state to a least collinear configuration reveals an effective positive biquadratic exchange provided by the structural disorder. On heating, doped samples regain the structure of a pure compound, thus allowing for an investigation of the remarkable competition between thermal and structural disorder.

  9. Spin-glass transition in bond-disordered Heisenberg antiferromagnets coupled with local lattice distortions on a pyrochlore lattice.

    PubMed

    Shinaoka, Hiroshi; Tomita, Yusuke; Motome, Yukitoshi

    2011-07-22

    Motivated by puzzling characteristics of spin-glass transitions widely observed in pyrochlore-based frustrated materials, we investigate the effects of coupling to local lattice distortions in a bond-disordered antiferromagnet on the pyrochlore lattice by extensive Monte Carlo simulations. We show that the spin-glass transition temperature T(f) is largely enhanced by the spin-lattice coupling and, furthermore, becomes almost independent of Δ in a wide range of the disorder strength Δ. The critical property of the spin-glass transition is indistinguishable from that of the canonical Heisenberg spin glass in the entire range of Δ. These peculiar behaviors are ascribed to a modification of the degenerate manifold from a continuous to semidiscrete one by spin-lattice coupling.

  10. Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice.

    PubMed

    Chern, Gia-Wei; Moessner, R

    2013-02-15

    Ever since the experiments which founded the field of highly frustrated magnetism, the kagome Heisenberg antiferromagnet has been the archetypical setting for the study of fluctuation induced exotic ordering. To this day the nature of its classical low-temperature state has remained a mystery: the nonlinear nature of the fluctuations around the exponentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex energy landscape has precluded numerical simulations at low temperature, T. Here we present an efficient Monte Carlo algorithm which removes the latter obstacle. Our simulations detect a low-temperature regime in which correlations asymptote to a remarkably small value as T→0. Feeding these results into an effective model and analyzing the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with a tripled unit cell.

  11. Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr2Ir1−xRuxO4

    DOE PAGES

    Cao, Yue; Liu, X.; Xu, Wenhu; ...

    2017-03-06

    Here, we study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr2Ir1–xRuxO4. The maximum energy of the magnetic excitation remains robust up to x = 0.77, with a gap opening at low dopings and increasing to over 150 meV at x = 0.77. At these higher Ru concentrations, the dispersive magnetic excitations in Sr2IrO4 are rendered essentially momentum independent. Up to a Ru concentration of x = 0.77, both experiments and first-principles calculations show the Ir Jeff = 1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder.more » Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.« less

  12. Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet

    SciTech Connect

    Stone, Matthew B; Podlesnyak, Andrey A; Ehlers, Georg; Huq, Ashfia; Samulon, Eric C; Shapiro, Max C; Fisher, Ian R

    2011-01-01

    We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered non-magnetic substituted spin-liquid antiferromagnet. The parent compound Ba3Mn2O8 is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic vanadium for the S = 1 manganese atoms, Ba3(Mn1-xVx)2O8, and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, x = 0.02 to 0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values. These excitations may be a precursor to a low-temperature random singlet phase which may exist in Ba3(Mn1-xVx)2O8.

  13. Memory matrix theory of the dc resistivity of a disordered antiferromagnetic metal with an effective composite operator

    NASA Astrophysics Data System (ADS)

    Freire, Hermann

    2017-09-01

    We perform the calculation of the dc resistivity as a function of temperature of the ;strange-metal; state that emerges in the vicinity of a spin-density-wave phase transition in the presence of weak disorder. This scenario is relevant to the phenomenology of many important correlated materials, such as, e.g., the pnictides, the heavy-fermion compounds and the cuprates. To accomplish this task, we implement the memory-matrix approach that allows the calculation of the transport coefficients of the model beyond the quasiparticle paradigm. Our computation is also inspired by the ɛ = 3 - d expansion in a hot-spot model embedded in d-space dimensions recently put forth by Sur and Lee (2015), in which they find a new low-energy non-Fermi liquid fixed point that is perturbatively accessible near three dimensions. As a consequence, we are able to establish here the temperature and doping dependence of the electrical resistivity at intermediate temperatures of a two-dimensional disordered antiferromagnetic metallic model with a composite operator that couples the order-parameter fluctuations to the entire Fermi surface. We argue that our present theory provides a good basis in order to unify the experimental transport data, e.g., in the cuprates and the pnictide superconductors, within a wide range of doping regimes.

  14. Order-disorder quantum phase transition in the quasi-one-dimensional spin-1/2 collinear antiferromagnetic Heisenberg model.

    PubMed

    Rufo, Sabrina; Mendonça, Griffith; Plascak, J A; de Sousa, J Ricardo

    2013-09-01

    The ground-state properties of the quasi-one-dimensional spin-1/2 antiferromagnetic Heisenberg model is investigated by using a variational method. Spins on chains along the x direction are antiferromagnetically coupled with exchange J>0, while spins between chains in the y direction are coupled either ferromagnetically (J' < 0) or antiferromagnetically (J' > 0). The staggered and the colinear antiferromagnetic magnetizations are computed and their dependence on the anisotropy parameter λ=|J'|/J is analyzed. It is found that an infinitesimal interchain coupling parameter is sufficient to stabilize a long-range order with either a staggered magnetization m_{s} (J' > 0) or a colinear antiferromagnetic magnetization m_{caf} (J' < 0), both behaving as ≃λ¹/² for λ → 0.

  15. Antiferromagnetic skyrmions

    NASA Astrophysics Data System (ADS)

    Tretiakov, Oleg; Barker, Joseph

    Skyrmions are topologically protected entities in magnetic materials which have the potential to be used in spintronics for information storage and processing. However, skyrmions in ferromagnets have some intrinsic difficulties which must be overcome to use them for spintronic applications, such as the inability to move straight along current. We show that skyrmions can also be stabilized and manipulated in antiferromagnetic materials. An antiferromagnetic skyrmion is a compound topological object with a similar but of opposite sign spin texture on each sublattice, which e.g. results in a complete cancelation of the Magnus force. We find that the composite nature of antiferromagnetic skyrmions gives rise to different dynamical behavior, both due to an applied current and temperature effects. O.A.T. and J.B. acknowledge support by the Grants-in-Aid for Scientific Research (Nos. 25800184, 25247056, 25220910 and 15H01009) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and SpinNet.

  16. Disordered SU(N) antiferromagnets and the renormalization of charged instanton gases in three dimensions

    NASA Astrophysics Data System (ADS)

    Murthy, Ganpathy

    1991-08-01

    Some generic properties of charged instanton gases are established in arbitrary dimension, including the fact that the self-energy, if it diverges, must diverge as the logarithm of the correlation length. A real-space renormalization is carried out in d=3 and recursion relations obtained under the assumption that the model can disorder without instantons. They reveal that for models with nontrivial Haldane phases, the correlation-length exponent is not altered by hedgehogs, and that generic instanton gases undergo first-order transitions in three dimensions. In special cases a second-order transition with continuously varying exponents is found.

  17. Antiferromagnetic interactions in 1D Heisenberg linear chains of 7-(4-fluorophenyl) and 7-phenyl-substituted 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals.

    PubMed

    Constantinides, Christos P; Koutentis, Panayiotis A; Rawson, Jeremy M

    2012-11-26

    7-(4-Fluorophenyl) and 7-phenyl-substituted 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals were characterized by X-ray diffraction analysis and variable-temperature magnetic susceptibility studies. The radicals pack in 1D π stacks of equally spaced slipped radicals with interplanar distances of 3.59 and 3.67 Å and longitudinal angles of 40.97 and 43.47°, respectively. Magnetic-susceptibility studies showed that both radicals exhibit antiferromagnetic interactions. Fitting the magnetic data revealed that the behavior is consistent with 1D regular linear antiferromagnetic chain with J=-12.9 cm(-1), zJ'=-0.4 cm(-1), g=2.0069 and J=-11.8 cm(-1), zJ'=-6.5 cm(-1), g=2.0071, respectively. Magnetic-exchange interactions in benzotriazinyl radicals are sensitive to the degree of slippage, and inter-radical separation and subtle changes in structure alter the fine balance between ferro- and antiferromagnetic interactions. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  18. Universal scaling of Néel temperature, staggered magnetization density, and spin-wave velocity of three-dimensional disordered and clean quantum antiferromagnets

    NASA Astrophysics Data System (ADS)

    Tan, D.-R.; Jiang, F.-J.

    2017-02-01

    The Néel temperature, staggered magnetization density, as well as the spin-wave velocity of a three-dimensional (3D) quantum Heisenberg model with antiferromagnetic disorder (randomness) are calculated using first-principles nonperturbative quantum Monte Carlo simulations. In particular, we examine the validity of universal scaling relations that are related to these three studied physical quantities. These relations are relevant to experimental data and are firmly established for clean (regular) 3D dimerized spin-1/2 Heisenberg models. Remarkably, our numerical results show that the considered scaling relations remain true for the investigated model with the introduced disorder. In addition, while the presence of disorder may change the physical properties of regular dimerized models, hence leading to different critical theories, both the obtained data of Néel temperature and staggered magnetization density in our study are fully compatible with the expected critical behavior for clean dimerized systems. As a result, it is persuasive to conclude that the related quantum phase transitions of the considered disordered model and its clean analogues are governed by the same critical theory, which is not always the case in general. Finally, we also find smooth scaling curves even emerging when both the data of the investigated disordered model as well as its associated clean system are taken into account concurrently. This in turn implies that, while in a restricted sense, the considered scaling relations for 3D spin-1/2 antiferromagnets are indeed universal.

  19. Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru

    NASA Astrophysics Data System (ADS)

    Klimczuk, T.; Wang, C. H.; Lawrence, J. M.; Xu, Q.; Durakiewicz, T.; Ronning, F.; Llobet, A.; Trouw, F.; Kurita, N.; Tokiwa, Y.; Lee, Han-Oh; Booth, C. H.; Gardner, J. S.; Bauer, E. D.; Joyce, J. J.; Zandbergen, H. W.; Movshovich, R.; Cava, R. J.; Thompson, J. D.

    2011-08-01

    We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence-band photoemission (PES) and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Γ7-doublet ground state and a Γ8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ6-excited state resides at 32 meV; however, the Γ8/Γ6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K there is a peak in specific heat that is too broad to represent a magnetic-phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand this peak also is too narrow to represent the Kondo effect in the Γ7-doublet ground state. On the basis of the field dependence of the specific heat, we argue that antiferromagnetic (AF) short-range order (SRO) (possibly coexisting with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

  20. Quantum critical point in the Sc-doped itinerant antiferromagnet TiAu

    NASA Astrophysics Data System (ADS)

    Svanidze, E.; Besara, T.; Wang, J. K.; Geiger, D.; Prochaska, L.; Santiago, J. M.; Lynn, J. W.; Paschen, S.; Siegrist, T.; Morosan, E.

    2017-06-01

    We present an experimental realization of a quantum critical point in an itinerant antiferromagnet composed of nonmagnetic constituents, TiAu. By partially substituting Ti with Sc in Ti1 -xScxAu , a doping amount of xc=0.13 ±0.01 induces a quantum critical point with minimal disorder effects. The accompanying non-Fermi liquid behavior is observed in both the resistivity ρ ∝T and specific heat Cp/T ∝-ln T , characteristic of a two-dimensional antiferromagnet. The quantum critical point is accompanied by an enhancement of the spin fluctuations, as indicated by the diverging Sommerfeld coefficient γ at x =xc .

  1. Small-cluster renormalization group in Ising and Blume-Emery-Griffiths models with ferromagnetic, antiferromagnetic, and quenched disordered magnetic interactions.

    PubMed

    Antenucci, F; Crisanti, A; Leuzzi, L

    2014-07-01

    The Ising and Blume-Emery-Griffiths (BEG) models' critical behavior is analyzed in two dimensions and three dimensions by means of a renormalization group scheme on small clusters made of a few lattice cells. Different kinds of cells are proposed for both ordered and disordered model cases. In particular, cells preserving a possible antiferromagnetic ordering under renormalization allow for the determination of the Néel critical point and its scaling indices. These also provide more reliable estimates of the Curie fixed point than those obtained using cells preserving only the ferromagnetic ordering. In all studied dimensions, the present procedure does not yield a strong-disorder critical point corresponding to the transition to the spin-glass phase. This limitation is thoroughly analyzed and motivated.

  2. Partially disordered antiferromagnetism and multiferroic behavior in a frustrated Ising system CoCl2–2SC(NH2)2

    DOE PAGES

    Mun, Eundeok; Weickert, Dagmar Franziska; Kim, Jaewook; ...

    2016-03-01

    We investigate partially disordered antiferromagnetism in CoCl2-2SC(NH2)2, in which ab-plane hexagonal layers are staggered along the c axis rather than stacked. A robust 1/3 state forms in applied magnetic fields in which the spins are locked, varying as a function of neither temperature nor field. By contrast, in zero field and applied fields at higher temperatures, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure and the specific heat, magnetization, and electric polarization down to T = 50mK and up to μ0H = 60T. The Co2+more » S = 3/2 spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements, allowing us to cleanly parametrize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Lastly, we find that magnetic exchange striction induces changes in the electric polarization up to 3μC/m2, and single-ion magnetic anisotropy effects induce a much larger electric polarization change of 300μC/m2.« less

  3. Substitutional and orientational disorder in organic crystals: a symmetry-adapted ensemble model.

    PubMed

    Habgood, Matthew; Grau-Crespo, Ricardo; Price, Sarah L

    2011-05-28

    Modelling of disorder in organic crystals is highly desirable since it would allow thermodynamic stabilities and other disorder-sensitive properties to be estimated for such systems. Two disordered organic molecular systems are modeled using a symmetry-adapted ensemble approach, in which the disordered system is treated as an ensemble of the configurations of a supercell with respect to substitution of one disorder component for another. Computation time is kept manageable by performing calculations only on the symmetrically inequivalent configurations. Calculations are presented on a substitutionally disordered system, the dichloro/dibromobenzene solid solution, and on an orientationally disordered system, eniluracil, and the resultant free energies, disorder patterns, and system properties are discussed. The results are found to be in agreement with experiment, when some physically implausible configurations are removed from the ensemble average for eniluracil, highlighting the dangers of a completely automated approach to organic crystal thermodynamics which ignores the barriers to equilibration once the crystal has been formed. This journal is © the Owner Societies 2011

  4. Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant

    SciTech Connect

    Kreyssig, Andreas; Beutier, Guillaume; Hiroto, Takanobu; Kim, Min Gyu; Tucker, Gregory S.; de Boissieu, Marc; Tamura, Ryuji; Goldman, Alan I.

    2014-09-22

    It has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have ‘aged’ at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb.

  5. Pressure-induced disordered substitution alloy in Sb2Te3.

    PubMed

    Zhao, Jinggeng; Liu, Haozhe; Ehm, Lars; Chen, Zhiqiang; Sinogeikin, Stanislav; Zhao, Yusheng; Gu, Genda

    2011-11-21

    A new type of disordered substitution alloy of Sb and Te at above 15.1 GPa was discovered by performing in situ high-pressure angle-dispersive X-ray diffraction experiments on antimony telluride (Sb(2)Te(3)), a topological insulator and thermoelectric material, at room temperature. In this disordered substitution alloy, Sb(2)Te(3) crystallizes into a monoclinic structure with the space group C2/m, which is different from the corresponding high-pressure phase of the similar isostructural compound Bi(2)Te(3). Above 19.8 GPa, Sb(2)Te(3) adopts a body-centered-cubic structure with the disordered atomic array in the crystal lattice. The in situ high-pressure experiments down to about 13 K show that Sb(2)Te(3) undergoes the same phase-transition sequence with increasing pressure at low temperature, with almost the same phase-transition pressures.

  6. Spin Disorder and Order in Quasi-2D Triangular Heisenberg Antiferromagnets: Comparative Study of FeGa2S4, Fe2Ga2S5, and NiGa2S4

    NASA Astrophysics Data System (ADS)

    Nakatsuji, S.; Tonomura, H.; Onuma, K.; Nambu, Y.; Sakai, O.; Maeno, Y.; Macaluso, R. T.; Chan, Julia Y.

    2007-10-01

    Our single crystal study reveals that the single-layer S=2 triangular Heisenberg antiferromagnet FeGa2S4 forms a frozen spin-disordered state, similar to the S=1 isostructural magnet NiGa2S4. In this state, the magnetic specific heat CM is not only insensitive to the field, but shows a T2 dependence that scales to CM of NiGa2S4, suggesting the same underlying mechanism of the 2D coherent behavior. In contrast, the bilayer system Fe2Ga2S5 exhibits a 3D antiferromagnetic order.

  7. Effects of coexisting spin disorder and antiferromagnetism on the magnetic behavior of nanostructured (Fe{sub 79}Mn{sub 21}){sub 1−x}Cu{sub x} alloys

    SciTech Connect

    Mizrahi, M. E-mail: cabrera@fisica.unlp.edu.ar; Cabrera, A. F. E-mail: cabrera@fisica.unlp.edu.ar; Desimoni, J.; Stewart, S. J.

    2014-06-07

    We report a magnetic study on nanostructured (Fe{sub 79}Mn{sub 21}){sub 1−x}Cu{sub x} (0.00 ≤ x ≤ 0.30) alloys using static magnetic measurements. The alloys are mainly composed by an antiferromagnetic fcc phase and a disordered region that displays a spin-glass-like behavior. The interplay between the antiferromagnetic and magnetically disordered phases establishes an exchange anisotropy that gives rise to a loop shift at temperatures below the freezing temperature of moments belonging to the disordered region. The loop shift is more noticeable as the Cu content increases, which also enhances the spin-glass-like features. Further, in the x = 0.30 alloy the alignment imposed by applied magnetic fields higher than 4 kOe prevail over the configuration determined by the frustration mechanism that characterizes the spin glass-like phase.

  8. Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral

    SciTech Connect

    Marisa, Mary E.; Zhou, Shiliang; Melot, Brent C.; Peaslee, Graham F.; Neilson, James R.

    2016-12-05

    Hydroxyapatite is an inorganic mineral closely resembling the mineral phase in bone. However, as a biological mineral, it is highly disordered, and its composition and atomistic structure remain poorly understood. Here, synchrotron X-ray total scattering and pair distribution function analysis methods provide insight into the nature of atomistic disorder in a synthetic bone mineral analogue, chemically substituted hydroxyapatite. By varying the effective hydrolysis rate and/or carbonate concentration during growth of the mineral, compounds with varied degrees of paracrystallinity are prepared. From advanced simulations constrained by the experimental pair distribution function and density functional theory, the paracrystalline disorder prevalent in these materials appears to result from accommodation of carbonate in the lattice through random displacement of the phosphate groups. Though many substitution modalities are likely to occur in concert, the most predominant substitution places carbonate into the mirror plane of an ideal phosphate site. Understanding the mineralogical imperfections of a biologically analogous hydroxyapatite is important not only to potential bone grafting applications but also to biological mineralization processes themselves.

  9. Disorder-Driven Quantum Phase Transition from Antiferromagnetic Metal to Insulating State in Multilayered High-Tc Cuprate (Cu,C)Ba2Ca4Cu5Oy

    NASA Astrophysics Data System (ADS)

    Mukuda, Hidekazu; Abe, Machiko; Shimizu, Sunao; Kitaoka, Yoshio; Iyo, Akira; Kodama, Yasuharu; Kito, Hijiri; Tanaka, Yasutomo; Tokiwa, Kazuyasu; Watanabe, Tsuneo

    2006-12-01

    We report on superconducting (SC) characteristics for the oxygen-reduced Cu-based five-layered high-temperature superconductor (Cu,C)Ba2Ca4Cu5Oy [Cu-1245(OPT)], which includes pyramidal outer planes (OPs) and square inner planes (IPs). As a result of a reduction in the carrier density, the superconductivity for Cu-1245(OPT) occurs at the nearly optimally doped OPs with Tc= 98 K. The onset of static antiferromagnetic (AFM) order at IPs is evidenced from the observation of zero-field Cu NMR at low temperatures, although the SC transition at OPs emerges below Tc=98 K. A disorder, which is actually mapped onto the underdoped IPs, is demonstrated to cause a quantum phase transition from AFM metal to insulating state in an underdoped regime. This finding reinforces that an AFM metallic phase exists between the AFM insulating phase and the SC phase for the ideally flat CuO2 plane without disorder, as reported for the IPs in optimally doped Hg-1245 with the same doping level as in Cu-1245(OPT).

  10. First-principles based calculation of phonon spectrain substitutionally disordered alloys

    NASA Astrophysics Data System (ADS)

    Ghosh, Subhradip

    2013-02-01

    A first-principles based solution to the longstanding problem of calculating the phonon spectra in substitutional disordered alloys where strong force-constant disorder plays a significantrole is provided by a combination of first-principles electronicstructure tools, physically reasonable models of force-constant in alloyenvironments, and the Itinerant Coherent-Potntial Approximation (ICPA) by Ghosh and co-workers (S. Ghosh et. al., Physical Review B 66, 214206 (2002)). Wehere present the salient features of such hybrid formalism and illustrate its capability by the computation of phonon spectrafor disordered alloys with large size mismatch of end point components. We demonstrate that the consideration of local environments insize-mismatched alloys is crucial in understanding the microscopicinterplay of forces between various pairs of chemical specie and a correctdepiction of these is important for computation of accurate phonondispersions in these systems.

  11. Robust spin transfer torque in antiferromagnetic tunnel junctions

    NASA Astrophysics Data System (ADS)

    Saidaoui, Hamed Ben Mohamed; Waintal, Xavier; Manchon, Aurélien

    2017-04-01

    We theoretically study the current-induced spin torque in antiferromagnetic tunnel junctions, composed of two semi-infinite antiferromagnetic layers separated by a tunnel barrier, in both clean and disordered regimes. We find that the torque enabling electrical manipulation of the Néel antiferromagnetic order parameter is out of plane, ˜n ×p , while the torque competing with the antiferromagnetic exchange is in plane, ˜n ×(p ×n ) . Here, p and n are the Néel order parameter direction of the reference and free layers, respectively. Their bias dependence shows behavior similar to that in ferromagnetic tunnel junctions, the in-plane torque being mostly linear in bias, while the out-of-plane torque is quadratic. Most importantly, we find that the spin transfer torque in antiferromagnetic tunnel junctions is much more robust against disorder than that in antiferromagnetic metallic spin valves due to the tunneling nature of spin transport.

  12. Crystal fields, disorder, and antiferromagnetic short-range order in (Yb{sub 0.24}Sn{sub 0.76})Ru

    SciTech Connect

    Klimczuk, T; Wang, C H; Lawrence, J M; Xu, Q; Durakiewicz, T; Ronning, F; Llobet, A; Trouw, F; Kurita, N; Tokiwa, Y; Lee, Han-oh; Booth, C H; Gardner, J S; Bauer, E D; Joyce, J J; Zandbergen, H W; Movshovich, R; Cava, R J; Thompson, J D

    2011-07-18

    We report extensive measurements on a new compound (Yb{sub 0.24}Sn{sub 0.76})Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L{sub 3} x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb{sup 3+} is split by the crystalline electric field (CEF) into a Γ{sub 7} doublet ground state and a Γ{sub 8} quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ{sub 6} excited state resides at 32 meV; however, the Γ{sub 8}/Γ{sub 6} transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ{sub 7} ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

  13. Vermicompost substitution influences growth, physiological disorders, fruit yield and quality of strawberry (Fragaria x ananassa Duch.).

    PubMed

    Singh, Rajbir; Sharma, R R; Kumar, Satyendra; Gupta, R K; Patil, R T

    2008-11-01

    Studies were conducted to determine the effect of vermicompost on growth, physiological disorders, fruit yield and quality of 'Chandler' strawberry. For this, 4 levels of vermicompost (2.5, 5.0, 7.5 and 10.0tha(-1)) were supplemented with inorganic fertilizers to balance fertilizer requirement of strawberry under semi-arid region of northern India. The vermicompost was incorporated into top 10cm layer of soil, which was supplemented on the basis of chemical analysis, with amount of inorganic N, P, K fertilizer calculated to equalize the recommended dose of nutrients. Vermicompost application increased plant spread (10.7%), leaf area (23.1%) and dry matter (20.7%), and increased total fruit yield (32.7%). Substitution of vermicompost drastically reduced the incidence of physiological disorders like albinism (16.1-4.5%); fruit malformation (11.5-4.0%) and occurrence of grey mould (10.4-2.1%) in strawberry indicating that vermicompost had significant role in reducing nutrient-related disorders and disease like Botrytis rot, and thereby increasing the marketable fruit yield up to 58.6% with better quality parameters. Fruit harvested from plant receiving vermicompost were firmer, have higher TSS, ascorbic acid content and lower acidity, and have attractive colour. All these parameters appeared to be dose dependent and best results were achieved @ 7.5tha(-1), however, beyond this dose of vermicompost, there was not significant influence on these parameters.

  14. Neuropsychological mechanisms of Digit Symbol Substitution Test impairment in Asperger Disorder.

    PubMed

    Yoran-Hegesh, Roni; Kertzman, Semion; Vishne, Tali; Weizman, Abraham; Kotler, Moshe

    2009-03-31

    Our aim was to investigate the neurocognitive mechanisms recruited by adolescents with Asperger Disorder (AD), in comparison to controls, and to detect the underlying mechanisms during the complex information processing required for the performance of the Digit Symbol Substitution Test (DSST). Male adolescents (n=23; mean age 15.1+/-3.6 years) with a DSM-IV diagnosis of AD were compared with a normal male control group with similar demographic characteristics (n=43; mean age: 15.1+/-3.6 years). A computerized neurocognitive battery was administered and included: Inspection Time (IT), Finger Tapping Test (FTT), Simple Reaction Time (SRT), Choice Reaction Time (CRT), Digit Running task (DRT), Stroop test and Digit Symbol Substitution Test (DSST). Adolescents with AD performed significantly worse than controls on the DSST. This impaired DSST performance was related to cognitive mechanisms different from those employed by normal controls. Motor slowness and inability to deal with increased amounts of information affected the performance of the AD group, while shifting of attention was the limiting factor in the controls. Both groups were similarly dependent on response selection. This study demonstrated differences in performance in complex cognitive tasks between adolescents with AD and normal controls that may be related to differences in neurocognitive mechanisms underlying information processing. Future neuroimaging studies are needed to clarify the neural network involved in the differences in cognitive performance between AD subjects and normal controls.

  15. Machine Learning methods in fitting first-principles total energies for substitutionally disordered solid

    NASA Astrophysics Data System (ADS)

    Gao, Qin; Yao, Sanxi; Widom, Michael

    2015-03-01

    Density functional theory (DFT) provides an accurate and first-principles description of solid structures and total energies. However, it is highly time-consuming to calculate structures with hundreds of atoms in the unit cell and almost not possible to calculate thousands of atoms. We apply and adapt machine learning algorithms, including compressive sensing, support vector regression and artificial neural networks to fit the DFT total energies of substitutionally disordered boron carbide. The nonparametric kernel method is also included in our models. Our fitted total energy model reproduces the DFT energies with prediction error of around 1 meV/atom. The assumptions of these machine learning models and applications of the fitted total energies will also be discussed. Financial support from McWilliams Fellowship and the ONR-MURI under the Grant No. N00014-11-1-0678 is gratefully acknowledged.

  16. Magnetization jumps and exchange bias induced by a partially disordered antiferromagnetic state in (FeTiO{sub 3}){sub 0.9}-(Fe{sub 2}O{sub 3}){sub 0.1}

    SciTech Connect

    Song, P.; Li, G. K.; Ma, L. Zhen, C. M.; Hou, D. L.; Wang, W. H.; Liu, E. K.; Chen, J. L.; Wu, G. H.

    2014-06-07

    Magnetization jumps (MJs) and the exchange bias (EB) effect are simultaneously observed in the mixed-spin oxide (FeTiO{sub 3}){sub 0.9}-(Fe{sub 2}O{sub 3}){sub 0.1} at 2.0 K. Dc and ac susceptibility measurements confirm a reentrant spin glass phase with a partially disordered antiferromagnetic (PDA) state below the irreversibility temperature (T{sub ir} = 60 K). Antiferromagnetic (AFM) Fe{sup 3+} clusters are nested in AFM Fe{sup 2+} lattices forming a triangular lattice, in which 2/3 of the magnetic moments order antiferromagnetically with each other leaving the remaining 1/3 “confused.” This geometric frustration in the triangular lattice leads to a PDA state that is the ground state of the AFM triangular configuration. The PDA state, in the presence of a critical trigger field, evolves into a ferromagnetic (FM) state, and induces the AFM spins of the Fe{sup 2+} ions to enter a FM state, resulting in the MJs. Meanwhile, the FM spins of Fe{sup 2+} can serve as the pinned phase, and the AFM spins of Fe{sup 3+} can serve as the pinning phase, resulting in the EB effect. Thus, we point out that the PDA state is very likely to be at the origin of the MJs and the EB effect.

  17. [Perception-production relations in substitution phonological processes in children with language disorders].

    PubMed

    Ygual-Fernández, Amparo; Cervera-Mérida, José Francisco

    2013-02-22

    It is generally agreed that the auditory perception skills of children with developmental language disorders are more limited than those of typically developing children. It is not easy to determine exactly how the capacity to discriminate and the capacity to pronounce phonemes influence each other in children with language disorders. For most authors, the inability to discriminate certain phonemes accurately causes a developmental delay in pronunciation, whereas others claim the influence is mutual. The aim of this study is to determine in which consonants perceptive difficulty is more likely to occur and in which cases there is a greater probability of difficulty when it comes to articulating them. The sample used in the study consisted of 86 children with a mean age of 4 years and 7 months. The phonological processes involved in simplifying speech were identified. Their errors were used as the basis on which to construct and apply a specific speech perception test. The relationship between the articulatory and perceptive skills of children with substitutive processes were analysed by means of two comparisons: first, in all the processes detected taken as a whole and, second, in the three most frequent substitutive processes. These analyses were carried out to determine whether the nature of the consonant implied a greater probability of perceptive difficulty. The findings provide information about a relation between the articulatory and perceptive skills, and about whether the nature of the consonant determines a higher probability of perceptive or articulatory difficulties. These results can be of value in the assessment, design and effectiveness of speech therapy programmes.

  18. Spin dynamics near a putative antiferromagnetic quantum critical point in Cu-substituted BaFe2As2 and its relation to high-temperature superconductivity

    SciTech Connect

    Kim, M. G.; Wang, M.; Tucker, G. S.; Valdivia, P. N.; Abernathy, D. L.; Chi, Songxue; Christianson, A. D.; Aczel, A. A.; Hong, T.; Heitmann, T. W.; Ran, S.; Canfield, P. C.; Bourret-Courchesne, E. D.; Kreyssig, A.; Lee, D. H.; Goldman, A. I.; McQueeney, R. J.; Birgeneau, R. J.

    2015-12-02

    We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe0.957Cu0.043)2As2, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe2As2 and superconducting Ba(Fe1–xNix)2As2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe0.957Cu0.043)2As2, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. In addition, we show that the spin-spin correlation length ξ(T) increases rapidly as the temperature is lowered and find ω/T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

  19. Microscopic model for exchange bias from grain-boundary disorder in a ferromagnet/antiferromagnet thin film with a nanocrystalline microstructure

    SciTech Connect

    Cortie, D. L.; Biternas, A. G.; Chantrell, R. W.; Wang, X. L.; Klose, F.

    2014-07-21

    Monte Carlo spin simulations were coupled to a Voronoi microstructure-generator to predict the magnitude and behavior of exchange bias in a ferromagnet/antiferromagnet (AF) thin film bilayer with a nanocrystalline microstructure. Our model accounts for the effects of irregular grain-shapes, finite-sized particles, and the possible presence of local random-fields originating from the antiferromagnet's grain-boundary regions. As the grain-boundary represents a crystal-structure distortion, we model the local effect on the exchange constants in the Gaussian approximation which can cause regions resembling a spin glass confined to an unusual 2D topology. Although an ensemble of completely disconnected AF grains isolated by non-magnetic barriers provides a small exchange bias, the introduction of a spin-glass network at the boundaries causes a four-fold enhancement in the magnitude of the loop-shift. This implies the importance of local grain-boundary behavior in defect-engineered antiferromagnets.

  20. A model to describe the inhomogeneous broadening of NQR lines in chlorohalobenzenes with orientational or substitutional disorder.

    PubMed

    Pérez, Silvina C; Zuriaga, Mariano

    2017-06-20

    A simple model to explain the NQR lineshape in solids with orientational disorder or substitutional disorder is presented. The particular case of m-chlorobromobenzene is studied. It is based on the assumption that Bromine atoms, of m-chlorobromobenzene molecules, behave as point defects in the m-dichlorobenzene lattice that modify the crystalline Electric Field Gradient. The model is also tested successfully in solid solutions of p-dichlorobenzene-p-dibromobenzene, where Bromine atoms of p-dibromobenzene molecules are assumed to be homogeneously distributed in the p-dichlorobenzene lattice. The lineshape, of others disordered chlorohalobenzenes, are also analyzed. Also, a characterization of m-chlorobromobenzene dynamics is included. In particular, there is no evidence of molecular reorientations as it is observed in the disordered phases of o-chlorobromobenzene. Copyright © 2017 Elsevier Inc. All rights reserved.

  1. STUDIES ON ANTIFERROMAGNETISM.

    DTIC Science & Technology

    The report describes studies on the antiferromagnetism of salts of metals of the first transition group. Low temperature heat capacities, x-ray and neutron diffraction, magnetic susceptibilities, visible, ultraviolet, and infrared spectra, and electronic and nuclear paramagnetic resonance were measured. The report lists the scientific publications which were issued as technical reports. (Author)

  2. Antiferromagnetism in chromium

    NASA Astrophysics Data System (ADS)

    Jaramillo, Rafael

    I present two experimental studies of the spin density wave antiferromagnetic order in elemental Chromium. The first addresses the response of the magnetic ground state to applied pressure. The spin and charge order parameters are probed at high pressure and low temperature in a diamond anvil cell using monochromatic X-ray diffraction. We find that the magnetism is suppressed exponentially with pressure, providing a canonical example of a weak-coupling, mean-field ground state, before terminating at a quantum phase transition. We confirm the harmonic relationship between the spin and charge degrees of freedom in the low temperature regime, and we identify the microscopic coupling between pressure and magnetism. The discovery of the long-sought-after quantum critical regime sets the stage for a complete study of antiferromagnetic quantum criticality in this clean model system. The second study addresses the thermodynamics and transport properties of antiferromagnetic domain structure. We find a robust thermal hysteresis in the longitudinal and Hall resistivities of sub-mm bulk Cr samples. The temperature limits of the hysteresis are correlated with domain wall fluctuations and freezing. The persistent sign of the hysteresis and the macroscopic return point memory warrant a new understanding of domain wall energetics. By combining electrical transport and X-ray microdiffraction measurements we are able to pinpoint the effects of antiferromagnetic domain walls on electron transport.

  3. Holes in Heisenberg antiferromagnets

    NASA Astrophysics Data System (ADS)

    Chen, Yang

    1990-05-01

    In this Brief Report we show that a recent model proposed by Shankar [Phys. Rev. Lett. 63, 203 (1989)], describing the motion of holes in quantum antiferromagnets is equivalent to the Schwinger model [Phys. Rev. 128, 2425 (1962)] in 1+1 dimensions. Some exact results are deduced. In addition to the superconducting long-range order found by Shankar, it is shown that there is a 2pF hole density wave existing with the superconducting pairing instability.

  4. Autism and diagnostic substitution: evidence from a study of adults with a history of developmental language disorder.

    PubMed

    Bishop, Dorothy V M; Whitehouse, Andrew J O; Watt, Helen J; Line, Elizabeth A

    2008-05-01

    Rates of diagnosis of autism have risen since 1980, raising the question of whether some children who previously had other diagnoses are now being diagnosed with autism. We applied contemporary diagnostic criteria for autism to adults with a history of developmental language disorder, to discover whether diagnostic substitution has taken place. A total of 38 adults (aged 15-31y; 31 males, seven females) who had participated in studies of developmental language disorder during childhood were given the Autism Diagnostic Observation Schedule--Generic. Their parents completed the Autism Diagnostic Interview--Revised, which relies largely on symptoms present at age 4 to 5 years to diagnose autism. Eight individuals met criteria for autism on both instruments, and a further four met criteria for milder forms of autistic spectrum disorder. Most individuals with autism had been identified with pragmatic impairments in childhood. Some children who would nowadays be diagnosed unambiguously with autistic disorder had been diagnosed with developmental language disorder in the past. This finding has implications for our understanding of the epidemiology of autism.

  5. Antiferromagnetic hedgehogs with superconducting cores

    SciTech Connect

    Goldbart, P.M.; Sheehy, D.E.

    1998-09-01

    Excitations of the antiferromagnetic state that resemble antiferromagnetic hedgehogs at large distances but are predominantly superconducting inside a core region are discussed within the context of Zhang{close_quote}s SO(5)-symmetry-based approach to the physics of high-temperature superconducting materials. Nonsingular, in contrast with their hedgehog cousins in pure antiferromagnetism, these texture excitations are what hedgehogs become when the antiferromagnetic order parameter is permitted to {open_quotes}escape{close_quotes} into superconducting directions. The structure of such excitations is determined in a simple setting, and a number of their experimental implications are examined. {copyright} {ital 1998} {ital The American Physical Society}

  6. Antiferromagnetic hedgehogs with superconducting cores

    NASA Astrophysics Data System (ADS)

    Goldbart, Paul M.; Sheehy, Daniel E.

    1998-09-01

    Excitations of the antiferromagnetic state that resemble antiferromagnetic hedgehogs at large distances but are predominantly superconducting inside a core region are discussed within the context of Zhang's SO(5)-symmetry-based approach to the physics of high-temperature superconducting materials. Nonsingular, in contrast with their hedgehog cousins in pure antiferromagnetism, these texture excitations are what hedgehogs become when the antiferromagnetic order parameter is permitted to ``escape'' into superconducting directions. The structure of such excitations is determined in a simple setting, and a number of their experimental implications are examined.

  7. Spin reorientation via antiferromagnetic coupling

    SciTech Connect

    Ranjbar, M.; Sbiaa, R.; Dumas, R. K.; Åkerman, J.; Piramanayagam, S. N.

    2014-05-07

    Spin reorientation in antiferromagnetically coupled (AFC) Co/Pd multilayers, wherein the thickness of the constituent Co layers was varied, was studied. AFC-Co/Pd multilayers were observed to have perpendicular magnetic anisotropy even for a Co sublayer thickness of 1 nm, much larger than what is usually observed in systems without antiferromagnetic coupling. When similar multilayer structures were prepared without antiferromagnetic coupling, this effect was not observed. The results indicate that the additional anisotropy energy contribution arising from the antiferromagnetic coupling, which is estimated to be around 6 × 10{sup 6} ergs/cm{sup 3}, induces the spin-reorientation.

  8. Digit Symbol Substitution test and future clinical and subclinical disorders of cognition, mobility and mood in older adults.

    PubMed

    Rosano, Caterina; Perera, Subashan; Inzitari, Marco; Newman, Anne B; Longstreth, William T; Studenski, Stephanie

    2016-09-01

    to examine whether psychomotor speed predicts individual and combined disorders in cognition, mobility and mood and if white matter hyperintensities explain these associations. longitudinal; Cardiovascular Health Study. 5,888 participants (57.6% women, 15.7% black, 75.1 (5.5), mean years (SD)). psychomotor speed (Digit Symbol Substitution Test (DSST)) and small vessel disease (white matter hyperintensities (WMH)) were measured in 1992-94. Global cognition (Modified Mini-Mental State (3MS) examination), mobility (gait speed (GS)) and mood (Center for Epidemiologic Studies Depression (CES-D) scale) were measured annually over 5 years and classified as clinical, subclinical or no disorders based on established values (3MS: 80 and 85 points; GS: 0.6 and 1.0 m/s; CES-D: 10 and 5 points). Analyses were adjusted for demographics, baseline status, education, diabetes, hypertension, ankle-arm index. among those with no disorder in cognition, mobility and mood (N = 619) in 1992-94, being in the lowest DSST quartile compared to the highest was associated with nearly twice the odds of developing 1+ clinical or subclinical disorders (N = 413) during follow-up. Associations were stronger for incident clinical disorders in cognition (OR: 8.44, p < 0.01) or mobility (OR: 9.09, p < 0.05) than for mood (OR: 1.88, p < 0.10). Results were similar after adjustment for WMH. slower psychomotor speed may serve as a biomarker of risk of clinical disorders of cognition, mobility and mood. While in part attributable to vascular brain disease, other potentially modifiable contributors may be present. Further studying the causes of psychomotor slowing with ageing might provide novel insights into age-related brain disorders. © The Author 2016. Published by Oxford University Press on behalf of the British Geriatrics Society. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

  9. Digit Symbol Substitution test and future clinical and subclinical disorders of cognition, mobility and mood in older adults

    PubMed Central

    Rosano, Caterina; Perera, Subashan; Inzitari, Marco; Newman, Anne B.; Longstreth, William T.; Studenski, Stephanie

    2016-01-01

    Objective to examine whether psychomotor speed predicts individual and combined disorders in cognition, mobility and mood and if white matter hyperintensities explain these associations. Design and setting longitudinal; Cardiovascular Health Study. Subjects 5,888 participants (57.6% women, 15.7% black, 75.1 (5.5), mean years (SD)). Methods psychomotor speed (Digit Symbol Substitution Test (DSST)) and small vessel disease (white matter hyperintensities (WMH)) were measured in 1992–94. Global cognition (Modified Mini-Mental State (3MS) examination), mobility (gait speed (GS)) and mood (Center for Epidemiologic Studies Depression (CES-D) scale) were measured annually over 5 years and classified as clinical, subclinical or no disorders based on established values (3MS: 80 and 85 points; GS: 0.6 and 1.0 m/s; CES-D: 10 and 5 points). Analyses were adjusted for demographics, baseline status, education, diabetes, hypertension, ankle–arm index. Results among those with no disorder in cognition, mobility and mood (N = 619) in 1992–94, being in the lowest DSST quartile compared to the highest was associated with nearly twice the odds of developing 1+ clinical or subclinical disorders (N = 413) during follow-up. Associations were stronger for incident clinical disorders in cognition (OR: 8.44, p < 0.01) or mobility (OR: 9.09, p < 0.05) than for mood (OR: 1.88, p < 0.10). Results were similar after adjustment for WMH. Conclusions slower psychomotor speed may serve as a biomarker of risk of clinical disorders of cognition, mobility and mood. While in part attributable to vascular brain disease, other potentially modifiable contributors may be present. Further studying the causes of psychomotor slowing with ageing might provide novel insights into age-related brain disorders. PMID:27496932

  10. Competing Bose-glass physics with disorder-induced Bose-Einstein condensation in the doped S =1 antiferromagnet Ni (Cl1-xBrx) 2-4 SC (NH2)2 at high magnetic fields

    NASA Astrophysics Data System (ADS)

    Dupont, Maxime; Capponi, Sylvain; Horvatić, Mladen; Laflorencie, Nicolas

    2017-07-01

    We study the interplay between disorder and interactions for emergent bosonic degrees of freedom induced by an external magnetic field in the Br-doped spin-gapped antiferromagnetic material Ni (Cl1-xBrx) 2-4 SC (NH2)2 (DTN X ). Building on nuclear magnetic resonance experiments at high magnetic field [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we describe the localization of isolated impurity states, providing a realistic theoretical modeling for DTN X . Going beyond single impurity localization we use quantum Monte Carlo simulations to explore many-body effects from which pairwise effective interactions lead to a (impurity-induced) Bose-Einstein condensation (BEC) revival [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017)]. We further address the question of the existence of a many-body localized Bose-glass (BG) phase in DTN X , which is found to compete with a series of a new kind of BEC regimes made out of the multi-impurity states. The global magnetic field-temperature phase diagram of DTN X reveals a very rich structure for low impurity concentration, with consecutive disorder-induced BEC minidomes separated by intervening many-body localized BG regimes. Upon increasing the impurity level, multiple mini-BEC phases start to overlap, while intermediate BG regions vanish.

  11. Antiferromagnetic Spin Seebeck Effect.

    PubMed

    Wu, Stephen M; Zhang, Wei; Kc, Amit; Borisov, Pavel; Pearson, John E; Jiang, J Samuel; Lederman, David; Hoffmann, Axel; Bhattacharya, Anand

    2016-03-04

    We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30  nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9  T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

  12. Antiferromagnetic Spin Seebeck Effect

    NASA Astrophysics Data System (ADS)

    Wu, Stephen M.; Zhang, Wei; KC, Amit; Borisov, Pavel; Pearson, John E.; Jiang, J. Samuel; Lederman, David; Hoffmann, Axel; Bhattacharya, Anand

    2016-03-01

    We report on the observation of the spin Seebeck effect in antiferromagnetic MnF2 . A device scale on-chip heater is deposited on a bilayer of MnF2 (110) (30 nm )/Pt (4 nm) grown by molecular beam epitaxy on a MgF2 (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF2 through the inverse spin Hall effect. The low temperature (2-80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T ) are applied parallel to the easy axis of the MnF2 thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

  13. Antiferromagnetic spin Seebeck effect.

    SciTech Connect

    Wu, Stephen M.; Zhang, Wei; KC, Amit; Borisov, Pavel; Pearson, John E.; Jiang, J. Samuel; Lederman, David; Hoffmann, Axel; Bhattacharya, Anand

    2016-03-03

    We report on the observation of the spin Seebeck effect in antiferromagnetic MnF2. A device scale on-chip heater is deposited on a bilayer of MnF2 (110) (30nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF2(110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF2 through the inverse spin Hall effect. The low temperature (2–80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9T) are applied parallel to the easy axis of the MnF2 thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

  14. Electrical switching of an antiferromagnet.

    PubMed

    Wadley, P; Howells, B; Železný, J; Andrews, C; Hills, V; Campion, R P; Novák, V; Olejník, K; Maccherozzi, F; Dhesi, S S; Martin, S Y; Wagner, T; Wunderlich, J; Freimuth, F; Mokrousov, Y; Kuneš, J; Chauhan, J S; Grzybowski, M J; Rushforth, A W; Edmonds, K W; Gallagher, B L; Jungwirth, T

    2016-02-05

    Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 10(6) ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics.

  15. Effects of cation disorder and size on metamagnetism in A-site substituted Pr0.5Ca0.5MnO3 system

    NASA Astrophysics Data System (ADS)

    Mavani, K. R.; Paulose, P. L.

    2005-04-01

    The effects of A-site cation disorder and size on metamagnetism of ABO3 type charge and orbital ordered Pr0.5Ca0.5MnO3 system have been studied by substituting Ba+2 for Ca+2 or La+3 for Pr+3. Substitution of 5% Ba+2 or 5% La+3 drastically reduces the critical magnetic field (Hc) for metamagnetism and induces successive steplike metamagnetic transitions at low temperatures. Interestingly, with further increase in substitution, Hc rises. We find that there is a sharp decrease in electrical resistivity corresponding to the metamagnetic transitions, which is indicative of strongly correlated magnetic and electronic transitions in these manganites.

  16. Magnetic phase diagrams of classical triangular and kagome antiferromagnets.

    PubMed

    Gvozdikova, M V; Melchy, P-E; Zhitomirsky, M E

    2011-04-27

    We investigate the effect of geometrical frustration on the H-T phase diagrams of the classical Heisenberg antiferromagnets on triangular and kagome lattices. The phase diagrams for the two models are obtained from large-scale Monte Carlo simulations. For the kagome antiferromagnet, thermal fluctuations are unable to lift degeneracy completely and stabilize translationally disordered multipolar phases. We find a substantial difference in the temperature scales of the order by disorder effect related to different degeneracy of the low- and the high-field classical ground states in the kagome antiferromagnet. In the low-field regime, the Kosterlitz-Thouless transition into a spin-nematic phase is produced by unbinding of half-quantum vortices.

  17. Piezospintronic effect in honeycomb antiferromagnets

    NASA Astrophysics Data System (ADS)

    Ulloa, Camilo; Troncoso, Roberto E.; Bender, Scott A.; Duine, R. A.; Nunez, A. S.

    2017-09-01

    The emission of pure spin currents by mechanical deformations, the piezospintronic effect, in antiferromagnets is studied. We characterize the piezospintronic effect in an antiferromagnetic honeycomb monolayer in response to external strains. It is shown that the strain tensor components can be evaluated in terms of the spin Berry phase. In addition, we propose an experimental setup to detect the piezospin current generated in the piezospintronic material through the inverse spin Hall effect. Our results apply to a wide family of two-dimensional antiferromagnetic materials without inversion symmetry, such as the transition-metal chalcogenophosphate materials M P X3 (M =V , Mn; X =S , Se, Te) and NiPSe3.

  18. Antiferromagnetic inclusions in lunar glass

    USGS Publications Warehouse

    Thorpe, A.N.; Senftle, F.E.; Briggs, Charles; Alexander, Corrine

    1974-01-01

    The magnetic susceptibility of 11 glass spherules from the Apollo 15, 16, and 17 fines and two specimens of a relatively large glass spherical shell were studied as a function of temperature from room temperature to liquid helium temperatures. All but one specimen showed the presence of antiferromagnetic inclusions. Closely spaced temperature measurements of the magnetic susceptibility below 77 K on five of the specimens showed antiferromagnetic temperature transitions (Ne??el transitions). With the exception of ilmenite in one specimen, these transitions did not correspond to any transitions in known antiferromagnetic compounds. ?? 1974.

  19. Various disordered ground states and 1/3 magnetization-plateau-like behavior in the S =1/2 Ti3 + kagome lattice antiferromagnets Rb2NaTi3F12 , Cs2NaTi3F12 , and Cs2KTi3F12

    NASA Astrophysics Data System (ADS)

    Goto, Masato; Ueda, Hiroaki; Michioka, Chishiro; Matsuo, Akira; Kindo, Koichi; Yoshimura, Kazuyoshi

    2016-09-01

    We have investigated the crystal structure and magnetic properties of three kagome lattice antiferromagnets, Rb2Na Ti3F12 , Cs2Na Ti3F12 , and Cs2K Ti3F12 , using single crystals. These compounds represent a S =1 /2 kagome system consisting of magnetic Ti3 + ions, which is expected to have negligibly small Dzyaloshinsky-Moriya interaction. The structural analyses revealed that each of the three compounds has a slightly distorted kagome lattice. The distortion of the kagome lattice becomes small as the ionic radii of constituent alkali metals increase. All three compounds have nearly the same Weiss temperature of -45 K, and the ground states are disordered and strongly depend on the distortion. The ground states of Rb2Na Ti3F12 , Cs2Na Ti3F12 , and Cs2K Ti3F12 are found to be a two-component state including approximately 1/3 nearly free spins, a gapless disordered state, and a gapped disordered state, respectively. Our experimental results suggest that the ground state of the ideal S =1 /2 Heisenberg kagome lattice antiferromagnet is gapped. In addition, the magnetization curves of Cs2Na Ti3F12 and Cs2K Ti3F12 show anomalies at approximately 1/3 of the full magnetic moment of Ti3 +, which are a notable observation of signs of the theoretically proposed 1/3 magnetization plateau in S =1 /2 kagome antiferromagnets.

  20. Surface spin disorder and spin-glass-like behaviour in manganese-substituted cobalt ferrite nanoparticles

    NASA Astrophysics Data System (ADS)

    Topkaya, R.; Akman, Ö.; Kazan, S.; Aktaş, B.; Durmus, Z.; Baykal, A.

    2012-10-01

    Manganese-substituted cobalt ferrite nanoparticles coated with triethylene glycol (TREG) have been prepared by the glycothermal reaction. The effect of Mn substitution and coating on temperature-dependent magnetic properties of the TREG-coated Mn x Co1- x Fe2O4 nanoparticles (0.0 ≤ x ≤ 0.8) with size of 5-7 nm has been investigated in the temperature range of 10-300 K in a magnetic field up to 9 T. After the irreversible processes of the magnetic hysteresis curves were completed, the high-field regions of these curves were fitted by using a `law of approach to saturation' to extract the magnetic properties, such as the effective anisotropy constant ( K eff) and the anisotropy field ( H A) etc. High coercive field of 12.6 kOe is observed in pure cobalt ferrite coated with TREG at 10 K. The low temperature unsaturated magnetization behaviour indicates the core-shell structure of the Mn x Co1- x Fe2O4 NPs. Zero-field-cooled (ZFC) and field-cooled (FC) measurements revealed superparamagnetic phase of TREG-coated Mn x Co1- x Fe2O4 nanoparticles at room temperature. The blocking and irreversibility temperatures obtained from ZFC-FC curves decrease at highest Mn concentration ( x = 0.8). The existence of spin-glass-like surface layer with freezing temperature of 215 K was established with the applied field dependence of the blocking temperatures following the de Almeida-Thouless line for the Mn0.6Co0.4Fe2O4 NPs. The shifted hysteresis loops with exchange bias field of 60 Oe and high-field irreversibility up to 60 kOe in FC M- H curve at 10 K show that spin-glass-like surface spins surrounds around ordered core material of the Mn0.6Co0.4Fe2O4 NPs. FMR measurement show that all the TREG-coated Mn x Co1- x Fe2O4 nanoparticles absorb microwave in broad field range of about ten thousands Oe. The spectra for all the samples have broad linewidth because of angular distributions of easy axis and internal fields of nanoparticles.

  1. From single nucleotide substitutions up to chromosomal deletions: genetic pause of leucism-associated disorders in animals.

    PubMed

    Fleck, Katharina; Erhardt, Georg; Lühken, Gesine

    2016-01-01

    Leucism is characterized by a complete or partial white skin and hair in combination with pigmented irides, which can be vivid blue or heterochromatic. This is due to a complete or partial lack of melanocytes. The underlying pathogenesis is a disturbed emigration or differentiation of neural crest-derived cells. Therefore, leucistic phenotypes can be associated with defects, which mainly impair sensory organs and nerves. In humans, a well-known example is the Waardenburg syndrome. Leucism-associated disorders were also described in mouse, rat, hamster, rabbit, mink, cat, dog, pig, sheep, llama, alpaca, cattle and horse. In some of these species already identified causal mutations affect the genes EDN3, EDNRB, KIT, MITF, PAX3, SILV and SOX10. Defect alleles represent different types of genetic variation, ranging from single nucleotide substitutions up to larger chromosomal deletions. Some of the defect alleles produce desired coat color patterns. In some but not all cases, available genetic tests enable breeders to avoid production of animals affected by a leucism-associated disorder.

  2. Stimulus control analysis of language disorders: A study of substitution between voiced and unvoiced consonants

    PubMed Central

    Brasolotto, Alcione G.; de Rose, Julio C.; Stoddard, Lawrence T.; de Souza, Deisy G.

    1993-01-01

    This study attempted to analyze defective stimulus control relations underlying persistent substitution between voiced and unvoiced consonants in the speech and writing of two children. A series of 20 tests was administered repeatedly. Some tests consisted of matching-to-sample tasks, with dictated words, printed words, or pictures as samples. Comparison stimuli were arranged in pairs of printed words or pictures, such that the only difference in their corresponding spoken words was the voicing of one consonant phoneme. In other tests, a stimulus (dictated word, printed word, or picture) was presented, and the subject was required to emit an oral response (repeat the dictated word, read the printed word, or name the picture) or a written response (write to dictation, copy the word, or write a picture name). Other tests required the subjects to make a same/different distinction in pairs of dictated words that did or did not differ in the voicing of a single phoneme. Results showed distinct deficit profiles for each subject, consisting of patterns of defective stimulus control relations. The subjects were able, however, to distinguish between voiced and unvoiced sounds and to produce these sounds. ImagesFig. 1Fig. 2 PMID:22477078

  3. E-Cigarettes for Immediate Smoking Substitution in Women Diagnosed with Cervical Dysplasia and Associated Disorders

    PubMed Central

    James, Shirley A.; Meier, Ellen M.; Wagener, Theodore L.; Smith, Katherine M.; Neas, Barbara R.; Beebe, Laura A.

    2016-01-01

    The aim of this study was to determine if 31 women with cervical dysplasia and associated conditions exacerbated by smoking would be successful substituting cigarettes with their choice of either nicotine replacement therapy (NRT) or electronic cigarettes (EC). Women received motivational interviewing and tried both NRT and ECs, choosing one method to use during a six-week intervention period. Daily cigarette consumption was measured at baseline, six, and 12 weeks, with differences analyzed by the Wilcoxon signed-rank test. Study analysis consisted only of women choosing to use ECs (29/31), as only two chose NRT. At the 12-week follow-up, the seven day point prevalence abstinence from smoking was 28.6%, and the median number of cigarettes smoked daily decreased from 18.5 to 5.5 (p < 0.0001). The median number of e-cigarette cartridges used dropped from 21 at the six-week follow-up to 12.5 at the 12-week follow-up. After initiating EC use, women at risk for cervical cancer were able to either quit smoking or reduce the number of cigarettes smoked per day. Although a controlled trial with a larger sample size is needed to confirm these initial results, this study suggests that using ECs during quit attempts may reduce cigarette consumption. PMID:26959042

  4. Composite spin crystal phase in antiferromagnetic chiral magnets

    NASA Astrophysics Data System (ADS)

    Osorio, S. A.; Rosales, H. D.; Sturla, M. B.; Cabra, D. C.

    2017-07-01

    We study the classical antiferromagnetic Heisenberg model on the triangular lattice with Dzyaloshinskii-Moriya interactions in a magnetic field. We focus in particular on the emergence of a composite spin crystal phase, dubbed an antiferromagnetic skyrmion lattice, that was recently observed for intermediate fields. This complex phase can be made up from three interpenetrated skyrmion lattices, one for each sublattice of the original triangular one. Following these recent numerical results, in this paper we explicitly construct the low-energy effective action that reproduces the correct phenomenology and could serve as a starting point to study the coupling to charge carriers, lattice vibrations, structural disorder, and transport phenomena.

  5. Quantum corrections to the conductivity of itinerant antiferromagnets

    NASA Astrophysics Data System (ADS)

    Muttalib, K. A.; Wölfle, P.

    2015-04-01

    We present a systematic calculation of the effects of scattering of electrons off spin waves on electron transport properties in itinerant antiferromagnetic thin films in two and three dimensions. We study various regimes set by the parameters related to the spin-wave gap, exchange energy, as well as the exchange splitting, in addition to the scales set by temperature and disorder. We find an interaction-induced quantum correction to the conductivity linear in temperature, similar to that obtained recently for ferromagnetic systems within a certain regime of disorder, although the disorder dependence is different. In addition, we explore the phase relaxation rates and the associated weak-localization corrections for both small and large spin-wave gaps. We obtain a wide variety of temperature and disorder dependence for various parameter regimes. These results should provide an alternative way to study magnetic properties of thin antiferromagnetic films, for which neutron scattering measurements could be difficult, by direct transport measurements.

  6. Huge positive magnetoresistance in antiferromagnetic double perovskite metals

    NASA Astrophysics Data System (ADS)

    Nand Singh, Viveka; Majumdar, Pinaki

    2014-07-01

    Metals with large positive magnetoresistance are rare. We demonstrate that antiferromagnetic metallic states, as have been predicted for the double perovskites, are excellent candidates for huge positive magnetoresistance. An applied field suppresses long range antiferromagnetic order leading to a state with short range antiferromagnetic correlations and strong electronic scattering. The field induced resistance ratio can be more than tenfold, at moderate field, in a structurally ordered system, and continues to be almost twofold even in systems with ˜25% antisite disorder. Although our explicit demonstration is in the context of a two- dimensional spin-fermion model of the double perovskites, the mechanism we uncover is far more general, complementary to the colossal negative magnetoresistance process, and would operate in other local moment metals that show a field driven suppression of non-ferromagnetic order.

  7. Huge positive magnetoresistance in antiferromagnetic double perovskite metals.

    PubMed

    Singh, Viveka Nand; Majumdar, Pinaki

    2014-07-23

    Metals with large positive magnetoresistance are rare. We demonstrate that antiferromagnetic metallic states, as have been predicted for the double perovskites, are excellent candidates for huge positive magnetoresistance. An applied field suppresses long range antiferromagnetic order leading to a state with short range antiferromagnetic correlations and strong electronic scattering. The field induced resistance ratio can be more than tenfold, at moderate field, in a structurally ordered system, and continues to be almost twofold even in systems with ∼ 25% antisite disorder. Although our explicit demonstration is in the context of a two- dimensional spin-fermion model of the double perovskites, the mechanism we uncover is far more general, complementary to the colossal negative magnetoresistance process, and would operate in other local moment metals that show a field driven suppression of non-ferromagnetic order.

  8. Robotic gait rehabilitation and substitution devices in neurological disorders: where are we now?

    PubMed

    Calabrò, Rocco Salvatore; Cacciola, Alberto; Bertè, Francesco; Manuli, Alfredo; Leo, Antonino; Bramanti, Alessia; Naro, Antonino; Milardi, Demetrio; Bramanti, Placido

    2016-04-01

    Gait abnormalities following neurological disorders are often disabling, negatively affecting patients' quality of life. Therefore, regaining of walking is considered one of the primary objectives of the rehabilitation process. To overcome problems related to conventional physical therapy, in the last years there has been an intense technological development of robotic devices, and robotic rehabilitation has proved to play a major role in improving one's ability to walk. The robotic rehabilitation systems can be classified into stationary and overground walking systems, and several studies have demonstrated their usefulness in patients after severe acquired brain injury, spinal cord injury and other neurological diseases, including Parkinson's disease, multiple sclerosis and cerebral palsy. In this review, we want to highlight which are the most widely used devices today for gait neurological rehabilitation, focusing on their functioning, effectiveness and challenges. Novel and promising rehabilitation tools, including the use of virtual reality, are also discussed.

  9. Electrical switching of an antiferromagnet

    NASA Astrophysics Data System (ADS)

    Jungwirth, Tomas

    Louis Néel pointed out in his Nobel lecture that while abundant and interesting from theoretical viewpoint, antiferromagnets did not seem to have any applications. Indeed, the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization make antiferromagnets hard to control by tools common in ferromagnets. Strong coupling would be achieved if the externally generated field had a sign alternating on the scale of a lattice constant at which moments alternate in AFMs. However, generating such a field has been regarded unfeasible, hindering the research and applications of these abundant magnetic materials. We have recently predicted that relativistic quantum mechanics may offer staggered current induced fields with the sign alternating within the magnetic unit cell which can facilitate a reversible switching of an antiferromagnet by applying electrical currents with comparable efficiency to ferromagnets. Among suitable materials is a high Néel temperature antiferromagnet, tetragonal-phase CuMnAs, which we have recently synthesized in the form of single-crystal epilayers structurally compatible with common semiconductors. We demonstrate electrical writing and read-out, combined with the insensitivity to magnetic field perturbations, in a proof-of-concept antiferromagnetic memory device. We acknowledge support from European Research Council Advanced Grant No. 268066.

  10. Kambersky Damping in L10 Magnetic Materials of Ordered and Disordered States with Substitutional Defects

    NASA Astrophysics Data System (ADS)

    Qu, Tao; Victora, Randall

    2015-03-01

    L10 phase alloys with high magnetic anisotropy play a key role in spintronic devices. The damping constant α represents the elimination of the magnetic energy and affects the efficiency of devices. However, the intrinsic Kambersky damping reported experimentally differs among investigators and the effect of defects on α is never investigated. Here, we apply Kambersky's torque correlation technique, within the tight-binding method, to L10 ordered and disordered alloys FePt, FePd,CoPt and CoPd. In the ordered phase, CoPt has the largest damping of 0.067 while FePd has the minimum value of 0.009 at room temperature. The calculated damping value of FePt and FePd agrees well with experiment. Artificially shifting Ef, as might be accomplished by doping with impurity atoms, shows that α follows the density of states (DOS) at Ef in these four L10 alloys. We introduce lattice defects through exchanging the positions of 3d and non-3d transition elements in 36 atom supercells. The damping increases with reduced degree of chemical order, owing to the enhanced spin-flip channel allowed by the broken symmetry. This prediction is confirmed by measurements in FePt. It is demonstrated that this corresponds to an enhanced DOS at the Fermi level, owing to the rounding of the DOS with loss of long-range order. This work was supported primarily by C-SPIN (one of the six SRC STAR-net Centers) and partly by the MRSEC Program under Contract No. DMR-0819885.

  11. Thermophoresis of an antiferromagnetic soliton

    NASA Astrophysics Data System (ADS)

    Kim, Se Kwon; Tchernyshyov, Oleg; Tserkovnyak, Yaroslav

    2015-07-01

    We study the dynamics of an antiferromagnetic soliton under a temperature gradient. To this end, we start by phenomenologically constructing the stochastic Landau-Lifshitz-Gilbert equation for an antiferromagnet with the aid of the fluctuation-dissipation theorem. We then derive the Langevin equation for the soliton's center of mass by the collective coordinate approach. An antiferromagentic soliton behaves as a classical massive particle immersed in a viscous medium. By considering a thermodynamic ensemble of solitons, we obtain the Fokker-Planck equation, from which we extract the average drift velocity of a soliton. The diffusion coefficient is inversely proportional to a small damping constant α , which can yield a drift velocity of tens of m/s under a temperature gradient of 1 K/mm for a domain wall in an easy-axis antiferromagnetic wire with α ˜10-4 .

  12. [Substitutive and dietetic approaches in childhood autistic disorder: interests and limits].

    PubMed

    Hjiej, H; Doyen, C; Couprie, C; Kaye, K; Contejean, Y

    2008-10-01

    Autism is a developmental disorder that requires specialized therapeutic approaches. Influenced by various theoretical hypotheses, therapeutic programs are typically structured on a psychodynamic, biological or educative basis. Presently, educational strategies are recommended in the treatment of autism, without excluding other approaches when they are necessary. Some authors recommend dietetic or complementary approaches to the treatment of autism, which often stimulates great interest in the parents but also provokes controversy for professionals. Nevertheless, professionals must be informed about this approach because parents are actively in demand of it. First of all, enzymatic disorders and metabolic errors are those most frequently evoked in the literature. The well-known phenylalanine hydroxylase deficit responsible for phenylketonuria has been described as being associated with autism. In this case, adapted diet prevents mental retardation and autistic symptoms. Some enzymatic errors are also corrected by supplementation with uridine or ribose for example, but these supplementations are the responsibility of specialized medical teams in the domain of neurology and cannot be applied by parents alone. Secondly, increased opoid activity due to an excess of peptides is also supposed to be at the origin of some autistic symptoms. Gluten-free or casein-free diets have thus been tested in controlled studies, with contradictory results. With such diets, some studies show symptom regression but others report negative side effects, essentially protein malnutrition. Methodological bias, small sample sizes, the use of various diagnostic criteria or heterogeneity of evaluation interfere with data analysis and interpretation, which prompted professionals to be cautious with such diets. The third hypothesis emphasized in the literature is the amino acid domain. Some autistic children lack some amino acids such as glutamic or aspartic acids for example and this deficiency

  13. Superconductivity, antiferromagnetism, and neutron scattering

    NASA Astrophysics Data System (ADS)

    Tranquada, John M.; Xu, Guangyong; Zaliznyak, Igor A.

    2014-01-01

    High-temperature superconductivity in both the copper-oxide and the iron-pnictide/chalcogenide systems occurs in close proximity to antiferromagnetically ordered states. Neutron scattering has been an essential technique for characterizing the spin correlations in the antiferromagnetic phases and for demonstrating how the spin fluctuations persist in the superconductors. While the nature of the spin correlations in the superconductors remains controversial, the neutron scattering measurements of magnetic excitations over broad ranges of energy and momentum transfers provide important constraints on the theoretical options. We present an overview of the neutron scattering work on high-temperature superconductors and discuss some of the outstanding issues.

  14. Intrinsic magnetization of antiferromagnetic textures

    NASA Astrophysics Data System (ADS)

    Tveten, Erlend G.; Müller, Tristan; Linder, Jacob; Brataas, Arne

    2016-03-01

    Antiferromagnets (AFMs) exhibit intrinsic magnetization when the order parameter spatially varies. This intrinsic spin is present even at equilibrium and can be interpreted as a twisting of the homogeneous AFM into a state with a finite spin. Because magnetic moments couple directly to external magnetic fields, the intrinsic magnetization can alter the dynamics of antiferromagnetic textures under such influence. Starting from the discrete Heisenberg model, we derive the continuum limit of the free energy of AFMs in the exchange approximation and explicitly rederive that the spatial variation of the antiferromagnetic order parameter is associated with an intrinsic magnetization density. We calculate the magnetization profile of a domain wall and discuss how the intrinsic magnetization reacts to external forces. We show conclusively, both analytically and numerically, that a spatially inhomogeneous magnetic field can move and control the position of domain walls in AFMs. By comparing our model to a commonly used alternative parametrization procedure for the continuum fields, we show that the physical interpretations of these fields depend critically on the choice of parametrization procedure for the discrete-to-continuous transition. This can explain why a significant amount of recent studies of the dynamics of AFMs, including effective models that describe the motion of antiferromagnetic domain walls, have neglected the intrinsic spin of the textured order parameter.

  15. Strong correlation induced charge localization in antiferromagnets

    PubMed Central

    Zhu, Zheng; Jiang, Hong-Chen; Qi, Yang; Tian, Chushun; Weng, Zheng-Yu

    2013-01-01

    The fate of a hole injected in an antiferromagnet is an outstanding issue of strongly correlated physics. It provides important insights into doped Mott insulators closely related to high-temperature superconductivity. Here, we report a systematic numerical study of t-J ladder systems based on the density matrix renormalization group. It reveals a surprising result for the single hole's motion in an otherwise well-understood undoped system. Specifically, we find that the common belief of quasiparticle picture is invalidated by the self-localization of the doped hole. In contrast to Anderson localization caused by disorders, the charge localization discovered here is an entirely new phenomenon purely of strong correlation origin. It results from destructive quantum interference of novel signs picked up by the hole, and since the same effect is of a generic feature of doped Mott physics, our findings unveil a new paradigm which may go beyond the single hole doped system. PMID:24002668

  16. Ising antiferromagnet on the 2-uniform lattices

    NASA Astrophysics Data System (ADS)

    Yu, Unjong

    2016-08-01

    The antiferromagnetic Ising model is investigated on the twenty 2-uniform lattices using the Monte Carlo method based on the Wang-Landau algorithm and the Metropolis algorithm to study the geometric frustration effect systematically. Based on the specific heat, the residual entropy, and the Edwards-Anderson freezing order parameter, the ground states of them were determined. In addition to the long-range-ordered phase and the spin ice phase found in the Archimedean lattices, two more phases were found. The partial long-range order is long-range order with exceptional disordered sites, which give extensive residual entropy. In the partial spin ice phase, the partial freezing phenomenon appears: A majority of sites are frozen without long-range order, but the other sites are fluctuating even at zero temperature. The spin liquid ground state was not found in the 2-uniform lattices.

  17. Effects of cation size disorder and lattice distortion on metamagnetism in phase-separated manganites

    NASA Astrophysics Data System (ADS)

    Mavani, K. R.; Paulose, P. L.

    2005-07-01

    The effects of A-site cation size disorder in ABO 3 type charge-ordered and antiferromagnetic Pr 0.5Ca 0.5MnO 3 system have been studied by substituting La 3+, Sr 2+ or Ba 2+, while keeping the valency of Mn ions and the tolerance factor ( t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO 6 octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO 6 octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites.

  18. Spin pumping torque in antiferromagnets

    NASA Astrophysics Data System (ADS)

    Semenov, Yuriy G.; Kim, Ki Wook

    2017-05-01

    A currentless, magnetic-field free mechanism for the Néel vector rotation in an antiferromagnet is proposed. An efficient torque is induced by spin pumping through charging/discharging of spin filtered electrons via a ferromagnetic layer in a spin capacitor structure consisting of the two heterogenous magnetic materials. The relatively long electron spin relaxation time in the antiferromagnet enables the electron spin polarizations to retain the exchange effective field sufficiently long to modulate the local magnetic moments and subsequently the magnetic state of the material. Precession of the sublattice magnetization is modeled based on the Néel vector formulation, theoretically demonstrating the feasibility of the physical mechanism (such as the reversal) with sub-aJ energy consumption. Its potential application to spintronic devices is also discussed.

  19. Brain-specific tryptophan hydroxylase 2 (TPH2): a functional Pro206Ser substitution and variation in the 5'-region are associated with bipolar affective disorder.

    PubMed

    Cichon, Sven; Winge, Ingeborg; Mattheisen, Manuel; Georgi, Alexander; Karpushova, Anna; Freudenberg, Jan; Freudenberg-Hua, Yun; Babadjanova, Gulia; Van Den Bogaert, Ann; Abramova, Lilia I; Kapiletti, Sofia; Knappskog, Per M; McKinney, Jeffrey; Maier, Wolfgang; Jamra, Rami Abou; Schulze, Thomas G; Schumacher, Johannes; Propping, Peter; Rietschel, Marcella; Haavik, Jan; Nöthen, Markus M

    2008-01-01

    The neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] controls a broad range of biological functions that are disturbed in affective disorder. In the brain, 5-HT production is controlled by tryptophan hydroxylase 2 (TPH2). In order to assess the possible contribution of TPH2 genetic variability to the aetiology of bipolar affective disorder (BPAD), we systematically investigated common and rare genetic variation in the TPH2 gene through a sequential sequencing and SNP-based genotyping approach. Our study sample comprised two cohorts of BPAD from Germany and Russia, totalling 883 patients and 1300 controls. SNPs located in a haplotype block covering the 5' region of the gene as well as a rare, non-synonymous SNP, resulting in a Pro206Ser substitution, showed significant association with bipolar disorder. The odds ratio for the minor allele in the pooled sample was 1.5 (95% CI 1.2-1.9) for rs11178997 (in the 5'-associated haplotype block) and 4.8 (95% CI 1.6-14.8) for rs17110563 encoding the Pro206Ser substitution. Examination of the functional effects of TPH2 Pro206Ser provided evidence for a reduced thermal stability and solubility of the mutated enzyme, suggesting reduced 5-HT production in the brain as a pathophysiological mechanism in BPAD.

  20. Antiferromagnetic spin flop and exchange bias

    NASA Astrophysics Data System (ADS)

    Nogués, J.; Morellon, L.; Leighton, C.; Ibarra, M. R.; Schuller, Ivan K.

    2000-03-01

    The effect of the antiferromagnetic spin flop on exchange bias has been investigated in antiferromagnetic (MnF2)-ferromagnetic (Fe) bilayers. Cooling and measuring in fields larger than the antiferromagnetic spin-flop field, HSF, causes an irreversible reduction of the magnitude of the exchange bias field, HE. This indicates that, contrary to what is normally assumed, the interface spin structure does not remain ``frozen in'' below TN if large enough fields are applied.

  1. Ferromagnetic and antiferromagnetic orderings in wurtzite diluted magnetic nanostructures

    NASA Astrophysics Data System (ADS)

    Tronc, P.; Kitaev, Yu. E.; Hayn, R.; Strelchuk, V.; Kolomys, O.

    2017-10-01

    Using a new approach based on symmetry analysis, we have determined the magnetic symmetry groups (Shubnikov groups) of ferro- and antiferromagnetic wurtzite nanostructures doped with magnetic atoms periodically distributed at cation sites as well as the direction of the spontaneous magnetic field. Quantum wells, nanorods (nanowires), nanotubes, and quantum dots have been considered. The direction of the spontaneous magnetic field is determined by magnetic atoms with higher C3v (3m) or Cs (m) site symmetry group (the site symmetry group is defined with respect to the doped nanostructure). When the magnetic-atom distribution becomes more or less disordered (Diluted Magnetic Semiconductors), it seems reasonable to assume that, in most of the cases, the symmetry in regions with a size of some wurtzite unit-cells remains close to that of a periodic one, hence possibly inducing local ferro- or antiferromagnetic properties. The regions can induce overall ferro- or antiferromagnetic states when they have common spontaneous magnetic field direction or, possibly, induce easy-magnetization direction or plane (direction or plane of maximal magnetization intensity for a given external magnetic field strength) when disorder imposes a spread of spontaneous magnetic field direction over the various regions. Of course, such an effect adds itself to magnetic properties eventually induced by grain boundaries, other crystalline phases, and defects such as vacancies, dislocations or interstitial atoms.

  2. The Solid Solution Sr(1-x)Ba(x)Ga2: Substitutional Disorder and Chemical Bonding Visited by NMR Spectroscopy and Quantum Mechanical Calculations.

    PubMed

    Pecher, Oliver; Mausolf, Bernhard; Lamberts, Kevin; Oligschläger, Dirk; Niewieszol, Carina; Englert, Ulli; Haarmann, Frank

    2015-09-28

    Complete miscibility of the intermetallic phases (IPs) SrGa2 and BaGa2 forming the solid solution Sr(1-x)Ba(x)Ga2 is shown by means of X-ray diffraction, thermoanalytical and metallographic studies. Regarding the distances of Sr/Ba sites versus substitution degree, a model of isolated substitution centres (ISC) for up to 10% cation substitution is explored to study the influence on the Ga bonding situation. A combined application of NMR spectroscopy and quantum mechanical (QM) calculations proves the electric field gradient (EFG) to be a sensitive measure of different bonding situations. The experimental resolution is boosted by orientation-dependent NMR on magnetically aligned powder samples, revealing in first approximation two different Ga species in the ISC regimes. EFG calculations using superlattice structures within periodic boundary conditions are in fair agreement with the NMR spectroscopy data and are discussed in detail regarding their application on disordered IPs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  3. Partially disordered antiferromagnetism and multiferroic behavior in a frustrated Ising system CoCl22SC(NH2)2

    SciTech Connect

    Mun, Eundeok; Weickert, Dagmar Franziska; Kim, Jaewook; Scott, Brian L.; Miclea, Corneliu Florin; Movshovich, Roman; Wilcox, Jason; Manson, Jamie; Zapf, Vivien S.

    2016-03-01

    We investigate partially disordered antiferromagnetism in CoCl2-2SC(NH2)2, in which ab-plane hexagonal layers are staggered along the c axis rather than stacked. A robust 1/3 state forms in applied magnetic fields in which the spins are locked, varying as a function of neither temperature nor field. By contrast, in zero field and applied fields at higher temperatures, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure and the specific heat, magnetization, and electric polarization down to T = 50mK and up to μ0H = 60T. The Co2+ S = 3/2 spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements, allowing us to cleanly parametrize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Lastly, we find that magnetic exchange striction induces changes in the electric polarization up to 3μC/m2, and single-ion magnetic anisotropy effects induce a much larger electric polarization change of 300μC/m2.

  4. Substitution of Li for Cu in Cu2ZnSnS4: Toward Wide Band Gap Absorbers with Low Cation Disorder for Thin Film Solar Cells.

    PubMed

    Lafond, A; Guillot-Deudon, C; Vidal, J; Paris, M; La, C; Jobic, S

    2017-03-06

    The substitution of lithium for copper in Cu2ZnSnS4 (CZTS) has been experimentally and theoretically investigated. Formally, the (Cu1-xLix)ZnSnS4 system exhibits two well-defined solid solutions. Indeed, single crystal structural analyses demonstrate that the low (x < 0.4) and high (x > 0.6) lithium-content compounds adopt the kesterite structure and the wurtz-kesterite structure, respectively. For x between 0.4 and 0.6, the two aforementioned structure types coexist. Moreover, (119)Sn NMR analyses carried out on a (Cu0.7Li0.3)2ZnSnS4 sample clearly indicate that lithium replaces copper preferentially on two of the three available 2-fold crystallographic sites commonly occupied by Cu and Zn in disordered kesterite. Furthermore, the observed individual lines in the NMR spectrum suggest that the propensity of Cu and Zn atoms to be randomly distributed over the 2c and 2d crystallographic sites is lowered when lithium is partially substituted for copper. Additionally, the first-principles calculations provide insights into the arrangement of Li atoms as a function of the Cu/Zn disorder and its effect on the structural (lattice parameters) and optical properties of CZTS (band gap evolution). Those calculations agree with the experimental observations and account for the evolutions of the unit cell parameters as well as for the increase of band gap when the Li-content increases. The calculation of the formation enthalpy of point defect unambiguously indicates that Li modifies the Cu/Zn disorder in a manner similar to the change of Cu/Zn disorder induced by Ag alloying. Overall, it was found that Li alloying is a versatile way of tuning the optoelectronic properties of CZTS making it a good candidate as wide band gap materials for the top cells of tandem solar cells.

  5. Antiferromagnetic state in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kharitonov, Maxim

    2012-11-01

    Motivated by the recent experiment of Velasco Jr. [J. Velasco Jr. , Nat. Nanotechnology1748-338710.1038/nnano.2011.251 7, 156 (2012)], we develop a mean-field theory of the interaction-induced antiferromagnetic (AF) state in bilayer graphene at charge neutrality point at arbitrary perpendicular magnetic field B. We demonstrate that the AF state can persist at all B. At higher B, the state continuously crosses over to the AF phase of the ν=0 quantum Hall ferromagnet, recently argued to be realized in the insulating ν=0 state. The mean-field quasiparticle gap is finite at B=0 and grows with increasing B, becoming quasilinear in the quantum Hall regime, in accord with the reported behavior of the transport gap. By adjusting the two free parameters of the model, we obtain a simultaneous quantitative agreement between the experimental and theoretical values of the key parameters of the gap dependence—its zero-field value and slope at higher fields. Our findings suggest that the insulating state observed in bilayer graphene in Ref. is antiferromagnetic (canted, once the Zeeman effect is taken into account) at all magnetic fields.

  6. Spin-orbit torque in two-dimensional antiferromagnetic topological insulators

    NASA Astrophysics Data System (ADS)

    Ghosh, S.; Manchon, A.

    2017-01-01

    We investigate spin transport in two-dimensional ferromagnetic (FTI) and antiferromagnetic (AFTI) topological insulators. In the presence of an in-plane magnetization AFTI supports zero energy modes, which enables topologically protected edge conduction at low energy. We address the nature of current-driven spin torque in these structures and study the impact of spin-independent disorder. Interestingly, upon strong disorder the spin torque develops an antidamping component (i.e., even upon magnetization reversal) along the edges, which could enable current-driven manipulation of the antiferromagnetic order parameter. This antidamping torque decreases when increasing the system size and when the system enters the trivial insulator regime.

  7. Coherent bremsstrahlung of relativistic electrons in antiferromagnets

    SciTech Connect

    Kunashenko, Yu.P.

    1988-08-01

    The coherent bremsstrahlung (CB) of relativistic electrons in antiferromagnetic crystals is studied theoretically. It is shown that the internal magnetic field of an antiferromagnet has a negligible effect on CB. The complete CB spectrum is calculated for hematite, ..cap alpha..-Fe/sub 2/O/sub 3/.

  8. Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB{sub 12}

    SciTech Connect

    Sluchanko, N. E. Azarevich, A. N.; Bogach, A. V.; Vlasov, I. I.; Glushkov, V. V.; Demishev, S. V.; Maksimov, A. A.; Tartakovskii, I. I.; Filatov, E. V.; Flachbart, K.; Gabani, S.; Filippov, V. B.; Shitsevalova, N. Yu.; Moshchalkov, V. V.

    2011-09-15

    Precision measurements of the specific heat and spectral intensity I({omega}) of Raman scattering for Lu{sup N}B{sub 12} single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I({omega}) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50-70 K, which appears when Lu{sup 3+} ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12-15 and 18-22 A, respectively, have been estimated. The vibrational density of states of LuB{sub 12} has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB{sub 12} compound at T < T*, as well as the features of the formation of magnetic structures in RB{sub 12} antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB{sub 12}.

  9. Purely antiferromagnetic magnetoelectric random access memory.

    PubMed

    Kosub, Tobias; Kopte, Martin; Hühne, Ruben; Appel, Patrick; Shields, Brendan; Maletinsky, Patrick; Hübner, René; Liedke, Maciej Oskar; Fassbender, Jürgen; Schmidt, Oliver G; Makarov, Denys

    2017-01-03

    Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50-fold reduction of the writing threshold compared with ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes, we construct a comprehensive model of the magnetoelectric selection mechanisms in thin films of magnetoelectric antiferromagnets, revealing misfit induced ferrimagnetism as an important factor. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in antiferromagnetic spintronics.

  10. Purely antiferromagnetic magnetoelectric random access memory

    NASA Astrophysics Data System (ADS)

    Kosub, Tobias; Kopte, Martin; Hühne, Ruben; Appel, Patrick; Shields, Brendan; Maletinsky, Patrick; Hübner, René; Liedke, Maciej Oskar; Fassbender, Jürgen; Schmidt, Oliver G.; Makarov, Denys

    2017-01-01

    Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50-fold reduction of the writing threshold compared with ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes, we construct a comprehensive model of the magnetoelectric selection mechanisms in thin films of magnetoelectric antiferromagnets, revealing misfit induced ferrimagnetism as an important factor. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in antiferromagnetic spintronics.

  11. Electric-field-induced spin disorder-to-order transition near a multiferroic triple phase point

    NASA Astrophysics Data System (ADS)

    Jang, Byung-Kweon; Lee, Jin Hong; Chu, Kanghyun; Sharma, Pankaj; Kim, Gi-Yeop; Ko, Kyung-Tae; Kim, Kwang-Eun; Kim, Yong-Jin; Kang, Kyungrok; Jang, Han-Byul; Jang, Hoyoung; Jung, Min Hwa; Song, Kyung; Koo, Tae Yeong; Choi, Si-Young; Seidel, Jan; Jeong, Yoon Hee; Ohldag, Hendrik; Lee, Jun-Sik; Yang, Chan-Ho

    2017-02-01

    The emergence of a triple phase point in a two-dimensional parameter space (such as pressure and temperature) can offer unforeseen opportunities for the coupling of two seemingly independent order parameters. On the basis of this, we demonstrate the electric control of magnetic order by manipulating chemical pressure: lanthanum substitution in the antiferromagnetic ferroelectric BiFeO3. Our demonstration relies on the finding that a multiferroic triple phase point of a single spin-disordered phase and two spin-ordered phases emerges near room temperature in Bi0.9La0.1FeO3 ferroelectric thin films. By using spatially resolved X-ray absorption spectroscopy, we provide direct evidence that the electric poling of a particular region of the compound near the triple phase point results in an antiferromagnetic phase while adjacent unpoled regions remain magnetically disordered, opening a promising avenue for magnetoelectric applications at room temperature.

  12. Magnetic dilution and domain selection in the X Y pyrochlore antiferromagnet Er2Ti2O7

    NASA Astrophysics Data System (ADS)

    Gaudet, J.; Hallas, A. M.; Maharaj, D. D.; Buhariwalla, C. R. C.; Kermarrec, E.; Butch, N. P.; Munsie, T. J. S.; Dabkowska, H. A.; Luke, G. M.; Gaulin, B. D.

    2016-08-01

    Below TN=1.1 K, the X Y pyrochlore Er2Ti2O7 orders into a k =0 noncollinear, antiferromagnetic structure referred to as the ψ2 state. The magnetic order in Er2Ti2O7 is known to obey conventional three-dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the ψ2 structure should be unstable to the formation of a related ψ3 magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er2 -xYxTi2O7 with x =0 (pure), 0.2 (10 %Y ) and 0.4 (20 % Y ), where magnetic Er3 + is substituted by nonmagnetic Y3 +. We find that the ψ2 ground state of pure Er2Ti2O7 is significantly affected by magnetic dilution. The characteristic domain selection associated with the ψ2 state, and the corresponding energy gap separating ψ2 from ψ3, vanish for Y3 + substitutions between 10 % Y and 20 % Y , far removed from the three-dimensional percolation threshold of ˜60 % Y . The resulting ground state for Er2Ti2O7 with magnetic dilutions from 20 % Y up to the percolation threshold is naturally interpreted as a frozen mosaic of ψ2 and ψ3 domains.

  13. Antiferromagnetic topological nodal line semimetals

    NASA Astrophysics Data System (ADS)

    Wang, Jing

    2017-08-01

    We study three-dimensional nodal line semimetals (NLSMs) with magnetic ordering and strong spin-orbit interaction. Two distinct classes of magnetic NLSMs are proposed. The first class is band-inversion NLSM where the accidental line node is induced by band inversion and locally protected by glide mirror plane and the combined time-reversal and inversion symmetries. This can be viewed as a trivial stacking of the two-dimensional antiferromagnetic Dirac semimetals. The second class is essential NLSM where the nodal features are filling enforced by specific magnetic symmetry group. We further provide two concrete tight-binding models for magnetic NLSMs which belong to these two different classes, respectively. We conclude with a brief discussion on the possible material venues and the experimental implications for such phases.

  14. Femtosecond optomagnetism in dielectric antiferromagnets

    NASA Astrophysics Data System (ADS)

    Bossini, D.; Rasing, Th

    2017-02-01

    Optical femtosecond manipulation of magnetic order is attractive for the development of new concepts for ultrafast magnetic recording. Theoretical and experimental investigations in this research area aim at establishing a physical understanding of magnetic media in light-induced non-equilibrium states. Such a quest requires one to adjust the theory of magnetism, since the thermodynamical concepts of elementary excitations and spin alignment determined by the exchange interaction are not applicable on the femtosecond time-scale after the photo-excitation. Here we report some key milestones concerning the femtosecond optical control of spins in dielectric antiferromagnets, whose spin dynamics is by nature faster than that of ferromagnets and can be triggered even without any laser heating. The recent progress of the opto-magnetic effect in the sub-wavelength regime makes this exciting research area even more promising, in terms of both fundamental breakthroughs and technological perspectives.

  15. Chemical disorder determines the deviation of the Slater-Pauling rule for Fe2MnSi-based Heusler alloys: evidences from neutron diffraction and density functional theory.

    PubMed

    Tedesco, J C G; Pedro, S S; Caraballo Vivas, R J; Cruz, C; Andrade, V M; Dos Santos, A M; Carvalho, A M G; Costa, M; Venezuela, P; Rocco, D L; Reis, M S

    2016-11-30

    Fe2MnSi fails to follow the Slater-Pauling rule. This phenomenon is thought to originate from either: (i) an antiferromagnetic arrangement of Mn ions at low temperature and/or (ii) chemical disorder. An important insight on this issue could be achieved by considering Fe2MnSi1-x Ga x compounds, thoroughly studied here by means of magnetization, neutron diffraction and density functional calculations (DFT). Our results indicate that chemical disorder (and not the antiferromagnetic arrangement) is responsible for the deviation of the Slater-Pauling rule on Fe2MnSi-based Heusler alloys. Furthermore, evidences suggest that Ga substitution into Si site favors the Fe/Mn disorder, further enhancing the observed deviation.

  16. Chemical disorder determines the deviation of the Slater-Pauling rule for Fe2MnSi-based Heusler alloys: evidences from neutron diffraction and density functional theory

    NASA Astrophysics Data System (ADS)

    Tedesco, J. C. G.; Pedro, S. S.; Caraballo Vivas, R. J.; Cruz, C.; Andrade, V. M.; dos Santos, A. M.; Carvalho, A. M. G.; Costa, M.; Venezuela, P.; Rocco, D. L.; Reis, M. S.

    2016-11-01

    Fe2MnSi fails to follow the Slater-Pauling rule. This phenomenon is thought to originate from either: (i) an antiferromagnetic arrangement of Mn ions at low temperature and/or (ii) chemical disorder. An important insight on this issue could be achieved by considering Fe2MnSi1-x Ga x compounds, thoroughly studied here by means of magnetization, neutron diffraction and density functional calculations (DFT). Our results indicate that chemical disorder (and not the antiferromagnetic arrangement) is responsible for the deviation of the Slater-Pauling rule on Fe2MnSi-based Heusler alloys. Furthermore, evidences suggest that Ga substitution into Si site favors the Fe/Mn disorder, further enhancing the observed deviation.

  17. Antiferromagnetic and xy ferro-orbital order in insulating SrRuO3 thin films with SrO termination.

    PubMed

    Autieri, C

    2016-10-26

    By means of first-principles calculations we study the structural, magnetic and electronic properties of SrRuO3 surface for the SrO termination. We find that the RuO6 octahedra and the structure of the SrO layers at the surface are strongly modified as well as the Ru-O-Ru bond angles. We find in the thin films a d xy ferro-orbital order. The d xy orbital becomes the lowest in energy as in other quasitwodimensional ruthenates. Such structural rearrangement, together with a band reduction, leads to a modification of the magnetic properties. We compare the Jahn-Teller effect between the ferromagnetic and antiferromagnetic phases. We show that an insulating G-type antiferromagnetic phase takes place in SrRuO3 thin films, substituting the metallic phase experimentally found in every bulk Sr-ruthenates. The single layer SrRuO3 presents many similarities with the Ca2RuO4 low temperature phase, these similarities disappear with a larger number of layers. A study of the ground state of the as function of the number of layers is presented, the competition between bandwidth and Coulomb repulsion determines the ground state. We propose the disorder as responsible for the exchange bias effect observed.

  18. Density matrix renormalization group numerical study of the kagome antiferromagnet.

    PubMed

    Jiang, H C; Weng, Z Y; Sheng, D N

    2008-09-12

    We numerically study the spin-1/2 antiferromagnetic Heisenberg model on the kagome lattice using the density-matrix renormalization group method. We find that the ground state is a magnetically disordered spin liquid, characterized by an exponential decay of spin-spin correlation function in real space and a magnetic structure factor showing system-size independent peaks at commensurate magnetic wave vectors. We obtain a spin triplet excitation gap DeltaE(S=1)=0.055+/-0.005 by extrapolation based on the large size results, and confirm the presence of gapless singlet excitations. The physical nature of such an exotic spin liquid is also discussed.

  19. Spin Superfluidity in Biaxial Antiferromagnetic Insulators

    NASA Astrophysics Data System (ADS)

    Qaiumzadeh, Alireza; Skarsvâg, Hans; Holmqvist, Cecilia; Brataas, Arne

    2017-03-01

    Antiferromagnets may exhibit spin superfluidity since the dipole interaction is weak. We seek to establish that this phenomenon occurs in insulators such as NiO, which is a good spin conductor according to previous studies. We investigate nonlocal spin transport in a planar antiferromagnetic insulator with a weak uniaxial anisotropy. The anisotropy hinders spin superfluidity by creating a substantial threshold that the current must overcome. Nevertheless, we show that applying a high magnetic field removes this obstacle near the spin-flop transition of the antiferromagnet. Importantly, the spin superfluidity can then persist across many micrometers, even in dirty samples.

  20. Antiferromagnetic spin flop and exchange bias

    SciTech Connect

    Nogues, J.; Morellon, L.; Leighton, C.; Ibarra, M. R.; Schuller, Ivan K.

    2000-03-01

    The effect of the antiferromagnetic spin flop on exchange bias has been investigated in antiferromagnetic (MnF{sub 2})-ferromagnetic (Fe) bilayers. Cooling and measuring in fields larger than the antiferromagnetic spin-flop field, H{sub SF}, causes an irreversible reduction of the magnitude of the exchange bias field, H{sub E}. This indicates that, contrary to what is normally assumed, the interface spin structure does not remain ''frozen in'' below T{sub N} if large enough fields are applied. (c) 2000 The American Physical Society.

  1. Classical Antiferromagnetism in Kinetically Frustrated Electronic Models

    NASA Astrophysics Data System (ADS)

    Sposetti, C. N.; Bravo, B.; Trumper, A. E.; Gazza, C. J.; Manuel, L. O.

    2014-05-01

    We study, by means of the density matrix renormalization group, the infinite U Hubbard model—with one hole doped away from half filling—in triangular and square lattices with frustrated hoppings, which invalidate Nagaoka's theorem. We find that these kinetically frustrated models have antiferromagnetic ground states with classical local magnetization in the thermodynamic limit. We identify the mechanism of this kinetic antiferromagnetism with the release of the kinetic energy frustration, as the hole moves in the established antiferromagnetic background. This release can occur in two different ways: by a nontrivial spin Berry phase acquired by the hole, or by the effective vanishing of the hopping amplitude along the frustrating loops.

  2. Domain walls in antiferromagnetically coupled multilayer films.

    PubMed

    Hellwig, Olav; Berger, Andreas; Fullerton, Eric E

    2003-11-07

    We report experimentally observed magnetic domain-wall structures in antiferromagnetically coupled multilayer films with perpendicular anisotropy. Our studies reveal a first-order phase transition from domain walls with no net moment to domain walls with ferromagnetic cores. The transition originates from the competition between dipolar and exchange energies, which we tune by means of layer thickness. Although observed in a synthetic antiferromagnetic system, such domain-wall structures may be expected to occur in A-type antiferromagnets with anisotropic exchange coupling.

  3. Solitonlike magnetization textures in noncollinear antiferromagnets

    NASA Astrophysics Data System (ADS)

    Ulloa, Camilo; Nunez, A. S.

    2016-04-01

    We show that proper control of magnetization textures can be achieved in noncollinear antiferromagnets. This opens the versatile toolbox of domain-wall manipulation in the context of a different family of materials. In this way, we show that noncollinear antiferromagnets are a good prospect for applications in the context of antiferromagnetic spintronics. As in many noncollinear antiferromagnets, the order parameter field takes values in SO(3). By performing a gradient expansion in the energy functional we derive an effective theory that accounts for the physics of the magnetization of long-wavelength excitations. We apply our formalism to static and dynamic textures such as domain walls and localized oscillations, and identify topologically protected textures that are spatially localized. Our results are applicable to the exchange-bias materials Mn3X , with X =Ir,Rh,Pt .

  4. Indirect control of antiferromagnetic domain walls with spin current.

    PubMed

    Wieser, R; Vedmedenko, E Y; Wiesendanger, R

    2011-02-11

    The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.

  5. CeCoAl4: An incommensurate antiferromagnet

    NASA Astrophysics Data System (ADS)

    Dhar, S. K.; Rama, B.; Ramakrishnan, S.

    1995-08-01

    The orthorhombic compound CeCoAl4 orders antiferromagnetically at a relatively high Néel temperature TN of 13 K. The resistivity of CeCoAl4 shows a sharp increase near the magnetic transition before decreasing with temperature below 12 K. We believe that such behavior is due to the energy gaps induced by the incommensurate antiferromagnetic order. We have also studied the solid solutions LaxCe1-xCoAl4 for x=0.1 and 0.2 and CeCo1-yTyAl4 for T=Ni, Cu, and Pd. The crystal structure changes to YNiAl4-type even at low values of y (y~=0.1 and T=Ni and Pd). We find that TN decreases when La or Cu is substituted for Ce and Co, respectively. The sharp increase in the resistivity near TN in CeCoAl4 is almost smeared out in these pseudoternaries.

  6. Synthetic antiferromagnetic nanoparticles with tunable susceptibilities

    PubMed Central

    Hu, Wei; Wilson, Robert J.; Earhart, Christopher M.; Koh, Ai Leen; Sinclair, Robert; Wang, Shan X.

    2009-01-01

    High-moment monodisperse disk-shaped Co–Fe magnetic nanoparticles, stable in aqueous solution, were physically fabricated by using nanoimprinted templates and vacuum deposition techniques. These multilayer synthetic antiferromagnetic nanoparticles exhibit nearly zero magnetic remanence and coercivity, and susceptibilities which can be tuned by exploiting interlayer magnetic interactions. In addition, a low cost method of scaling up the production of sub-100 nm synthetic antiferromagnetic nanoparticles is demonstrated. PMID:19529797

  7. Kinetic antiferromagnetism in the triangular lattice.

    PubMed

    Haerter, Jan O; Shastry, B Sriram

    2005-08-19

    We show that the motion of a single hole in the infinite-U Hubbard model with frustrated hopping leads to weak metallic antiferromagnetism of kinetic origin. An intimate relationship is demonstrated between the simplest versions of this problem in one and two dimensions, and two of the most subtle many body problems, namely, the Heisenberg Bethe ring in one dimension and the two-dimensional triangular lattice Heisenberg antiferromagnet.

  8. Magnon Spin Nernst Effect in Antiferromagnets

    NASA Astrophysics Data System (ADS)

    Zyuzin, Vladimir A.; Kovalev, Alexey A.

    2016-11-01

    We predict that a temperature gradient can induce a magnon-mediated spin Hall response in an antiferromagnet with nontrivial magnon Berry curvature. We develop a linear response theory which gives a general condition for a Hall current to be well defined, even when the thermal Hall response is forbidden by symmetry. We apply our theory to a honeycomb lattice antiferromagnet and discuss a role of magnon edge states in a finite geometry.

  9. Constructing a magnetic handle for antiferromagnetic manganites

    SciTech Connect

    Glavic, Artur; Dixit, Hemant; Cooper, Valentino R.; Aczel, Adam A.

    2016-04-27

    An intrinsic property of antiferromagnetic materials is the compensation of the magnetic moments from the individual atoms that prohibits the direct interaction of the spin lattice with an external magnetic field. To overcome this limitation we have created artificial spin structures by heteroepitaxy between two bulk antiferromagnets SrMnO 3 and NdMnO 3 . We demonstrate that charge transfer at the interface results in the creation of thin ferromagnetic layers adjacent to A -type antiferromagnetism in thick NdMnO 3 layers. A novel interference based neutron diffraction technique and polarized neutron reflectometry are used to confirm the presence of ferromagnetism in the SrMnO 3 layers and to probe the relative alignment of antiferromagnetic spins induced by the coupling at the ferro- to antiferromagnet interface. A density functional theory analysis of the driving forces for the exchange reveals strong ferromagnetic interfacial coupling through quantifiable short range charge transfer. Our results confirm a layer-by-layer control of magnetic arrangements that constitutes a promising step on a path towards isothermal magnetic control of antiferromagnetic arrangements as would be necessary in spin-based heterostructures like multiferroic devices.

  10. Constructing a magnetic handle for antiferromagnetic manganites

    DOE PAGES

    Glavic, Artur; Dixit, Hemant; Cooper, Valentino R.; ...

    2016-04-27

    An intrinsic property of antiferromagnetic materials is the compensation of the magnetic moments from the individual atoms that prohibits the direct interaction of the spin lattice with an external magnetic field. To overcome this limitation we have created artificial spin structures by heteroepitaxy between two bulk antiferromagnets SrMnO 3 and NdMnO 3 . We demonstrate that charge transfer at the interface results in the creation of thin ferromagnetic layers adjacent to A -type antiferromagnetism in thick NdMnO 3 layers. A novel interference based neutron diffraction technique and polarized neutron reflectometry are used to confirm the presence of ferromagnetism in themore » SrMnO 3 layers and to probe the relative alignment of antiferromagnetic spins induced by the coupling at the ferro- to antiferromagnet interface. A density functional theory analysis of the driving forces for the exchange reveals strong ferromagnetic interfacial coupling through quantifiable short range charge transfer. Our results confirm a layer-by-layer control of magnetic arrangements that constitutes a promising step on a path towards isothermal magnetic control of antiferromagnetic arrangements as would be necessary in spin-based heterostructures like multiferroic devices.« less

  11. Roughness effects in uncompensated antiferromagnets

    SciTech Connect

    Charilaou, M.; Hellman, F.

    2015-02-28

    Monte Carlo simulations show that roughness in uncompensated antiferromagnets decreases not just the surface magnetization but also the net magnetization and particularly strongly affects the temperature dependence. In films with step-type roughness, each step creates a new compensation front that decreases the global net magnetization. The saturation magnetization decreases non-monotonically with increasing roughness and does not scale with the surface area. Roughness in the form of surface vacancies changes the temperature-dependence of the magnetization; when only one surface has vacancies, the saturation magnetization will decrease linearly with surface occupancy, whereas when both surfaces have vacancies, the magnetization is negative and exhibits a compensation point at finite temperature, which can be tuned by controlling the occupancy. Roughness also affects the spin-texture of the surfaces due to long-range dipolar interactions and generates non-collinear spin configurations that could be used in devices to produce locally modified exchange bias. These results explain the strongly reduced magnetization found in magnetometry experiments and furthers our understanding of the temperature-dependence of exchange bias.

  12. Quantum annealing with antiferromagnetic fluctuations.

    PubMed

    Seki, Yuya; Nishimori, Hidetoshi

    2012-05-01

    We introduce antiferromagnetic quantum fluctuations into quantum annealing in addition to the conventional transverse-field term. We apply this method to the infinite-range ferromagnetic p-spin model, for which the conventional quantum annealing has been shown to have difficulties in finding the ground state efficiently due to a first-order transition. We study the phase diagram of this system both analytically and numerically. Using the static approximation, we find that there exists a quantum path to reach the final ground state from the trivial initial state that avoids first-order transitions for intermediate values of p. We also study numerically the energy gap between the ground state and the first excited state and find evidence for intermediate values of p for which the time complexity scales polynomially with the system size at a second-order transition point along the quantum path that avoids first-order transitions. These results suggest that quantum annealing would be able to solve this problem with intermediate values of p efficiently, in contrast to the case with only simple transverse-field fluctuations.

  13. Spin superconductivity in the frustrated two-dimensional antiferromagnet in the square lattice

    NASA Astrophysics Data System (ADS)

    Lima, L. S.

    2017-02-01

    We use the SU(2) Schwinger boson formalism to study the spin transport in the two-dimensional S = 1 / 2 frustrated Heisenberg antiferromagnet in a square lattice, considering the second-neighbors interactions in the diagonal. We have obtained a spin superfluid behavior for the spin transport to this system similar to obtained recently to the triangular lattice. We consider an antiferromagnetic inter-chain coupling on the diagonal, J2 > 0 , and the nearest-neighbor coupling antiferromagnetic J1 > 0 . We also have in the critical temperature T0, where the correlation length ξ → 0 , that the system suffers a transition from an ordered ground state to a disordered ground state.

  14. Ab initio investigation of competing antiferromagnetic structures in low Si-content FeMn(PSi) alloy.

    PubMed

    Li, Guijiang; Eriksson, Olle; Johansson, Börje; Vitos, Levente

    2016-06-02

    The antiferromagnetic structures of a low Si-content FeMn(PSi) alloy were investigated by first principles calculations. One possible antiferromagnetic structure in supercell along the c-axis was revealed in FeMnP0.75Si0.25 alloy. It was found that atomic disorder occupation between Fe atom on 3f and Mn atoms on 3g sites is responsible for the formation of antiferromagnetic structures. Furthermore the magnetic competition and the coupling between possible AFM supercells along the c and a-axis can promote a non-collinear antiferromagnetic structure. These theoretical investigations help to deeply understand the magnetic order in FeMn(PSi) alloys and benefit to explore the potential magnetocaloric materials in Fe2P-type alloys.

  15. Synthesis and characterization of the new two-dimensional Heisenberg antiferromagnet double perovskite BaLaCuSbO6.

    PubMed

    Blanco, M Cecilia; Paz, Sergio Alexis; Nassif, Vivian M; Guimpel, Julio J; Carbonio, Raúl E

    2015-06-21

    The BaLaCuSbO(6) double perovskite has been successfully synthesized by solid state reaction under an air atmosphere. Its structure was refined using powder neutron diffraction in the monoclinic space group I2/m with a 4% antisite disorder on the B cations. Magnetic measurements give signs of 2D-antiferromagnetic behaviour with TN around 64 K. The Jahn-Teller distortion produced by Cu(2+) ions favours a crystallographic tetragonal distortion and consequently the in-plane super-superexchange antiferromagnetic interactions, J(90°), are favoured over the in-plane J(180°) antiferromagnetic exchange interactions. Both, J and J' magnetic interactions have been evaluated according to a Heisenberg antiferromagnetic rectangular model using an approximation to Curie's law in powers of J/T, being |J| around 10 times stronger than |J'|.

  16. Exchange bias mechanism at the ferromagnetic/antiferromagnetic interface with rotatable antiferromagnetic spins: A Monte Carlo study

    NASA Astrophysics Data System (ADS)

    Hu, Yong; Wang, Yuying; Chi, Xiaodan; Li, Xuesi; Du, An; Shi, Feng

    2017-08-01

    We perform modified Monte Carlo simulations on a ferromagnetic/antiferromagnetic bilayer structure with adjustable antiferromagnetic anisotropy and degree of exchange coupling. Generally, both the antiferromagnetic anisotropy and the degree of exchange coupling at the ferromagnetic/antiferromagnetic interface are difficult to be directly detected experimentally. However, they may play crucial roles in establishing the exchange bias properties through determining whether the antiferromagnetic spins at the interface are rotatable or pinned. Therefore, we precisely calculated the numbers of rotatable and pinned antiferromagnetic spins at the interface and analyzed their contribution to exchange bias and coercivity in the specified ranges of antiferromagnetic anisotropy and degree of exchange coupling. The simulation results may help to clarify the experimental controversies concerning the occurrence of exchange bias effect prior to the detection of pinned uncompensated antiferromagnetic spins. They can also be used to properly interpret the dependence of exchange bias on several-nanometer antiferromagnetic layer thicknesses.

  17. Antiferromagnetic Domain Wall Motion Driven by Spin-Orbit Torques.

    PubMed

    Shiino, Takayuki; Oh, Se-Hyeok; Haney, Paul M; Lee, Seo-Won; Go, Gyungchoon; Park, Byong-Guk; Lee, Kyung-Jin

    2016-08-19

    We theoretically investigate the dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet-heavy-metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin waves in the terahertz frequency range. The interplay between spin-orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals from antiferromagnets.

  18. Antiferromagnetic domain wall motion driven by spin-orbit torques

    PubMed Central

    Shiino, Takayuki; Oh, Se-Hyeok; Haney, Paul M.; Lee, Seo-Won; Go, Gyungchoon; Park, Byong-Guk; Lee, Kyung-Jin

    2016-01-01

    We theoretically investigate dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet/heavy metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin-waves in the terahertz frequency range. The interplay between spin-orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals from antiferromagnets. PMID:27588878

  19. Substitute Solutions.

    ERIC Educational Resources Information Center

    Jones, Kevin R.; Hawkins, Amber

    2000-01-01

    In summer 1999, a group of Park City, Utah, school administrators, personnel directors, human-resource specialists, and substitute teacher coordinators brainstormed on improving the recruitment, training, and retention of substitute teachers. Providing effective preservice and on-the-job training and professional recognition are key suggestions.…

  20. An itinerant antiferromagnetic metal without magnetic constituents

    PubMed Central

    Svanidze, E.; Wang, Jiakui K.; Besara, T.; Liu, L.; Huang, Q.; Siegrist, T.; Frandsen, B.; Lynn, J. W.; Nevidomskyy, Andriy H.; Gamża, Monika B.; Aronson, M. C.; Uemura, Y. J.; Morosan, E.

    2015-01-01

    The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems. PMID:26166042

  1. An itinerant antiferromagnetic metal without magnetic constituents.

    PubMed

    Svanidze, E; Wang, Jiakui K; Besara, T; Liu, L; Huang, Q; Siegrist, T; Frandsen, B; Lynn, J W; Nevidomskyy, Andriy H; Gamża, Monika B; Aronson, M C; Uemura, Y J; Morosan, E

    2015-07-13

    The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant-electron systems.

  2. Purely antiferromagnetic magnetoelectric random access memory

    PubMed Central

    Kosub, Tobias; Kopte, Martin; Hühne, Ruben; Appel, Patrick; Shields, Brendan; Maletinsky, Patrick; Hübner, René; Liedke, Maciej Oskar; Fassbender, Jürgen; Schmidt, Oliver G.; Makarov, Denys

    2017-01-01

    Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50-fold reduction of the writing threshold compared with ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes, we construct a comprehensive model of the magnetoelectric selection mechanisms in thin films of magnetoelectric antiferromagnets, revealing misfit induced ferrimagnetism as an important factor. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in antiferromagnetic spintronics. PMID:28045029

  3. An itinerant antiferromagnetic metal without magnetic constituents

    SciTech Connect

    Svanidze, E.; Wang, Jiakui K.; Besara, T.; Liu, L.; Huang, Q.; Siegrist, T.; Frandsen, B.; Lynn, J. W.; Nevidomskyy, Andriy H.; Gamża, Monika B.; Aronson, M. C.; Uemura, Y. J.; Morosan, E.

    2015-07-13

    The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. In conclusion, this itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems.

  4. An itinerant antiferromagnetic metal without magnetic constituents

    DOE PAGES

    Svanidze, E.; Wang, Jiakui K.; Besara, T.; ...

    2015-07-13

    The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn2 and Sc3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemedmore » crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. In conclusion, this itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems.« less

  5. Field-Induced Multiple Reentrant Quantum Phase Transitions in Randomly Dimerized Antiferromagnetic S=1/2 Heisenberg Chains

    NASA Astrophysics Data System (ADS)

    Hida, Kazuo

    2006-07-01

    The multiple reentrant quantum phase transitions in the S=1/2 antiferromagnetic Heisenberg chains with random bond alternation in the magnetic field are investigated by the density matrix renormalization group method combined with interchain mean field approximation. It is assumed that odd numbered bonds are antiferromagnetic with strength J and even numbered bonds can take the values JS and JW (JS > J > JW > 0) randomly with the probabilities p and 1- p, respectively. The pure version ( p=0 and 1) of this model has a spin gap but exhibits a field-induced antiferromagnetism in the presence of interchain coupling if Zeeman energy due to the magnetic field exceeds the spin gap. For 0 < p < 1, antiferromagnetism is induced by randomness at the small field region where the ground state is disordered due to the spin gap in the pure version. At the same time, this model exhibits randomness-induced plateaus at several values of magnetization. The antiferromagnetism is destroyed on the plateaus. As a consequence, we find a series of reentrant quantum phase transitions between transverse antiferromagnetic phases and disordered plateau phases with the increase of magnetic field for a moderate strength of interchain coupling. Above the main plateaus, the magnetization curve consists of a series of small plateaus and jumps between them. It is also found that antiferromagnetism is induced by infinitesimal interchain coupling at the jumps between the small plateaus. We conclude that this antiferromagnetism is supported by the mixing of low-lying excited states by the staggered interchain mean field even though the spin correlation function is short ranged in the ground state of each chain.

  6. [Bone substitutes].

    PubMed

    Jordana, Fabienne; Le Visage, Catherine; Weiss, Pierre

    2017-01-01

    Bone substitutes, used to fill a defect after a surgery or a trauma, provide a mechanical support and might induce bone healing. They constitute an alternative to autogenous bone grafts, the 'gold standard' which remains the reference despite its risk of postoperative complications. The clinician choice of a bone substitute is based on the required bone volume, the handling (injectability, malleability) and mechanical properties (setting time, viscosity, resorbability among others) of the material. Bone substitutes are commonly used in orthopedic surgery, neurosurgery, stomatology and dental applications. Their use increases steadily, with the recent clinical development of injectable forms. In addition, novel technologies by subtractive or additive techniques allow today the production of controlled architecture materials. Here, we present a bone substitutes classification according to their origin (natural or synthetic) and chemical composition, and the most common use of these substitutes. © 2017 médecine/sciences – Inserm.

  7. Skin Substitutes

    PubMed Central

    Howe, Nicole; Cohen, George

    2014-01-01

    In a relatively short timespan, a wealth of new skin substitutes made of synthetic and biologically derived materials have arisen for the purpose of wound healing of various etiologies. This review article focuses on providing an overview of skin substitutes including their indications, contraindications, benefits, and limitations. The result of this overview was an appreciation of the vast array of options available for clinicians, many of which did not exist a short time ago. Yet, despite the rapid expansion this field has undergone, no ideal skin substitute is currently available. More research in the field of skin substitutes and wound healing is required not only for the development of new products made of increasingly complex biomolecular material, but also to compare the existing skin substitutes. PMID:25371771

  8. Room Temperature Antiferromagnetic Ordering of Nanocrystalline Tb1.90Ni0.10O3

    NASA Astrophysics Data System (ADS)

    Mandal, J.; Dalal, M.; Sarkar, B. J.; Chakrabarti, P. K.

    2017-02-01

    Nanocrystalline Ni-doped terbium oxide (Tb1.90Ni0.10O3) has been synthesized by the co-precipitation method followed by annealing at 700°C for 6 h in vacuum. The crystallographic phase and the substitution of Ni2+ ions in the lattice of Tb2O3 are confirmed by Rietveld analysis of the x-ray diffraction pattern using the software MAUD. High-resolution transmission electron microscopy is also carried out to study the morphology of the sample. Magnetic measurements are carried out at different temperatures from 5 K to 300 K using a superconducting quantum interference device (SQUID) magnetometer. The dependence of the magnetization of Tb1.90Ni0.10O3 as a function of temperature ( M- T) and magnetic field ( M- H) suggests the presence of both paramagnetic and antiferromagnetic phase at room temperature, but antiferromagnetic phase dominates below ˜120 K. The lack of saturation in the M- H curve and good fitting of the M- T curve by the Johnston formula also indicate the presence of both paramagnetic and antiferromagnetic phase at room temperature. Interestingly, an antiferromagnetic to ferromagnetic phase transition is observed below ˜40 K. The result also shows a high value of magnetization at 5 K.

  9. A temperature-induced order-disorder phase transition in a 4-substituted 4,2':6',4''-terpyridine.

    PubMed

    Granifo, Juan; Westermeyer, Marleen; Riquelme, Maricel; Gaviño, Rubén; Suárez, Sebastián; Halac, Emilia B; Baggio, Ricardo

    2015-12-01

    Crystals of 4'-(isoquinolin-4-yl)-4,2':6',4''-terpyridine (iqtp), C24H16N4, grown from an ethanol solution, undergo a reversible first-order single-crystal to single-crystal phase transition at Tc in the range 273-275 K, from a disordered higher-temperature phase [form (I)] in the space group P21/c, with one single molecule in the asymmetric unit, to an ordered lower-temperature one [form (II)] in the space group P21/n, with two independent molecules in the asymmetric unit. There is a group-subgroup relationship linking (I)-(II), due to cell doubling and the disappearance of a number of symmetry operations. In addition to X-ray diffraction, the transition has been monitored by Raman spectroscopy and differential scanning calorimetry, the latter disclosing an enthalpy change of 0.72 (6) kJ mol(-1). Variations of the unit-cell parameters with temperature between 170 and 293 K are presented. The evolution of diffraction spots in the vicinity of the transition temperature shows the coexistence of both phases, confirming the first-order character of the transition. Structural details of both phases are analyzed and intermolecular interactions compared in order to investigate the mechanism of the phase transition. A three-dimensional Hirshfeld surface analysis was performed to corroborate the significant changes in the intermolecular features.

  10. Effect of side-chain asymmetry on the intermolecular structure and order-disorder transition in alkyl-substituted polyfluorenes

    NASA Astrophysics Data System (ADS)

    Knaapila, M.; Stepanyan, R.; Torkkeli, M.; Haase, D.; Fröhlich, N.; Helfer, A.; Forster, M.; Scherf, U.

    2016-04-01

    We study relations among the side-chain asymmetry, structure, and order-disorder transition (ODT) in hairy-rod-type poly(9,9-dihexylfluorene) (PF6) with two identical side chains and atactic poly(9-octyl-9-methyl-fluorene) (PF1-8) with two different side chains per repeat. PF6 and PF1-8 organize into alternating side-chain and backbone layers that transform into an isotropic phase at TODT(PF 6 ) and TbiODT(PF 1 -8 ) . We interpret polymers in terms of monodisperse and bidisperse brushes and predict scenarios TODT

  11. Itinerant Antiferromagnetism in RuO2

    DOE PAGES

    Berlijn, Tom; Snijders, Paul C.; Delaire, Oliver A.; ...

    2017-02-15

    Bulk rutile RuO2 has long been considered a Pauli paramagnet. Here, in this article, we report that RuO2 exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05μB as evidenced by polarized neutron diffraction. Density functional theory plus U(DFT+U) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutilemore » crystal field. The combination of high Néel temperature and small itinerant moments make RuO2 unique among ruthenate compounds and among oxide materials in general.« less

  12. Antiferromagnetic Spin Wave Field-Effect Transistor

    PubMed Central

    Cheng, Ran; Daniels, Matthew W.; Zhu, Jian-Gang; Xiao, Di

    2016-01-01

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. Our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale. PMID:27048928

  13. Itinerant Antiferromagnetism in RuO2

    NASA Astrophysics Data System (ADS)

    Berlijn, T.; Snijders, P. C.; Delaire, O.; Zhou, H.-D.; Maier, T. A.; Cao, H.-B.; Chi, S.-X.; Matsuda, M.; Wang, Y.; Koehler, M. R.; Kent, P. R. C.; Weitering, H. H.

    2017-02-01

    Bulk rutile RuO2 has long been considered a Pauli paramagnet. Here we report that RuO2 exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05 μB as evidenced by polarized neutron diffraction. Density functional theory plus U (DFT +U ) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Néel temperature and small itinerant moments make RuO2 unique among ruthenate compounds and among oxide materials in general.

  14. Antiferromagnetic Spin Wave Field-Effect Transistor

    SciTech Connect

    Cheng, Ran; Daniels, Matthew W.; Zhu, Jian-Gang; Xiao, Di

    2016-04-06

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. In conclusion, our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.

  15. Antiferromagnetic Spin Wave Field-Effect Transistor

    DOE PAGES

    Cheng, Ran; Daniels, Matthew W.; Zhu, Jian-Gang; ...

    2016-04-06

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. In conclusion, our findings open up the exciting possibilitymore » of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.« less

  16. Antiferromagnetic Spin Wave Field-Effect Transistor

    NASA Astrophysics Data System (ADS)

    Cheng, Ran; Daniels, Matthew W.; Zhu, Jian-Gang; Xiao, Di

    2016-04-01

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. Our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.

  17. Paramagnetic and Antiferromagnetic Spin Seebeck Effect

    NASA Astrophysics Data System (ADS)

    Wu, Stephen

    We report on the observation of the longitudinal spin Seebeck effect in both antiferromagnetic and paramagnetic insulators. By using a microscale on-chip local heater, it is possible to generate a large thermal gradient confined to the chip surface without a large increase in the total sample temperature. This technique allows us to easily access low temperatures (200 mK) and high magnetic fields (14 T) through conventional dilution refrigeration and superconducting magnet setups. By exploring this regime, we detect the spin Seebeck effect through the spin-flop transition in antiferromagnetic MnF2 when a large magnetic field (>9 T) is applied along the easy axis direction. Using the same technique, we are also able to resolve a spin Seebeck effect from the paramagnetic phase of geometrically frustrated antiferromagnet Gd3Ga5O12 (gadolinium gallium garnet) and antiferromagnetic DyScO3 (DSO). Since these measurements occur above the ordering temperatures of these two materials, short-range magnetic order is implicated as the cause of the spin Seebeck effect in these systems. The discovery of the spin Seebeck effect in these two materials classes suggest that both antiferromagnetic spin waves and spin excitations from short range magnetic order may be used to generate spin current from insulators and that the spin wave spectra of individual materials are highly important to the specifics of the longitudinal spin Seebeck effect. Since insulating antiferromagnets and paramagnets are far more common than the typical insulating ferrimagnetic materials used in spin Seebeck experiments, this discovery opens up a large new class of materials for use in spin caloritronic devices. All authors acknowledge support of the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials, was supported by the U.S. DOE, BES under Contract No. DE-AC02-06CH11357.

  18. Antiferromagnetic domains in epitaxial thin films

    NASA Astrophysics Data System (ADS)

    Scholl, Andreas

    2002-03-01

    Interface and surface effects play a central role in modern magnet structures. Magnetic exchange coupling and bias, spin injection across the boundary between magnetic and non-magnetic layers, and the surface and interface anisotropy in multilayers are examples for interface phenomena that are utilized in magneto-electronics. In particular, the microscopic origin of exchange bias at ferromagnet/antiferromagnet interfaces is still an unsolved problem despite of intense research, driven by the important application of exchange bias in hard disk read-heads and magnetic RAM. Knowledge of the microscopic magnetic structure in antiferromagnetic thin films and surfaces is of crucial importance for a better understanding of the exchange bias effect. Microscopic experiments on magnetically coupled ferromagnet/antiferromagnet layers using X-ray Photoemission Electron Microscopy (X-PEEM) now provide a new insight into the microscopic processes at this important interface. Using a combination of x-ray magnetic dichroism (XMD) contrast and microscopic electron yield detection we have resolved the magnetic domain structure in LaFeO3 and NiO thin films and crystals. The antiferromagnetic domain structure is linked to the crystallographic structure of the material and vanishes approaching the magnetic ordering temperature. Ferromagnetic films grown on the antiferromagnetic substrate show a corresponding ferromagnetic domain structure, an uniaxial exchange anistropy and a local bias which increases with decreasing domain size, suggesting a statistical origin of the bias effect. The role of uncompensated interface spins will also be discussed. We will present first experiments on magnetic interlayer coupling across metallic antiferromagnets, which suggest a similar origin of bias in full-metallic exchange bias system. A. Scholl et al., Science 287, 1014 (2000), F. Nolting et al., Nature 405, 767 (2000), H. Ohldag et al., Phys. Rev. Lett. 86, 2878 (2001)

  19. Competing antiferromagnetic and spin-glass phases in a hollandite structure

    NASA Astrophysics Data System (ADS)

    Crespo, Y.; Andreanov, A.; Seriani, N.

    2013-07-01

    We introduce a simple lattice model with Ising spins as a zeroth-order approximation of the hollandite-type magnetic compounds. We argue that geometrical frustration of the lattice in combination with nearest-neighbor antiferromagnetic (AFM) interactions are responsible for the appearance of a spin-glass phase in presence of disorder. We investigate this system numerically using parallel tempering. The model reproduces magnetic transitions present in some oxides with hollandite structure and displays a rich phenomenology: in the absence of disorder we have identified five different ground states, depending on the relative strength and sign of the interactions: one ferromagnetically ordered, three antiferromagnetically ordered, and one disordered, macroscopically degenerate ground state. Remarkably, for the sets of AFM couplings having an AFM ground state in the clean system, there exists a critical value of the disorder above which the ground state becomes a spin glass while maintaining all the couplings antiferromagnetically. This model presents this kind of transition with nearest-neighbor frustrated AFM interactions. We argue that this model is useful for understanding the relation between AFM coupling, disorder, and appearance of spin-glass phases.

  20. Observation of antiferromagnetic correlations in UBe 13

    NASA Astrophysics Data System (ADS)

    Neumann, K. U.; Capellmann, H.; Fisk, Z.; Smith, J. L.; Ziebeck, K. R. A.

    1986-11-01

    The wavevector and energy dependence of the paramagnetic response in the normal phase of the Heavy Fermion system UBe 13 has been investigated between 10 K and 300 K using polarized neutrons and polarization analysis. At 10 K the response was found to be enhanced at non zero wave-vectors indicating the presence of strong antiferromagnetic correlations. The peaks in the scattering occured at positions expected for incipient type G antiferromagnetism of the simple cubic uranium sublattice. At room temperature the spatial correlations completely disappeared and the response was wave vector independent. Constant Q scans carried out at 10 K confirmed the Lorentzian dependence proposed by Goldman et al. [1].

  1. NMR characterization of sulphur substitution effects in the KxFe2-ySe2-zSz high-Tc superconductor

    DOE PAGES

    Torchetti, D. A.; Imai, T.; Lei, H. C.; ...

    2012-04-17

    We present a⁷⁷ Se NMR study of the effect of S substitution in the high-Tc superconductor KxFe2-ySe2-zSz in a temperature range up to 250 K. We examine two S concentrations, with z=0.8 (Tc~ 26 K) and z=1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the KxFe₂Se₂ sample due to local disorder in the Se environment. Our Knight shift ⁷⁷K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None ofmore » the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near Tc in 1/T₁T, as seen in FeSe.« less

  2. Theory of spin transport in antiferromagnets (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Manchon, Aurélien; Saidaoui, Hamed; Akosa, Collins

    2016-10-01

    Antiferromagnets (AF) have long remained an intriguing and exotic state of matter, their application being restricted to enabling interfacial exchange bias in spin-valves. Their role in the expanding field of applied spintronics has been mostly passive and the in-depth investigation of their basic properties considered as fundamental condensed matter physics. A conceptual breakthrough was achieved ten years ago with the proposal that spin transfer torque could be used to electrically control the direction of the order parameter of AF spin valves, henceforth making these materials potential candidates for low energy spin devices. In spite of substantial theoretical efforts and experimental attempts to observe such a torque, the difficulty to independently detect the direction of the AF order parameter has remained a major obstacle. In this talk, I will first introduce the original concept of spin transfer torque in AF spin-valves, demonstrating that it is strongly limited by the spin decoherence and dramatically vanishes in the presence of disorder, leaving little hope to observe this effect experimentally. Then, I will present the newly proposed concept of spin-orbit torque that utilizes bulk or interfacial the spin-orbit coupling in non-centrosymmetric magnets to directly generate a torque on the AF order parameter. This torque, being local, is much more robust against impurities, as will be demonstrated for the specific case of interfacial Rashba spin-orbit coupling. Finally, I will discuss about spin motive force and torques in antiferromagnetic textures, intriguing effects that remained to be experimentally observed.

  3. TEMPERATURE DEPENDENCE OF THE ANTIFERROMAGNETIC ANISOTROPY IN MNF2,

    DTIC Science & Technology

    Existing data on the temperature dependence of the sublattice magnetization and of the antiferromagnetic resonance frequency of MnF2, together with...new antiferromagnetic resonance data, are used to determine the temperature dependence of the antiferromagnetic anisotropy energy. The experimental

  4. Antiferromagnetic phase transition of K-Rb alloy nanoclusters incorporated in sodalite

    NASA Astrophysics Data System (ADS)

    Nakano, Takehito; Ishida, Yuko; Hanazawa, Atsufumi; Nozue, Yasuo

    2013-06-01

    We prepared Rb-rich K-Rb alloy nanoclusters arrayed in the regular nanospace of aluminosilicate sodalite which has a bcc arrangement of cages. The average chemical formula of the cluster is (K1.5Rb2.5)3+, where one unpaired s-electron is shared by four alkali cations and is confined in a cage. The magnetic susceptibility and the electron spin resonance clearly show an antiferromagnetic phase transition at a Néel temperature T N of approximately 90-100 K. The observed T N is higher than that in K{4/3+} ( T N = 72 K) and (K3Rb)3+ clusters ( T N = 80 K) in sodalites. This result indicates a systematic enhancement of the antiferromagnetic exchange coupling between the adjacent nanoclusters by substituting Rb atoms for K ones. The size and the spatial distribution of the s-electron wave function in the nanocluster play a key role in the exchange coupling.

  5. Doping-Induced Quantum Critical Point in an Itinerant Antiferromagnet TiAu

    NASA Astrophysics Data System (ADS)

    Santiago, Jessica; Svanidze, Eteri; Besara, Tiglet; Siegrist, Theo; Morosan, Emilia

    The recently discovered itinerant magnet TiAu is the first antiferromagnet composed of non-magnetic constituents. The spin density wave ground state develops below TN ~36 K, about an order of magnitude smaller than in Cr. Achieving a quantum critical point in this material would provide a better understanding of weak itinerant antiferromagnets, while giving long sought-after insights into the effects of spin fluctuations in itinerant electron systems. While the application of pressure increases the ordering temperature TN, partial substitution of Ti provides an alternative avenue towards achieving a quantum critical point. The non-Fermi liquid behavior accompanies the quantum phase transition, as evidenced by the divergent specific heat coefficient and linear temperature dependence of the resistivity. The transition is accompanied by enhanced electron-electron correlations as well as strong spin-fluctuations, providing an experimental avenue for the verification of the self-consistent theory of spin fluctuations.

  6. Spin Hall Effects in Metallic Antiferromagnets

    DOE PAGES

    Zhang, Wei; Jungfleisch, Matthias B.; Jiang, Wanjun; ...

    2014-11-04

    In this paper, we investigate four CuAu-I-type metallic antiferromagnets for their potential as spin current detectors using spin pumping and inverse spin Hall effect. Nontrivial spin Hall effects were observed for FeMn, PdMn, and IrMn while a much higher effect was obtained for PtMn. Using thickness-dependent measurements, we determined the spin diffusion lengths of these materials to be short, on the order of 1 nm. The estimated spin Hall angles of the four materials follow the relationship PtMn > IrMn > PdMn > FeMn, highlighting the correlation between the spin-orbit coupling of nonmagnetic species and the magnitude of the spinmore » Hall effect in their antiferromagnetic alloys. These experiments are compared with first-principles calculations. Finally, engineering the properties of the antiferromagnets as well as their interfaces can pave the way for manipulation of the spin dependent transport properties in antiferromagnet-based spintronics.« less

  7. Spin Hall Effects in Metallic Antiferromagnets

    SciTech Connect

    Zhang, Wei; Jungfleisch, Matthias B.; Jiang, Wanjun; Pearson, John E.; Hoffmann, Axel; Freimuth, Frank; Mokrousov, Yuriy

    2014-11-04

    In this paper, we investigate four CuAu-I-type metallic antiferromagnets for their potential as spin current detectors using spin pumping and inverse spin Hall effect. Nontrivial spin Hall effects were observed for FeMn, PdMn, and IrMn while a much higher effect was obtained for PtMn. Using thickness-dependent measurements, we determined the spin diffusion lengths of these materials to be short, on the order of 1 nm. The estimated spin Hall angles of the four materials follow the relationship PtMn > IrMn > PdMn > FeMn, highlighting the correlation between the spin-orbit coupling of nonmagnetic species and the magnitude of the spin Hall effect in their antiferromagnetic alloys. These experiments are compared with first-principles calculations. Finally, engineering the properties of the antiferromagnets as well as their interfaces can pave the way for manipulation of the spin dependent transport properties in antiferromagnet-based spintronics.

  8. Magnetoelastic effects on antiferromagnetic phase transitions (invited)

    SciTech Connect

    Wolf, W.P.; Huan, C.H.A.

    1988-04-15

    The effect of elastic strains on antiferromagnetic phase transitions is considered. For cases in which the magnetic and chemical unit cells coincide, the combination of a strain and an applied field is found to lead to the possibility of a linear magnetoelastic (LME) coupling which may induce antiferromagnetic order, even in the normally paramagnetic phase. Such an effect can, in principle, destroy any second-order phase transition. An order of magnitude estimate shows that the effect is small but not negligible, and that it may explain a number of unusual effects observed in dysprosium aluminum garnet, including anomalous neutron scattering, magnetic hysteresis and magnetostriction. Similar strain-induced effects may be important in many other antiferromagnets, including CoF/sub 2/, FeF/sub 2/, MnF/sub 2/, and ..cap alpha..Fe/sub 2/O/sub 3/, as well as in mixed crystals with the same structures. Strain gradients may produce similar effects in other antiferromagnets which are magnetoelectric, including DyPO/sub 4/, DyAlO/sub 3/, and Cr/sub 2/O/sub 3/.

  9. Direct measurement of antiferromagnetic domain fluctuations.

    PubMed

    Shpyrko, O G; Isaacs, E D; Logan, J M; Feng, Yejun; Aeppli, G; Jaramillo, R; Kim, H C; Rosenbaum, T F; Zschack, P; Sprung, M; Narayanan, S; Sandy, A R

    2007-05-03

    Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value-consistent with quantum fluctuations-on cooling below 40 K.

  10. Electrical resistivity of substitutionally disordered hcp Fe-Si and Fe-Ni alloys: Chemically-induced resistivity saturation in the Earth's core

    NASA Astrophysics Data System (ADS)

    Gomi, Hitoshi; Hirose, Kei; Akai, Hisazumi; Fei, Yingwei

    2016-10-01

    The thermal conductivity of the Earth's core can be estimated from its electrical resistivity via the Wiedemann-Franz law. However, previously reported resistivity values are rather scattered, mainly due to the lack of knowledge with regard to resistivity saturation (violations of the Bloch-Grüneisen law and the Matthiessen's rule). Here we conducted high-pressure experiments and first-principles calculations in order to clarify the relationship between the resistivity saturation and the impurity resistivity of substitutional silicon in hexagonal-close-packed (hcp) iron. We measured the electrical resistivity of Fe-Si alloys (iron with 1, 2, 4, 6.5, and 9 wt.% silicon) using four-terminal method in a diamond-anvil cell up to 90 GPa at 300 K. We also computed the electronic band structure of substitutionally disordered hcp Fe-Si and Fe-Ni alloy systems by means of Korringa-Kohn-Rostoker method with coherent potential approximation (KKR-CPA). The electrical resistivity was then calculated from the Kubo-Greenwood formula. These experimental and theoretical results show excellent agreement with each other, and the first principles results show the saturation behavior at high silicon concentration. We further calculated the resistivity of Fe-Ni-Si ternary alloys and found the violation of the Matthiessen's rule as a consequence of the resistivity saturation. Such resistivity saturation has important implications for core dynamics. The saturation effect places the upper limit of the resistivity, resulting in that the total resistivity value has almost no temperature dependence. As a consequence, the core thermal conductivity has a lower bound and exhibits a linear temperature dependence. We predict the electrical resistivity at the top of the Earth's core to be 1.12 ×10-6 Ωm, which corresponds to the thermal conductivity of 87.1 W/m/K. Such high thermal conductivity suggests high isentropic heat flow, leading to young inner core age (<0.85 Gyr old) and high initial

  11. Economic impact of therapeutic substitution of a brand selective serotonin reuptake inhibitor with an alternative generic selective serotonin reuptake inhibitor in patients with major depressive disorder.

    PubMed

    Wu, Eric Q; Yu, Andrew P; Lauzon, Veronique; Ramakrishnan, Karthik; Marynchenko, Maryna; Ben-Hamadi, Rym; Blum, Steven; Erder, M Haim

    2011-04-01

    To reduce pharmacy costs, managed care organizations encourage therapeutic substitution from brand to a generic product. However, little is known about whether these cost-containment strategies can also potentially lower total expenditures for payers in treatment of major depressive disorder (MDD). To compare economic outcomes of patients with MDD who were switched from a brand selective serotonin reuptake inhibitor (SSRI) to an alternative generic SSRI for nonmedical reasons versus patients who continued on the brand SSRI. Adult MDD patients in the Ingenix Impact Database (2003-2007) were considered "switchers" if they received treatment with a brand SSRI and were later switched to an alternative generic SSRI for nonmedical reasons. Patients who remained on the brand SSRI (nonswitchers) were matched 1:1 with switchers. All-cause, mental health-related, and MDD-related rates of hospitalizations/emergency department (ED) visits and costs over 6 months were compared both descriptively and by using adjusted regression models. A subgroup analysis on patients who were switched from escitalopram (Lexapro) to an alternative generic SSRI was also performed. The study included 4449 matched pairs. Compared with nonswitchers, switchers had higher risk of all-cause, mental health-related, and MDD-related use of hospitalizations/ED visits (OR 1.15, 1.34, and 1.54, respectively; all p < 0.01) and higher risk-adjusted mental health-related and MDD-related medical costs ($219 and $222, respectively; both p < 0.05). Subgroup analysis on escitalopram showed similar results; switchers experienced higher risk of any-cause, mental health-related, and MDD-related use of hospitalizations/ED visits (OR 1.21, 1.41, and 1.53, respectively; all p < 0.01) and higher risk-adjusted MDD-related medical costs ($151; p < 0.05). Compared with patients who continued on their patented SSRIs, patients who switched to a generic SSRI incurred more resource use of hospitalizations/ED visits and higher

  12. Performance of synthetic antiferromagnetic racetrack memory: domain wall versus skyrmion

    NASA Astrophysics Data System (ADS)

    Tomasello, R.; Puliafito, V.; Martinez, E.; Manchon, A.; Ricci, M.; Carpentieri, M.; Finocchio, G.

    2017-08-01

    A storage scheme based on racetrack memory, where the information can be coded in a domain or a skyrmion, seems to be an alternative to conventional hard disk drive for high density storage. Here, we perform a full micromagnetic study of the performance of synthetic antiferromagnetic (SAF) racetrack memory in terms of velocity and sensitivity to defects by using experimental parameters. We find that, to stabilize a SAF skyrmion, the Dzyaloshinskii-Moriya interaction in the top and the bottom ferromagnet should have an opposite sign. The velocity of SAF skyrmions and SAF Néel domain walls are of the same order and can reach values larger than 1200 m s-1 if a spin-orbit torque from the spin-Hall effect with opposite sign is applied to both ferromagnets. The presence of disordered anisotropy in the form of randomly distributed grains introduces a threshold current for both SAF skyrmions and SAF domain walls motions.

  13. Spectral evolution with doping of an antiferromagnetic Mott state

    NASA Astrophysics Data System (ADS)

    Wu, Huan-Kuang; Lee, Ting-Kuo

    2017-01-01

    Since the discovery of half-filled cuprate to be a Mott insulator, the excitation spectra above the chemical potential for the unoccupied states has attracted much research attention. There were many theoretical works using different numerical techniques to study this problem, but many have reached different conclusions. One of the reasons is the lack of very detailed high-resolution experimental results for the theories to be compared with. Recently, the scanning tunneling spectroscopy [P. Cai et al., Nat. Phys. 12, 1047 (2016), 10.1038/nphys3840; C. Ye et al., Nat. Commun. 4, 1365 (2013), 10.1038/ncomms2369] on lightly doped Mott insulator with an antiferromagnetic order found the presence of in-gap states with energy of order half an eV above the chemical potential. The measured spectral properties with doping are not quite consistent with earlier theoretical works. Although the experiment has disorder and localization effect, but for the energy scale we will study here, a model without disorder is sufficed to illustrate the underlying physics. We perform a diagonalization method on top of the variational Monte Carlo calculation to study the evolution of antiferromagnetic Mott state with doped hole concentration in the Hubbard model. Our results found in-gap states that behave similarly with ones reported by STS. These in-gap states acquire a substantial amount of dynamical spectral weight transferred from the upper Hubbard band. The in-gap states move toward chemical potential with increasing spectral weight as doping increases. Our result also provides information about the energy scale of these in-gap states in relation with the Coulomb coupling strength U .

  14. Remanent Magnetization: Signature of Many-Body Localization in Quantum Antiferromagnets.

    PubMed

    Ros, V; Müller, M

    2017-06-09

    We study the remanent magnetization in antiferromagnetic, many-body localized quantum spin chains, initialized in a fully magnetized state. Its long time limit is an order parameter for the localization transition, which is readily accessible by standard experimental probes in magnets. We analytically calculate its value in the strong-disorder regime exploiting the explicit construction of quasilocal conserved quantities of the localized phase. We discuss analogies in cold atomic systems.

  15. Magnetotransport and Antiferromagnetic Behavior in ErP Epitaxial Layers on GaInP(001)

    NASA Astrophysics Data System (ADS)

    Nakamura, A.; Ito, T.; Ohnishi, H.; Koizumi, A.; Takeda, Y.

    2007-04-01

    We have investigated transport and magnetic properties in ErP ultrathin films grown on the GaInP (001) surface. Hall resistance and transverse magnetoresistance measurements yield the carrier density of ˜1.4×1020 cm-3 and the electron and hole mobilities of 1250-1450 cm2/Vs at 10 K. The resistivity anomaly in the longitudinal megnetoresistance is interpreted in terms of the spin-disorder scattering in the antiferromagnetic phase below 3.5 K.

  16. Breakdown of antiferromagnet order in polycrystalline NiFe/NiO bilayers probed with acoustic emission

    NASA Astrophysics Data System (ADS)

    Lebyodkin, M. A.; Lebedkina, T. A.; Shashkov, I. V.; Gornakov, V. S.

    2017-07-01

    Magnetization reversal of polycrystalline NiFe/NiO bilayers was investigated using magneto-optical indicator film imaging and acoustic emission techniques. Sporadic acoustic signals were detected in a constant magnetic field after the magnetization reversal. It is suggested that they are related to elastic waves excited by sharp shocks in the NiO layer with strong magnetostriction. Their probability depends on the history and number of repetitions of the field cycling, thus testifying the thermal-activation nature of the long-time relaxation of an antiferromagnetic order. These results provide evidence of spontaneous thermally activated switching of the antiferromagnetic order in NiO grains during magnetization reversal in ferromagnet/antiferromagnet (FM/AFM) heterostructures. The respective deformation modes are discussed in terms of the thermal fluctuation aftereffect in the Fulcomer and Charap model which predicts that irreversible breakdown of the original spin orientation can take place in some antiferromagnetic grains with disordered anisotropy axes during magnetization reversal of exchange-coupled FM/AFM structures. The spin reorientation in the saturated state may induce abrupt distortion of isolated metastable grains because of the NiO magnetostriction, leading to excitation of shock waves and formation of plate (or Lamb) waves.

  17. Entanglement entropy and massless phase in the antiferromagnetic three-state quantum chiral clock model

    NASA Astrophysics Data System (ADS)

    Dai, Yan-Wei; Cho, Sam Young; Batchelor, Murray T.; Zhou, Huan-Qiang

    2017-01-01

    The von Neumann entanglement entropy is used to estimate the critical point hc/J ≃0.143 (3 ) of the mixed ferro-antiferromagnetic three-state quantum Potts model H =∑i[J (XiXi+1 2+Xi2Xi +1) -h Ri] , where Xi and Ri are standard three-state Potts spin operators and J >0 is the antiferromagnetic coupling parameter. This critical point value gives improved estimates for two Kosterlitz-Thouless transition points in the antiferromagnetic (β <0 ) region of the Δ -β phase diagram of the three-state quantum chiral clock model, where Δ and β are, respectively, the chirality and coupling parameters in the clock model. These are the transition points βc≃-0.143 (3 ) at Δ =1/2 between incommensurate and commensurate phases and βc≃-7.0 (1 ) at Δ =0 between disordered and incommensurate phases. The von Neumann entropy is also used to calculate the central charge c of the underlying conformal field theory in the massless phase h ≤hc . The estimate c ≃1 in this phase is consistent with the known exact value at the particular point h /J =-1 corresponding to the purely antiferromagnetic three-state quantum Potts model. The algebraic decay of the Potts spin-spin correlation in the massless phase is used to estimate the continuously varying critical exponent η .

  18. Griffiths singularities in the random quantum Ising antiferromagnet: A tree tensor network renormalization group study

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Ping; Kao, Ying-Jer; Chen, Pochung; Lin, Yu-Cheng

    2017-08-01

    The antiferromagnetic Ising chain in both transverse and longitudinal magnetic fields is one of the paradigmatic models of a quantum phase transition. The antiferromagnetic system exhibits a zero-temperature critical line separating an antiferromagnetic phase and a paramagnetic phase; the critical line connects an integrable quantum critical point at zero longitudinal field and a classical first-order transition point at zero transverse field. Using a strong-disorder renormalization group method formulated as a tree tensor network, we study the zero-temperature phase of the quantum Ising chain with bond randomness. We introduce a new matrix product operator representation of high-order moments, which provides an efficient and accurate tool for determining quantum phase transitions via the Binder cumulant of the order parameter. Our results demonstrate an infinite-randomness quantum critical point in zero longitudinal field accompanied by pronounced quantum Griffiths singularities, arising from rare ordered regions with anomalously slow fluctuations inside the paramagnetic phase. The strong Griffiths effects are signaled by a large dynamical exponent z >1 , which characterizes a power-law density of low-energy states of the localized rare regions and becomes infinite at the quantum critical point. Upon application of a longitudinal field, the quantum phase transition between the paramagnetic phase and the antiferromagnetic phase is completely destroyed. Furthermore, quantum Griffiths effects are suppressed, showing z <1 , when the dynamics of the rare regions is hampered by the longitudinal field.

  19. Discrete-Symmetry Breaking and Novel Critical Phenomena in an Antiferromagnetic Planar (XY) Model in Two Dimensions

    NASA Astrophysics Data System (ADS)

    Lee, D. H.; Joannopoulos, J. D.; Negele, J. W.; Landau, D. P.

    1984-02-01

    Landau-Ginzburg-Wilson symmetry analyses and Monte Carlo calculations for the classical antiferromagnetic planar (XY) model on a triangular lattice reveal a wealth of interesting critical phenomena. From this simple model arise a zero-field transition to a state of long-range order, a new mechanism for spin disordering, and a critical point associated with a possible new universality class.

  20. Solvent substitution

    SciTech Connect

    Not Available

    1990-01-01

    The DOE Environmental Restoration and Waste Management Office of Technology Development and the Air Force Engineering and Services Center convened the First Annual International Workshop on Solvent Substitution on December 4--7, 1990. The primary objectives of this joint effort were to share information and ideas among attendees in order to enhance the development and implementation of required new technologies for the elimination of pollutants associated with industrial use of hazardous and toxic solvents; and to aid in accelerating collaborative efforts and technology transfer between government and industry for solvent substitution. There were workshop sessions focusing on Alternative Technologies, Alternative Solvents, Recovery/Recycling, Low VOC Materials and Treatment for Environmentally Safe Disposal. The 35 invited papers presented covered a wide range of solvent substitution activities including: hardware and weapons production and maintenance, paint stripping, coating applications, printed circuit boards, metal cleaning, metal finishing, manufacturing, compliance monitoring and process control monitoring. This publication includes the majority of these presentations. In addition, in order to further facilitate information exchange and technology transfer, the US Air Force and DOE solicited additional papers under a general Call for Papers.'' These papers, which underwent review and final selection by a peer review committee, are also included in this combined Proceedings/Compendium. For those involved in handling, using or managing hazardous and toxic solvents, this document should prove to be a valuable resource, providing the most up-to-date information on current technologies and practices in solvent substitution. Individual papers are abstracted separated.

  1. Dirac fermions in an antiferromagnetic semimetal

    SciTech Connect

    Tang, Peizhe; Zhou, Quan; Xu, Gang; Zhang, Shou-Cheng

    2016-08-08

    Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry and inversion symmetry. Here in this paper, we show that Dirac fermions can exist in one type of antiferromagnetic system, where both and are broken but their combination is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections and demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.

  2. Antiferromagnetic anisotropy determination by spin Hall magnetoresistance

    NASA Astrophysics Data System (ADS)

    Wang, Hua; Hou, Dazhi; Qiu, Zhiyong; Kikkawa, Takashi; Saitoh, Eiji; Jin, Xiaofeng

    2017-08-01

    An electric method for measuring magnetic anisotropy in antiferromagnetic insulators (AFIs) is proposed. When a metallic film with strong spin-orbit interactions, e.g., platinum (Pt), is deposited on an AFI, its resistance should be affected by the direction of the AFI Néel vector due to the spin Hall magnetoresistance (SMR). Accordingly, the direction of the AFI Néel vector, which is affected by both the external magnetic field and the magnetic anisotropy, is reflected in resistance of Pt. The magnetic field angle dependence of the resistance of Pt on AFI is calculated by considering the SMR, which indicates that the antiferromagnetic anisotropy can be obtained experimentally by monitoring the Pt resistance in strong magnetic fields. Calculations are performed for realistic systems such as Pt/Cr2O3, Pt/NiO, and Pt/CoO.

  3. Antiferromagnetic order in hybrid electromagnetic metamaterials

    NASA Astrophysics Data System (ADS)

    Miroshnichenko, Andrey E.; Filonov, Dmitry; Lukyanchuk, Boris; Kivshar, Yuri

    2017-08-01

    We demonstrate experimentally a new type of order in optical magnetism resembling the staggered structure of spins in antiferromagnetic ordered materials. We study hybrid electromagnetic metasurfaces created by assembling hybrid meta-atoms formed by metallic split-ring resonators and dielectric particles with a high refractive index, both supporting optically-induced magnetic dipole resonances of different origin. Each pair (or ‘metamolecule’) is characterized by two interacting magnetic dipole moments with the distance-dependent magnetization resembling the spin exchange interaction in magnetic materials. By directly mapping the structure of the electromagnetic fields, we demonstrate experimentally that strong coupling between the optically-induced magnetic moments of different origin can flip the magnetisation orientation in a metamolecule creating an antiferromagnetic lattice of staggered optically-induced magnetic moments in hybrid metasurfaces.

  4. Dirac fermions in an antiferromagnetic semimetal

    DOE PAGES

    Tang, Peizhe; Zhou, Quan; Xu, Gang; ...

    2016-08-08

    Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry and inversion symmetry. Here in this paper, we show that Dirac fermions can exist in one type of antiferromagnetic system, where both and are broken but their combination is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections andmore » demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.« less

  5. Weyl magnons in breathing pyrochlore antiferromagnets

    SciTech Connect

    Li, Fei-Ye; Li, Yao-Dong; Kim, Yong Baek; Balents, Leon; Yu, Yue; Chen, Gang

    2016-09-21

    Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.

  6. Weyl magnons in breathing pyrochlore antiferromagnets

    DOE PAGES

    Li, Fei-Ye; Li, Yao-Dong; Kim, Yong Baek; ...

    2016-09-21

    Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by appliedmore » fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.« less

  7. Weyl magnons in breathing pyrochlore antiferromagnets

    PubMed Central

    Li, Fei-Ye; Li, Yao-Dong; Kim, Yong Baek; Balents, Leon; Yu, Yue; Chen, Gang

    2016-01-01

    Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems. PMID:27650053

  8. Antiferromagnetic Ising Model in Hierarchical Networks

    NASA Astrophysics Data System (ADS)

    Cheng, Xiang; Boettcher, Stefan

    2015-03-01

    The Ising antiferromagnet is a convenient model of glassy dynamics. It can introduce geometric frustrations and may give rise to a spin glass phase and glassy relaxation at low temperatures [ 1 ] . We apply the antiferromagnetic Ising model to 3 hierarchical networks which share features of both small world networks and regular lattices. Their recursive and fixed structures make them suitable for exact renormalization group analysis as well as numerical simulations. We first explore the dynamical behaviors using simulated annealing and discover an extremely slow relaxation at low temperatures. Then we employ the Wang-Landau algorithm to investigate the energy landscape and the corresponding equilibrium behaviors for different system sizes. Besides the Monte Carlo methods, renormalization group [ 2 ] is used to study the equilibrium properties in the thermodynamic limit and to compare with the results from simulated annealing and Wang-Landau sampling. Supported through NSF Grant DMR-1207431.

  9. Dirac fermions in an antiferromagnetic semimetal

    NASA Astrophysics Data System (ADS)

    Tang, Peizhe; Zhou, Quan; Xu, Gang; Zhang, Shou-Cheng

    2016-12-01

    Analogues of the elementary particles have been extensively searched for in condensed-matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low-energy excitations in materials now known as Dirac semimetals. All of the currently known Dirac semimetals are non-magnetic with both time-reversal symmetry and inversion symmetry . Here we show that Dirac fermions can exist in one type of antiferromagnetic system, where both and are broken but their combination is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyse the robustness of the Dirac points under symmetry protections and demonstrate its distinctive bulk dispersions, as well as the corresponding surface states, by ab initio calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism.

  10. Novel domain wall dynamics in synthetic antiferromagnets

    NASA Astrophysics Data System (ADS)

    Yang, See-Hun; Parkin, Stuart

    2017-08-01

    In this article, we review fascinating new mechanisms on recently observed remarkable current driven domain wall motion in nanowires formed from perpendicularly magnetized synthetic antiferromagnets interfaced with heavy metallic layers, sources of spin-orbit torques. All the associated torques such as volumetric adiabatic and non-adiabatic spin-transfer-torque, spin-orbit torques, shape anisotropy field torques, Dzyaloshinkii-Moriya interaction torques and most importantly a new powerful torque, exchange coupling torque, will be discussed based on an analytical model that provides an intuitive description of domain wall dynamics in synthetic ferromagnets as well as synthetic antiferromagnets. In addition, the current driven DW motion in the presence of in-plane fields will be investigated, thus deepening our knowledge about the role of the exchange coupling torque, which will be of potential use for application to various novel spintronic devices.

  11. Modeling anisotropic magnetoresistance in layered antiferromagnets

    NASA Astrophysics Data System (ADS)

    Santos, D. L. R.; Pinheiro, F. A.; Velev, J.; Chshiev, M.; Castro, J. d.'Albuquerque e.; Lacroix, C.

    2017-06-01

    We have investigated the electronic transport and the anisotropic magnetoresistance in systems consisting of pairs of antiferromagnetically aligned layers separated by a non-magnetic layer, across which an antiferromagnetic coupling between the double layers is established. Calculations have been performed within the framework of the tight-binding model, taking into account the exchange coupling within the ferromagnetic layers and the Rashba spin-orbit interaction. Conductivities have been evaluated in the ballistic regime, based on Kubo formula. We have systematically studied the dependence of the conductivity and of the anisotropic magnetoresistance on several material and structural parameters, such as the orientation of the magnetic moments relative to the crystalline axis, band filling, out-of-plane hopping and spin-orbit parameter.

  12. Half-metallic diluted antiferromagnetic semiconductors.

    PubMed

    Akai, H; Ogura, M

    2006-07-14

    The possibility of half-metallic antiferromagnetism, a special case of ferrimagnetism with a compensated magnetization, in the diluted magnetic semiconductors is highlighted on the basis of the first-principles electronic structure calculation. As typical examples, the electrical and magnetic properties of II-VI compound semiconductors doped with 3d transition metal ion pairs--(V, Co) and (Fe, Cr)--are discussed.

  13. Spin Seebeck Effect Signals from Antiferromagnets

    NASA Astrophysics Data System (ADS)

    Prakash, Arati; Brangham, Jack; Yang, Fengyuan; Heremans, Joseph

    The Longitudinal Spin Seebeck Effect (LSSE), in which a heat current stimulates spin propagation across an interface between a magnetic material and a normal metal, is well established and observed in ferromagnetic systems. Data have been presented indicating that antiferromagnetic systems could also give rise to LSSE signals. We report here on LSSE signal measured on the Pt/NiO/YIG structure, where NiO is an antiferromagnet. This system is reported to exhibit antiferromagnonic transport. We explore the dependence of the signal on the thickness of the NiO and YIG layers. We also report its temperature dependence, which was not explored before. The results are interpreted in terms of the temperature dependence of the magnon density of states. It appears that magnon modes with energies below about 40 K are most involved in the process, as was the case to the LSSE on YIG itself. Preliminary results using other antiferromagnets and other inverse spin-Hall layers look promising and will also be reported Work supported by ARO- MURI W911NF-14-1-0016.

  14. Spin transfer in antiferromagnets (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Moriyama, Takahiro

    2016-10-01

    Since antiferromagnets (AFMs) have no spontaneous magnetization unlike ferromagnetic materials, it is not easy to manipulate the magnetic moments in AFMs by external magnetic field. However, recent theoretical studies suggest that it is possible to manipulate the magnetization in AFMs by spin-transfer-torque in a similar manner to ferromagnetic materials. In this study, we perform spin-toque ferromagnetic resonance (ST-FMR) measurements on FeNi/NiO/Pt multilayers to experimentally investigate the interaction between the spin current and the magnetic moments of antiferromagnetic NiO. The spin current is injected to the NiO by the spin Hall effect in Pt. The monotonous change in the FMR linewidth of this system with respect to the spin current can be interpreted in a way that the spin current is transferred through the NiO and interacts with the FeNi. This intriguing spin current transport can be explained by the angular momentum transfer mediated by the antiferromagnetic magnons. The results assure that the spin current exerts a torque on the NiO magnetic moments and excites their dynamics. In the talk, recent results will be also discussed.

  15. Analysis of the antiferromagnetic phase transitions of the 2D Kondo lattice

    NASA Astrophysics Data System (ADS)

    Jones, Barbara

    2010-03-01

    The Kondo lattice continues to present an interesting and relevant challenge, with its interactions between Kondo, RKKY, and coherent order. We present our study[1] of the antiferromagnetic quantum phase transitions of a 2D Kondo-Heisenberg square lattice. Starting from the nonlinear sigma model as a model of antiferromagnetism, we carry out a renormalization group analysis of the competing Kondo-RKKY interaction to one-loop order in an ɛ-expansion. We find a new quantum critical point (QCP) strongly affected by Kondo fluctuations. Near this QCP, there is a breakdown of hydrodynamic behavior, and the spin waves are logarithmically frozen out. The renormalization group results allow us to propose a new phase diagram near the antiferromagnetic fixed point of this 2D Kondo lattice model. The T=0 phase diagram contains four phases separated by a tetracritical point, the new QCP. For small spin fluctuations, we find a stable local magnetic moment antiferromagnet. For stronger coupling, region II is a metallic quantum disordered paramagnet. We find in region III a paramagnetic phase driven by Kondo interactions, with possible ground states of a heavy fermion liquid or a Kondo driven spin-liquid. The fourth phase is a spiral phase, or a large-Fermi-surface antiferromagnetic phase. We will describe these phases in more detail, including possible experimental confirmation of the spiral phase. The existence of the tetracritical point found here would be expected to affect the phase diagram at finite temperatures as well. In addition, It is hoped that these results, and particularly the Kondo interaction paramagnetic phase, will serve to bridge to solutions starting from the opposite limit, of a Kondo effect leading to a heavy fermion ground state. Work in collaboration with T. Tzen Ong. [4pt] [1] T. Ong and B. A. Jones, Phys. Rev. Lett. 103, 066405 (2009).

  16. Antiferromagnetic order and spin dynamics in iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Dai, Pengcheng

    2015-07-01

    High-transition temperature (high-Tc) superconductivity in the iron pnictides or chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxide superconductors. This raises a fundamental question concerning the role of magnetism in the superconductivity of these materials. Neutron scattering, a powerful probe to study the magnetic order and spin dynamics, plays an essential role in determining the relationship between magnetism and superconductivity in high-Tc superconductors. The rapid development of modern neutron time-of-flight spectrometers allows a direct determination of the spin dynamical properties of iron-based superconductors throughout the entire Brillouin zone. In this paper, an overview is presented of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin-excitation spectra as a function of electron and hole doping and isoelectronic substitution. Spin dynamical properties of iron-based superconductors are compared with those of copper oxide and heavy fermion superconductors and the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity are discussed.

  17. Modelling compensated antiferromagnetic interfaces with MuMax3

    NASA Astrophysics Data System (ADS)

    De Clercq, Jonas; Leliaert, Jonathan; Van Waeyenberge, Bartel

    2017-10-01

    We show how compensated antiferromagnetic interfaces can be implemented in the micromagnetic simulation program MuMax3. We demonstrate that we can model spin flop coupling as a uniaxial anisotropy for small canting angles and how we can take into account the exact energy terms for strong coupling between a ferromagnet and a compensated antiferromagnet. We also investigate athermal training in biaxial antiferromagnets and reproduce the training effect in a polycrystalline IrMn/CoFe bilayer.

  18. Dynamics of antiferromagnetic skyrmion driven by the spin Hall effect

    NASA Astrophysics Data System (ADS)

    Jin, Chendong; Song, Chengkun; Wang, Jianbo; Liu, Qingfang

    2016-10-01

    Magnetic skyrmion moved by the spin-Hall effect is promising for the application of the generation racetrack memories. However, the Magnus force causes a deflected motion of skyrmion, which limits its application. Here, we create an antiferromagnetic skyrmion by injecting a spin-polarized pulse in the nanostripe and investigate the spin Hall effect-induced motion of antiferromagnetic skyrmion by micromagnetic simulations. In contrast to ferromagnetic skyrmion, we find that the antiferromagnetic skyrmion has three evident advantages: (i) the minimum driving current density of antiferromagnetic skyrmion is about two orders smaller than the ferromagnetic skyrmion; (ii) the velocity of the antiferromagnetic skyrmion is about 57 times larger than the ferromagnetic skyrmion driven by the same value of current density; (iii) antiferromagnetic skyrmion can be driven by the spin Hall effect without the influence of Magnus force. In addition, antiferromagnetic skyrmion can move around the pinning sites due to its property of topological protection. Our results present the understanding of antiferromagnetic skyrmion motion driven by the spin Hall effect and may also contribute to the development of antiferromagnetic skyrmion-based racetrack memories.

  19. Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

    NASA Astrophysics Data System (ADS)

    Zhou, X.; Ma, L.; Shi, Z.; Fan, W. J.; Evans, R. F. L.; Zheng, Jian-Guo; Chantrell, R. W.; Mangin, S.; Zhang, H. W.; Zhou, S. M.

    2015-03-01

    In this work, disordered-IrMn3/insulating-Y3Fe5O12 exchange-biased bilayers are studied. The behavior of the net magnetic moment ΔmAFM in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The ΔmAFM is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis loss are induced by partial rotation and irreversible switching of the ΔmAFM. In the athermal training effect, the state of the ΔmAFM cannot be recovered after one cycle of hysteresis loop. This work highlights the fundamental role of the ΔmAFM in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices.

  20. Pressure-tuned spin and charge ordering in an itinerant antiferromagnet.

    PubMed

    Feng, Yejun; Jaramillo, R; Srajer, G; Lang, J C; Islam, Z; Somayazulu, M S; Shpyrko, O G; Pluth, J J; Mao, H-K; Isaacs, E D; Aeppli, G; Rosenbaum, T F

    2007-09-28

    Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.

  1. Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

    PubMed Central

    Zhou, X.; Ma, L.; Shi, Z.; Fan, W. J.; Evans, R. F. L.; Zheng, Jian-Guo; Chantrell, R. W.; Mangin, S.; Zhang, H. W.; Zhou, S. M.

    2015-01-01

    In this work, disordered-IrMn3/insulating-Y3Fe5O12 exchange-biased bilayers are studied. The behavior of the net magnetic moment ΔmAFM in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The ΔmAFM is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis loss are induced by partial rotation and irreversible switching of the ΔmAFM. In the athermal training effect, the state of the ΔmAFM cannot be recovered after one cycle of hysteresis loop. This work highlights the fundamental role of the ΔmAFM in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices. PMID:25777540

  2. Identification of acquired coagulation disorders and effects of target-controlled coagulation factor substitution on the incidence and severity of spontaneous intracranial bleeding during veno-venous ECMO therapy.

    PubMed

    Kalbhenn, J; Wittau, N; Schmutz, A; Zieger, B; Schmidt, R

    2015-11-01

    Intracranial haemorrhage is a redoubtable complication during extracorporeal membrane oxygenation (ECMO) therapy. The underlying mechanisms of haemorrhagic diathesis are still not completely understood. This study was performed to evaluate a coagulation protocol for the regular analysis of acquired coagulation disorders and the systematic substitution of coagulation factors to reach predefined target values. We hypothesised that using this strategy would lead to the identification of acquired bleeding disorders which cannot be monitored with standard coagulation tests and that substitution of the respective factors in a target-controlled approach could have an impact on the incidence and severity of intracranial haemorrhage. A protocol for the analysis of acquired coagulation disorders and the subsequent administration of associated factor concentrates was introduced. Previously, coagulation management was mainly based on clinical bleeding signs as the trigger for the administration of blood products. In this investigation, nineteen consecutive patients before (control group) and twenty consecutive patients after the implementation of the protocol (intervention group) have been included in the study. Eighty-eight percent of the patients developed factor XIII deficiency, 79% acquired von Willebrand syndrome, 40% fibrinogen deficiency and 54% of the patients showed a decline in platelet count >20% within the first 24 hours of ECMO therapy. In 6 out of 19 (31%) patients in the control group and in 2 patients out of 20 (10%) in the intervention group, intracranial haemorrhage was detected. Whilst 5 of 6 patients in the control group died because of fatal bleeding, both of the patients in the intervention group recovered with a favourable neurologic outcome. Veno-venous ECMO therapy leads to thrombocytopenia, factor XIII and fibrinogen deficiency as well as acquired von Willebrand syndrome. The implementation of a coagulation protocol including a standardized

  3. Switching of antiferromagnetic chains with magnetic pulses

    NASA Astrophysics Data System (ADS)

    Tao, Kun; Polyakov, Oleg P.; Stepanyuk, Valeri S.

    2016-04-01

    Recent experimental studies have demonstrated the possibility of information storage in short antiferromagnetic chains on an insulator substrate [S. Loth et al., Science 335, 196 (2012), 10.1126/science.1214131]. Here, using the density functional theory and atomistic spin dynamics simulations, we show that a local magnetic control of such chains with a magnetic tip and magnetic pulses can be used for fast switching of their magnetization. Furthermore, by changing the position of the tip one can engineer the magnetization dynamics of the chains.

  4. Spin-mechanical inertia in antiferromagnets

    NASA Astrophysics Data System (ADS)

    Cheng, Ran; Wu, Xiaochuan; Xiao, Di

    2017-08-01

    Angular-momentum conservation has served as a guiding principle in the interplay between spin dynamics and mechanical rotations. However, in an antiferromagnet with vanishing magnetization, new fundamental rules are required to properly describe spin-mechanical phenomena. Here we show that the Néel order dynamics affects the mechanical motion of a rigid body by modifying its inertia tensor in the presence of strong magnetocrystalline anisotropy. This effect depends on temperature when magnon excitations are considered. Such a spin-mechanical inertia can produce measurable consequences at small scales.

  5. Ferrimagnetism in a transverse Ising antiferromagnet

    NASA Astrophysics Data System (ADS)

    Kaneyoshi, T.

    2016-05-01

    The phase diagrams and temperature dependences of total magnetization mT in a transverse Ising antiferromagnet consisting of alternating two (A and B) layers are studied by the uses of the effective-field theory with correlations and the mean-field-theory. A lot of characteristic phenomena, namely ferrimagnetic behaviors, have been found in the mT, when the crystallographically equivalent conditions between the A and B layers are broken. The appearance of a compensation point has been found below its transition temperature.

  6. High-Tc spin superfluidity in antiferromagnets.

    PubMed

    Bunkov, Yu M; Alakshin, E M; Gazizulin, R R; Klochkov, A V; Kuzmin, V V; L'vov, V S; Tagirov, M S

    2012-04-27

    We report the observation of the unusual behavior of induction decay signals in antiferromagnetic monocrystals with Suhl-Nakamura interactions. The signals show the formation of the Bose-Einstein condensation (BEC) of magnons and the existence of spin supercurrent, in complete analogy with the spin superfluidity in the superfluid (3)He and the atomic BEC of quantum gases. In the experiments described here, the temperature of the magnon BEC is a thousand times larger than in the superfluid (3)He. It opens a possibility to apply the spin supercurrent for various magnetic spintronics applications.

  7. Correlated electron state in CeCu2Si2 controlled through Si to P substitution

    NASA Astrophysics Data System (ADS)

    Lai, Y.; Saunders, S. M.; Graf, D.; Gallagher, A.; Chen, K.-W.; Kametani, F.; Besara, T.; Siegrist, T.; Shekhter, A.; Baumbach, R. E.

    2017-08-01

    CeCu2Si2 is an exemplary correlated electron metal that features two domes of unconventional superconductivity in its temperature-pressure phase diagram. The first dome surrounds an antiferromagnetic quantum critical point, whereas the more exotic second dome may span the termination point of a line of f -electron valence transitions. This behavior has received intense interest, but what has been missing are ways to access the high pressure behavior under milder conditions. Here we study Si → P chemical substitution, which compresses the unit cell volume but simultaneously weakens the hybridization between the f - and conduction electron states and encourages complex magnetism. At concentrations that show magnetism, applied pressure suppresses the magnetic ordering temperature and superconductivity is recovered for samples with low disorder. These results reveal that the electronic behavior in this system is controlled by a nontrivial combination of effects from unit cell volume and electronic shell filling. Guided by this topography, we discuss prospects for uncovering a valence fluctuation quantum phase transition in the broader family of Ce-based ThCr2Si2 -type materials through chemical substitution.

  8. Topological gapless phases in nonsymmorphic antiferromagnets

    NASA Astrophysics Data System (ADS)

    Brzezicki, Wojciech; Cuoco, Mario

    2017-04-01

    We investigate the nature of the electronic states in a variety of nonsymmorphic collinear antiferromagnets with glide reflection symmetry, a combination of mirror and half-lattice translation. In particular, the study refers to a class of systems with two-band itinerant electrons that are spin-orbit coupled and interacting with a magnetic background having a zigzag pattern. We describe the symmetry properties of the model system by focusing on the role of nonsymmorphic transformations arising from the antiferromagnetic structure of the spin ordering. Gapless phases with Dirac points having different types of symmetry-protection as well as electronic structures with triple and quadruple band-crossing points are obtained. A glide semimetal is shown to be converted into a gapless phase with Dirac points protected by inversion and time-inversion symmetry combination. Interestingly, we find a relation between the states in the glide sectors that provides a general mechanism to get multiple band touching points. The split of the multiple Fermi points drives the transition from a point node to a line node semimetal or to a metal with nontrivial winding around the Fermi pockets and an electronic structure that is tied to the presence of glide symmetric Dirac points. Besides a new perspective of ordered states in complex materials, our findings indicate relevant paths to topological gapless phases and edge states in a wide class of magnetic systems.

  9. Antiferromagnetic phase diagram of the cuprate superconductors

    NASA Astrophysics Data System (ADS)

    Nunes, L. H. C. M.; Teixeira, A. W.; Marino, E. C.

    2017-02-01

    Taking the spin-fermion model as the starting point for describing the cuprate superconductors, we obtain an effective nonlinear sigma-field hamiltonian, which takes into account the effect of doping in the system. We obtain an expression for the spin-wave velocity as a function of the chemical potential. For appropriate values of the parameters we determine the antiferromagnetic phase diagram for the YBa2Cu3O6+x compound as a function of the dopant concentration in good agreement with the experimental data. Furthermore, our approach provides a unified description for the phase diagrams of the hole-doped and the electron doped compounds, which is consistent with the remarkable similarity between the phase diagrams of these compounds, since we have obtained the suppression of the antiferromagnetic phase as the modulus of the chemical potential increases. The aforementioned result then follows by considering positive values of the chemical potential related to the addition of holes to the system, while negative values correspond to the addition of electrons.

  10. Antiferromagnetic Dirac semimetals in two dimensions

    NASA Astrophysics Data System (ADS)

    Wang, Jing

    2017-03-01

    The search for symmetry-protected two-dimensional (2D) Dirac semimetals analogous to graphene is important both for fundamental and practical interest. The 2D Dirac cones are protected by crystalline symmetries and magnetic ordering may destroy their robustness. Here we propose a general framework to classify stable 2D Dirac semimetals in spin-orbit coupled systems having the combined time-reversal and inversion symmetries, and show the existence of the stable Dirac points in 2D antiferromagnetic semimetals. Compared to 3D Dirac semimetals which fall into two distinct classes, Dirac semimetals in 2D with combined time-reversal and inversion symmetries belong to a single class which is closely related to the nonsymmorphic space-group symmetries. We further provide a concrete model in antiferromagnetic semimetals which supports symmetry-protected 2D Dirac points. The symmetry breaking in such systems leads to 2D chiral topological states such as quantum anomalous Hall insulator and chiral topological superconductor phases.

  11. Orientational transitions in antiferromagnetic liquid crystals

    NASA Astrophysics Data System (ADS)

    Zakhlevnykh, A. N.; Petrov, D. A.

    2016-09-01

    The orientational phases in an antiferromagnetic liquid crystal (ferronematic) based on the nematic liquid crystal with the negative anisotropy of diamagnetic susceptibility are studied in the framework of the continuum theory. The ferronematic was assumed to be compensated; i.e., in zero field, impurity ferroparticles with the magnetic moments directed parallel and antiparallel to the director are equiprobably distributed in it. It is established that under the action of a magnetic field the ferronematic undergoes orientational transitions compensated (antiferromagnetic) phase-non-uniform phase-saturation (ferrimagnetic) phase. The analytical expressions for threshold fields of the transitions as functions of material parameters are obtained. It is shown that with increasing magnetic impurity segregation parameter, the threshold fields of the transitions significantly decrease. The bifurcation diagram of the ferronematic orientational phases is built in terms of the energy of anchoring of magnetic particles with the liquid-crystal matrix and magnetic field. It is established that the Freedericksz transition is the second-order phase transition, while the transition to the saturation state can be second- or first-order. In the latter case, the suspension exhibits orientational bistability. The orientational and magnetooptical properties of the ferronematic in different applied magnetic fields are studied.

  12. Different patterns of sexual dysfunctions associated with psychiatric disorders and psychopharmacological treatment. Results of an investigation by semistructured interview of schizophrenic and neurotic patients and methadone-substituted opiate addicts.

    PubMed

    Teusch, L; Scherbaum, N; Böhme, H; Bender, S; Eschmann-Mehl, G; Gastpar, M

    1995-05-01

    Little is known about sexual dysfunctions associated with psychiatric disorders and psychopharmacological treatment. In the present study schizophrenic patients (n = 45, mostly under neuroleptic treatment), neurotic patients (n = 50, mostly treated without medication), methadone-substituted opiate addicts (n = 37), and normal controls (n = 41) were included. They were interviewed with the aid of a sex-differentiated semistructured questionnaire on sexual function. All the methadone-substituted opiate addicts and nearly all the schizophrenic patients suffered from dysfunctions in at least one criterion. The three clinical groups differed significantly from the controls in sexual interest, emotional arousal, physiological arousal (erectile function/vaginal lubrication), performance (ejaculatory function/vaginism, dyspareunia), and orgasm satisfaction. Characteristic patterns of dysfunction were found in the male patients. The schizophrenic patients had significantly more dysfunctions of interest, physiological arousal, performance, and orgasm than the controls. Emotional arousal, erectile and ejaculatory functions, and orgasm satisfaction were impaired more frequently in the male schizophrenics than in the neurotic patients. Reduced sexual interest, emotional arousal, and orgasm satisfaction were reported more frequently by the methadone-substituted opiate addicts than by the neurotic men. Emotional arousal was even more frequently reduced than in the schizophrenic men. There was no correlation between sexual dysfunction and particular neuroleptics or neuroleptic or methadone dosage. The results are compared with the literature and suggestions made for further investigations.

  13. Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures

    NASA Astrophysics Data System (ADS)

    He, Qing Lin; Kou, Xufeng; Grutter, Alexander J.; Yin, Gen; Pan, Lei; Che, Xiaoyu; Liu, Yuxiang; Nie, Tianxiao; Zhang, Bin; Disseler, Steven M.; Kirby, Brian J.; Ratcliff, William, II; Shao, Qiming; Murata, Koichi; Zhu, Xiaodan; Yu, Guoqiang; Fan, Yabin; Montazeri, Mohammad; Han, Xiaodong; Borchers, Julie A.; Wang, Kang L.

    2017-01-01

    Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

  14. Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures.

    PubMed

    He, Qing Lin; Kou, Xufeng; Grutter, Alexander J; Yin, Gen; Pan, Lei; Che, Xiaoyu; Liu, Yuxiang; Nie, Tianxiao; Zhang, Bin; Disseler, Steven M; Kirby, Brian J; Ratcliff Ii, William; Shao, Qiming; Murata, Koichi; Zhu, Xiaodan; Yu, Guoqiang; Fan, Yabin; Montazeri, Mohammad; Han, Xiaodong; Borchers, Julie A; Wang, Kang L

    2017-01-01

    Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures exhibiting Néel order in an antiferromagnetic CrSb and ferromagnetic order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of magnetic topological insulators. Proximity effects are shown to induce an interfacial spin texture modulation and establish an effective long-range exchange coupling mediated by antiferromagnetism, which significantly enhances the magnetic ordering temperature in the superlattice. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics.

  15. Dynamic zero modes of Dirac fermions and competing singlet phases of antiferromagnetic order

    NASA Astrophysics Data System (ADS)

    Goswami, Pallab; Si, Qimiao

    2017-06-01

    In quantum spin systems, singlet phases often develop in the vicinity of an antiferromagnetic order. Typical settings for such problems arise when itinerant fermions are also present. In this paper, we develop a theoretical framework for addressing such competing orders in an itinerant system, described by Dirac fermions strongly coupled to an O(3) nonlinear sigma model. We focus on two spatial dimensions, where upon disordering the antiferromagnetic order by quantum fluctuations the singular tunneling events also known as (anti)hedgehogs can nucleate competing singlet orders in the paramagnetic phase. In the presence of an isolated hedgehog configuration of the nonlinear sigma model field, we show that the fermion determinant vanishes as the dynamic Euclidean Dirac operator supports fermion zero modes of definite chirality. This provides a topological mechanism for suppressing the tunneling events. Using the methodology of quantum chromodynamics, we evaluate the fermion determinant in the close proximity of magnetic quantum phase transition, when the antiferromagnetic order-parameter field can be described by a dilute gas of hedgehogs and antihedgehogs. We show how the precise nature of emergent singlet order is determined by the overlap between dynamic fermion zero modes of opposite chirality, localized on the hedgehogs and antihedgehogs. For a Kondo-Heisenberg model on the honeycomb lattice, we demonstrate the competition between spin Peierls order and Kondo singlet formation, thereby elucidating its global phase diagram. We also discuss other physical problems that can be addressed within this general framework.

  16. Unconventional resistivity at the border of metallic antiferromagnetism in NiS2

    NASA Astrophysics Data System (ADS)

    Niklowitz, P. G.; Alireza, P. L.; Steiner, M. J.; Lonzarich, G. G.; Braithwaite, D.; Knebel, G.; Flouquet, J.; Wilson, J. A.

    2008-03-01

    We report low-temperature and high-pressure measurements of the electrical resistivity ρ(T) of the antiferromagnetic compound NiS2 in its high-pressure metallic state. The form of ρ(T,p) suggests the presence of a quantum phase transition at a critical pressure pc=76±5kbar . Near pc , the temperature variation of ρ(T) is similar to that observed in NiS2-xSex near the critical composition x=1 , where metallic antiferromagnetism is suppressed at ambient pressure. In both cases, ρ(T) varies approximately as T1.5 over a wide range below 100K . This lets us assume that the high-pressure metallic phase of stoichiometric NiS2 also develops itinerant antiferromagnetism, which becomes suppressed at pc . However, on closer analysis, the resistivity exponent in NiS2 exhibits an undulating variation with temperature not seen in NiSSe (x=1) . This difference in behavior may be due to the effects of spin-fluctuation scattering of charge carriers on cold and hot spots of the Fermi surface in the presence of quenched disorder, which is higher in NiSSe than in stoichiometric NiS2 .

  17. Multiplicative logarithmic corrections to quantum criticality in three-dimensional dimerized antiferromagnets

    NASA Astrophysics Data System (ADS)

    Qin, Yan Qi; Normand, B.; Sandvik, Anders W.; Meng, Zi Yang

    2015-12-01

    We investigate the quantum phase transition in an S =1 /2 dimerized Heisenberg antiferromagnet in three spatial dimensions. By performing large-scale quantum Monte Carlo simulations and detailed finite-size scaling analyses, we obtain high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the antiferromagnetically ordered Néel phase. This transition breaks O(N ) symmetry with N =3 in D =3 +1 dimensions. This is the upper critical dimension, where multiplicative logarithmic corrections to the leading mean-field critical properties are expected; we extract these corrections, establishing their precise forms for both the zero-temperature staggered magnetization ms and the Néel temperature TN. We present a scaling ansatz for TN, including logarithmic corrections, which agrees with our data and indicates exact linearity with ms, implying a complete decoupling of quantum and thermal fluctuation effects even arbitrarily close to the quantum critical point. We also demonstrate the predicted N -independent leading and subleading logarithmic corrections in the size dependence of the staggered magnetic susceptibility. These logarithmic scaling forms have not previously been identified or verified by unbiased numerical methods, and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as TlCuCl3.

  18. Ferroelectric strain modulation of antiferromagnetic moments in Ni/NiO ferromagnet/antiferromagnet heterostructures

    NASA Astrophysics Data System (ADS)

    Zhang, Yu-Jun; Chen, Jia-Hui; Li, Liang-Liang; Ma, Jing; Nan, Ce-Wen; Lin, Yuan-Hua

    2017-05-01

    Electric field manipulation of magnetic properties has attracted a lot of research interest recently in solid-state physics. However, ferroelectric strain modulation of antiferromagnetic (AFM) layer is rarely studied in ferromagnet/antiferromagnet/ferroelectric heterostructures. In this paper, we prepared a Ni/NiO(001) heterostructure on ferroelectric Pb (Mg1/3N b2 /3 ) 0.7T i0.3O3(001 ) substrates and observed an out-of-plane electric field modulation of exchange bias and magnetic anisotropy in the Ni layer. The exchange bias was easily eliminated by an electric field cycle, which was due to the AFM domain switching induced by piezoelectric strain in the NiO layer. Synchrotron x ray linear dichroism results confirmed the AFM moment alignment induced by ferroelectric strain as well. Our work showed a promising strategy to manipulate AFM moments and domains, serving the blooming AFM spintronics.

  19. Critical two-dimensional Ising model with free, fixed ferromagnetic, fixed antiferromagnetic, and double antiferromagnetic boundaries.

    PubMed

    Wu, Xintian; Izmailyan, Nickolay

    2015-01-01

    The critical two-dimensional Ising model is studied with four types boundary conditions: free, fixed ferromagnetic, fixed antiferromagnetic, and fixed double antiferromagnetic. Using bond propagation algorithms with surface fields, we obtain the free energy, internal energy, and specific heat numerically on square lattices with a square shape and various combinations of the four types of boundary conditions. The calculations are carried out on the square lattices with size N×N and 30

  20. Kinetic arrest induced antiferromagnetic order in hexagonal FeMnP{sub 0.75}Si{sub 0.25} alloy

    SciTech Connect

    Li, Guijiang Li, Wei; Schönecker, Stephan; Li, Xiaoqing; Delczeg-Czirjak, Erna K.; Kvashnin, Yaroslav O.; Eriksson, Olle; Johansson, Börje; Vitos, Levente

    2014-12-29

    The magnetic state of the FeMnP{sub 0.75}Si{sub 0.25} alloy was investigated by first principles calculations. The coexistence of ferromagnetic and antiferromagnetic phases in FeMnP{sub 0.75}Si{sub 0.25} with the same hexagonal crystal structure was revealed. It was found that kinetic arrest during the transition from the high temperature disordered paramagnetic phase to the low temperature ordered ferromagnetic phase results in the intermediate metastable and partially disordered antiferromagnetic phase. We propose that the ratio of the ferromagnetic and antiferromagnetic phases in the FeMnP{sub 0.75}Si{sub 0.25} sample can be tuned by adjusting the kinetic process of atomic diffusion. The investigations suggest that careful control of the kinetic diffusion process provides another tuning parameter to design candidate magnetocaloric materials.

  1. Spin supercurrent in the canted antiferromagnetic phase

    NASA Astrophysics Data System (ADS)

    Hama, Yusuke; Tsitsishvili, George; Ezawa, Zyun F.

    2013-03-01

    The spin and layer (pseudospin) degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor ν=2. A complex Goldstone mode emerges describing such a combined degree of freedom. In the zero tunneling-interaction limit (ΔSAS→0), its phase field provokes a supercurrent carrying both spin and charge within each layer. The Hall resistance is predicted to become anomalous precisely as in the ν=1 bilayer system in the counterflow and drag experiments. Furthermore, it is shown that the total current flowing in the bilayer system is a supercurrent carrying solely spins in the counterflow geometry. It is intriguing that all these phenomena occur only in imbalanced bilayer systems.

  2. Ising antiferromagnet on the Archimedean lattices

    NASA Astrophysics Data System (ADS)

    Yu, Unjong

    2015-06-01

    Geometric frustration effects were studied systematically with the Ising antiferromagnet on the 11 Archimedean lattices using the Monte Carlo methods. The Wang-Landau algorithm for static properties (specific heat and residual entropy) and the Metropolis algorithm for a freezing order parameter were adopted. The exact residual entropy was also found. Based on the degree of frustration and dynamic properties, ground states of them were determined. The Shastry-Sutherland lattice and the trellis lattice are weakly frustrated and have two- and one-dimensional long-range-ordered ground states, respectively. The bounce, maple-leaf, and star lattices have the spin ice phase. The spin liquid phase appears in the triangular and kagome lattices.

  3. Antiferromagnetic ordering in MnF(salen).

    PubMed

    Čižmár, Erik; Risset, Olivia N; Wang, Tong; Botko, Martin; Ahir, Akhil R; Andrus, Matthew J; Park, Ju-Hyun; Abboud, Khalil A; Talham, Daniel R; Meisel, Mark W; Brown, Stuart E

    2016-06-15

    Antiferromagnetic order at [Formula: see text] K has been identified in Mn(III)F(salen), salen  =  H14C16N2O2, an S  =  2 linear-chain system. Using single crystals, specific heat studies performed in magnetic fields up to 9 T revealed the presence of a field-independent cusp at the same temperature where (1)H NMR studies conducted at 42 MHz observed dramatic changes in the spin-lattice relaxation time, T 1, and in the linewidths. Low-field (less than 0.1 T) magnetic susceptibility studies of single crystals and randomly-arranged microcrystalline samples reveal subtle features associated with the transition.

  4. Antiferromagnetic ordering in MnF(salen)

    NASA Astrophysics Data System (ADS)

    Čižmár, Erik; Risset, Olivia N.; Wang, Tong; Botko, Martin; Ahir, Akhil R.; Andrus, Matthew J.; Park, Ju-Hyun; Abboud, Khalil A.; Talham, Daniel R.; Meisel, Mark W.; Brown, Stuart E.

    2016-06-01

    Antiferromagnetic order at {{T}\\text{N}}=23 K has been identified in Mn(III)F(salen), salen  =  H14C16N2O2, an S  =  2 linear-chain system. Using single crystals, specific heat studies performed in magnetic fields up to 9 T revealed the presence of a field-independent cusp at the same temperature where 1H NMR studies conducted at 42 MHz observed dramatic changes in the spin-lattice relaxation time, T 1, and in the linewidths. Low-field (less than 0.1 T) magnetic susceptibility studies of single crystals and randomly-arranged microcrystalline samples reveal subtle features associated with the transition.

  5. Antiferromagnetically Induced Photoemission Band in the Cuprates

    NASA Astrophysics Data System (ADS)

    Haas, Stephan; Moreo, Adriana; Dagotto, Elbio

    1995-05-01

    Strong antiferromagnetic correlations in models of high critical temperature (high- Tc) cuprates produce quasiparticlelike features in photoemission (PES) calculations above the Fermi momentum pF corresponding to weakly interacting electrons. This effect, discussed before by Kampf and Schrieffer [Phys. Rev. B 41, 6399 (1990)], is analyzed here using computational techniques in strong coupling. It is concluded that weight above pF should be observable in PES data for underdoped compounds, while in the overdoped regime it will be hidden in the experimental background. At optimal doping the signal is weak. The order of magnitude of our results is compatible with experimental data by Aebi et al. [Phys. Rev. Lett. 72, 2757 (1994)] for Bi2Sr2CaCu2O8.

  6. Antiferromagnetic resonance in Rb1C60

    NASA Astrophysics Data System (ADS)

    Bennati, M.; Griffin, R. G.; Knorr, S.; Grupp, A.; Mehring, M.

    1998-08-01

    High-frequency (94 and 140 GHz) ESR was used to investigate the magnetic properties of the low-dimensional conductor Rb1C60. Below 35 K new features of the electron spin resonance are distinguished from the CESR signal of the conducting phase. The analysis of the resonance linewidth and line shift allows a clear identification of a frequency-dependent antiferromagnetic resonance line (AFMR) below 25 K. The characteristic temperature TN for the ordering transition is 25 K. Between 25 K

  7. Magnetic susceptibilities of antiferromagnetic Re4+ compounds

    NASA Astrophysics Data System (ADS)

    Chatterjee, Ibha; Desai, V. P.

    1981-11-01

    The low-temperature (0-30 K) antiferromagnetic susceptibilites of hexachloro- and hexabromorhenates (K2ReCl6 and K2ReBr6) are explained by using correlated effective-field theory and considering XY symmetry of the exchange Hamiltonian. The theory gives a good account of the observed magnetic susceptibilities of these compounds and the sublattice magnetization of the K2ReCl6 compound. The nearest- and next-nearest-neighbor exchange integrals for these compounds are J1=-1.32 cm-1, J2=0.20 cm-1 for K2ReCl6 and J1=-1.82 cm-1, J2=0.15 cm-1 for K2ReBr6.

  8. Quasiparticle bandstructure of antiferromagnetic EuTe

    NASA Astrophysics Data System (ADS)

    Mathi Jaya, S.; Nolting, W.

    1997-11-01

    The temperature-dependent electronic quasiparticle spectrum of the antiferromagnetic semiconductor EuTe is derived by use of a combination of a many-body model procedure with a tight-binding - `linear muffin tin orbital' (TB - LMTO) band structure calculation. The central part is the d - f model for a single band electron (`test electron') being exchange coupled to the antiferromagnetically ordered localized moments of the Eu ions. The single-electron Bloch energies of the d - f model are taken from a TB - LMTO calculation for paramagnetic EuTe. The d - f model is evaluated by a recently proposed moment conserving Green function technique to get the temperature-dependent sublattice - quasiparticle bandstructure (S - QBS) and sublattice - quasiparticle density of states (S - QDOS) of the unoccupied 5d - 6s energy bands. Unconventional correlation effects and the appearance of characteristic quasiparticles (`magnetic polarons') are worked out in detail. The temperature dependence of the S - QDOS and S - QBS is mainly provoked by the spectral weights of the energy dispersions. Minority- and majority-spin spectra coincide for all temperatures but with different densities of states. Upon cooling from 0953-8984/9/47/012/img1 to T = 0 K the lower conduction band edge exhibits a small blue shift of -0.025 eV in accordance with the experiment. Quasiparticle damping manifesting itself in a temperature-dependent broadening of the spectral density peaks arises from spin exchange processes between (5d - 6s) conduction band electrons and localized 4f moments.

  9. Antiferromagnetic long-range spin ordering in Fe- and NiF e2 -doped BaTi O3 multiferroic layers

    NASA Astrophysics Data System (ADS)

    Barbier, A.; Aghavnian, T.; Badjeck, V.; Mocuta, C.; Stanescu, D.; Magnan, H.; Rountree, C. L.; Belkhou, R.; Ohresser, P.; Jedrecy, N.

    2015-01-01

    We report on the Fe doping and on the comparative Ni-Fe codoping with composition close to NiF e2 of fully oxidized BaTi O3 layers (˜20 nm) elaborated by atomic oxygen plasma assisted molecular beam epitaxy; specifically any role of oxygen vacancies can be excluded in our films. Additionally to the classical in situ laboratory tools, the films were thoroughly characterized by synchrotron radiation x-ray diffraction and x-ray absorption spectroscopy. For purely Fe-doped layers, the native tetragonal perovskite structure evolves rapidly toward cubiclike up to 5% doping level above which the crystalline order disappears. On the contrary, low codoping levels (˜5 %NiF e2 ) fairly improve the thin film crystalline structure and surface smoothness; high levels (˜27%) lead to more crystallographically disordered films, although the tetragonal structure is preserved. Synchrotron radiation magnetic dichroic measurements reveal that metal clustering does not occur, that the Fe valence evolves from Fe2 + for low Fe doping levels to Fe3 + for high doping levels, and that the introduction of Ni favors the occurrence of the Fe2 + valence in the films. For the lower codoping levels it seems that Fe2 + substitutes Ba2 +, whereas Ni2 + always substitutes Ti4 +. Ferromagnetic long-range ordering can be excluded with great sensitivity in all samples as deduced from our x-ray magnetic absorption circular dichroic measurements. On the contrary, our linear dichroic x-ray absorption results support antiferromagnetic long-range ordering while piezoforce microscopy gives evidence of a robust ferroelectric long-range ordering showing that our films are excellent candidates for magnetic exchange coupled multiferroic applications.

  10. A nonmagnetic impurity in a 2D quantum critical antiferromagnet

    NASA Astrophysics Data System (ADS)

    Troyer, Matthias

    2003-03-01

    We compute the properties of a mobile hole and a static impurity injected into a two-dimensional antiferromagnet or superconductor in the vicinity of a magnetic quantum critical point. A static S=1/2 impurity doped into a quantum-disordered spin gap system induces a local moment with spin S=1/2 and a corresponding Curie-like impurity susceptibility, while the same impurity in a Néel ordered state only gives a finite impurity susceptibility. For the quantum critical system however an interesting field-theoretical prediction has been made that there the impurity spin susceptibility still has a Curie-like divergence, but with a universal effective spin that is neither an integer nor a half-odd integer [1]. In large-scale quantum Monte Carlo (QMC) simulations using the loop algorithm we calculate the impurity susceptibility and find that, unfortunately, this effect is not observable since the renormalization of the effective spin away from S=1/2 is minimal. Other predictions of the field theory, such as a new critical exponent η' describing the time-dependent impurity spin correlations can however be confirmed [2]. Next we compute the spectral function of a hole injected into a 2D antiferromagnet or superconductor in the vicinity of a magnetic quantum critical point [3]. We show that, near van Hove singularities, the problem maps onto that of a static vacancy. This allows the calculation of the spectral function in a QMC simulation without encountering the negative sign problem. We find a vanishing quasiparticle residue at the critical point, a new exponent η_h0.080.04 describing the frequency dependence of the spectral function G_h(ω)(ɛ_0-ω)-1+ηh and discuss possible relevance to photoemission spectra of cuprate superconductors near the antinodal points. ^1 S. Sachdev, C. Buragohain and M. Vojta, Science 286, 2479 (1999). ^2 M. Troyer, in Prog. Theor. Phys. Suppl. 145 (2002); M. Körner and M. Troyer, ibid. ^3 S. Sachdev, M. Troyer, and M. Vojta, Phys. Rev

  11. Devil's staircase continuum in the chiral clock spin glass with competing ferromagnetic-antiferromagnetic and left-right chiral interactions.

    PubMed

    Çağlar, Tolga; Berker, A Nihat

    2017-04-01

    The chiral clock spin-glass model with q=5 states, with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations, is studied in d=3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature, antiferromagnetic bond concentration p, random chirality strength, and right-chirality concentration c. The system has a ferromagnetic phase, a multitude of different chiral phases, a chiral spin-glass phase, and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases, where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase, bordered by fragments of regular and temperature-inverted devil's staircases, are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.

  12. Devil's staircase continuum in the chiral clock spin glass with competing ferromagnetic-antiferromagnetic and left-right chiral interactions

    NASA Astrophysics Data System (ADS)

    Ćaǧlar, Tolga; Berker, A. Nihat

    2017-04-01

    The chiral clock spin-glass model with q =5 states, with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations, is studied in d =3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature, antiferromagnetic bond concentration p , random chirality strength, and right-chirality concentration c . The system has a ferromagnetic phase, a multitude of different chiral phases, a chiral spin-glass phase, and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases, where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase, bordered by fragments of regular and temperature-inverted devil's staircases, are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.

  13. Paramagnetic to antiferromagnetic transition in epitaxial tetragonal CuMnAs (invited)

    SciTech Connect

    Hills, V.; Wadley, P. Campion, R. P.; Beardsley, R.; Edmonds, K. W.; Gallagher, B. L.; Novak, V.; Ouladdiaf, B.; Jungwirth, T.

    2015-05-07

    In this paper, we use neutron scattering and electrical transport to investigate the paramagnetic to antiferromagnetic phase transition in tetragonal CuMnAs films on GaP(001). X-ray diffraction and cross-sectional transmission electron microscopy measurements show that the films are chemically ordered with high structural quality. The temperature dependence of the structurally forbidden (100) neutron scattering peak is used to determine the Néel temperature, T{sub N}. We then demonstrate the presence of a clear peak in the temperature derivative of the resistivity around T{sub N}. The effect of disorder-induced broadening on the shape of the peak is discussed.

  14. Quantum selection of order in an XXZ antiferromagnet on a Kagome lattice.

    PubMed

    Chernyshev, A L; Zhitomirsky, M E

    2014-12-05

    Selection of the ground state of the kagome-lattice XXZ antiferromagnet by quantum fluctuations is investigated by combining nonlinear spin-wave and real-space perturbation theories. The two methods unanimously favor q=0 over sqrt[3]×sqrt[3] magnetic order in a wide range of the anisotropy parameter 0≤Δ≲0.72. Both approaches are also in accord on the magnitude of the quantum order-by-disorder effect generated by topologically nontrivial, looplike spin-flip processes. A tentative S-Δ phase diagram of the model is proposed.

  15. Kapitza problem for the magnetic moments of synthetic antiferromagnetic systems

    SciTech Connect

    Dzhezherya, Yu. I.; Demishev, K. O.; Korenivskii, V. N.

    2012-08-15

    The dynamics of magnetization in synthetic antiferromagnetic systems with the magnetic dipole coupling in a rapidly oscillating field has been examined. It has been revealed that the system can behave similar to the Kapitza pendulum. It has been shown that an alternating magnetic field can be efficiently used to control the magnetic state of a cell of a synthetic antiferromagnet. Analytical relations have been obtained between the parameters of such an antiferromagnet and an external magnetic field at which certain quasistationary states are implemented.

  16. Microscopic and macroscopic signatures of antiferromagnetic domain walls.

    PubMed

    Jaramillo, R; Rosenbaum, T F; Isaacs, E D; Shpyrko, O G; Evans, P G; Aeppli, G; Cai, Z

    2007-03-16

    Magnetotransport measurements on small single crystals of Cr, the elemental antiferromagnet, reveal the hysteretic thermodynamics of the domain structure. The temperature dependence of the transport coefficients is directly correlated with the real-space evolution of the domain configuration as recorded by x-ray microprobe imaging, revealing the effect of antiferromagnetic domain walls on electron transport. A single antiferromagnetic domain wall interface resistance is deduced to be of order 5 x 10(-5) mu Omega cm(2) at a temperature of 100 K.

  17. Magnonic analog of relativistic Zitterbewegung in an antiferromagnetic spin chain

    NASA Astrophysics Data System (ADS)

    Wang, Weiwei; Gu, Chenjie; Zhou, Yan; Fangohr, Hans

    2017-07-01

    We theoretically investigate the spin-wave (magnon) excitations in a classical antiferromagnetic spin chain with easy-axis anisotropy. We obtain a Dirac-like equation by linearizing the Landau-Lifshitz-Gilbert equation in this antiferromagnetic system, in contrast to the ferromagnetic system in which a Schrödinger-type equation is derived. The Hamiltonian operator in the Dirac-like equation is a pseudo-Hermitian. We compute and demonstrate relativistic Zitterbewegung (trembling motion) in the antiferromagnetic spin chain by measuring the expectation values of the wave-packet position.

  18. Piezo-antiferromagnetic effect of sawtooth-like graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Zhao, Shangqian; Lu, Yan; Zhang, Yuchun; Lu, Wengang; Liang, Wenjie; Wang, Enge

    2014-05-01

    A type of sawtooth-like graphene nanoribbon (SGNR) with piezo-antiferromagnetic effect is studied numerically. The ground state of the studied SGNR changes from nonmagnetic state to antiferromagnetic state with uniaxial strain. The changes of the spin-charge distributions during the stretching are investigated. The Hubbard model reveals that the hopping integrals between the π-orbitals of the carbon atoms are responsible to the piezo-antiferromagnetic effect. The study sheds light on the application of graphene-based structures to nanosensors and spintronic devices.

  19. Inverse freezing in a cluster Ising spin-glass model with antiferromagnetic interactions.

    PubMed

    Silva, C F; Zimmer, F M; Magalhaes, S G; Lacroix, C

    2012-11-01

    Inverse freezing is analyzed in a cluster spin-glass (SG) model that considers infinite-range disordered interactions between magnetic moments of different clusters (intercluster interaction) and short-range antiferromagnetic coupling J(1) between Ising spins of the same cluster (intracluster interaction). The intercluster disorder J is treated within a mean-field theory by using a framework of one-step replica symmetry breaking. The effective model obtained by this treatment is computed by means of an exact diagonalization method. With the results we build phase diagrams of temperature T/J versus J(1)/J for several sizes of clusters n(s) (number of spins in the cluster). The phase diagrams show a second-order transition from the paramagnetic phase to the SG order at the freezing temperature T(f) when J(1)/J is small. The increase in J(1)/J can then destroy the SG phase. It decreases T(f)/J and introduces a first-order transition. In addition, inverse freezing can arise at a certain range of J(1)/J and large enough n(s). Therefore, the nontrivial frustration generated by disorder and short-range antiferromagnetic coupling can introduce inverse freezing spontaneously.

  20. Isotope effect and cation disorder in manganites

    NASA Astrophysics Data System (ADS)

    Babushkina, N. A.; Chistotina, E. A.; Balagurov, A. M.; Pomjakushin, V. Yu.; Gorbenko, O. Yu.; Kaul, A. R.; Kartavtseva, M. S.

    2006-05-01

    The measurements of temperature dependence of electrical resistivity ρ(T) and magnetic susceptibility χ(T) as well as neutron diffraction studies were performed for three groups of R1-xSrxMnO3 manganites. Each group was characterized by the same average ionic radius of rare earth R, but by different degree of cation disorder σ2=xiri2-2. For each composition, the isotope-substituted samples ( O16→O18) were also prepared. It was shown that the increase in σ2 at fixed leads to appreciable changes in electrical and magnetic properties, as well as to a more pronounced isotope effect. Large values of σ2 give rise to a significant scatter in the values of the electron hopping integral for neighboring Mn sites. This favors the electron localization and the tendency to antiferromagnetism in the phase-separated state, thus enhancing the effect of oxygen isotope substitution.

  1. Giant magnetic effects and oscillations in antiferromagnetic Josephson weak links

    SciTech Connect

    Gorkov, L.; Kresin, Vladimir

    2001-04-01

    Josephson junctions with an antiferromagnetic metal as a link are described. The junction can be switched off by a relatively small magnetic field. The amplitude of the current oscillates as a function of the field.

  2. Long-range interactions in antiferromagnetic quantum spin chains

    NASA Astrophysics Data System (ADS)

    Bravo, B.; Cabra, D. C.; Gómez Albarracín, F. A.; Rossini, G. L.

    2017-08-01

    We study the role of long-range dipolar interactions on antiferromagnetic spin chains, from the classical S →∞ limit to the deep quantum case S =1 /2 , including a transverse magnetic field. To this end, we combine different techniques such as classical energy minima, classical Monte Carlo, linear spin waves, bosonization, and density matrix renormalization group (DMRG). We find a phase transition from the already reported dipolar ferromagnetic region to an antiferromagnetic region for high enough antiferromagnetic exchange. Thermal and quantum fluctuations destabilize the classical order before reaching magnetic saturation in both phases, and also close to zero field in the antiferromagnetic phase. In the extreme quantum limit S =1 /2 , extensive DMRG computations show that the main phases remain present with transition lines to saturation significatively shifted to lower fields, in agreement with the bosonization analysis. The overall picture maintains a close analogy with the phase diagram of the anisotropic XXZ spin chain in a transverse field.

  3. A new phase diagram for layered antiferromagnetic films.

    PubMed

    Hellwig, Olav; Kirk, Taryl L; Kortright, Jeffrey B; Berger, Andreas; Fullerton, Eric E

    2003-02-01

    Magnetic multilayer films provide convenient model systems for studying the physics of antiferromagnetic films and surfaces. Here we report on the magnetic reversal and domain structure in antiferromagnetically coupled Co/Pt multilayers that are isomorphic to layered antiferromagnetic films with perpendicular magnetic anisotropy. We observe two distinct remanent states and reversal modes of the system. In mode 1 the magnetization in each layer reverses independently, producing an antiferromagnetic remanent state that shows full lateral correlation and vertical anticorrelation across the interlayers. In mode 2 the reversal in adjacent layers is locally synchronized with a remanent state that is vertically correlated but laterally anticorrelated in ferromagnetic stripe domains. Theoretical energy calculations of the two ground states identify a new phase boundary that is in good agreement with our experimental results.

  4. Characterization of the Dilute Ising Antiferromagnet

    SciTech Connect

    Wiener, Timothy

    2000-09-12

    A spin glass is a magnetic ground state in which ferromagnetic and antiferromagnetic exchange interactions compete, thereby creating frustration and a multidegenerate state with no long range order. An Ising system is a system where the spins are constrained to lie parallel or antiparallel to a primary axis. There has been much theoretical interest in the past ten years in the effects of applying a magnetic field transverse to the primary axis in an Ising spin glass at low temperatures and thus study phase transitions at the T=0 limit. The focus of this study is to search for and characterize a new Ising spin glass system. This is accomplished by site diluting yttrium for terbium in the crystalline material TbNi2Ge2. The first part of this work gives a brief overview of the physics of rare earth magnetism and an overview of experimental characteristics of spin glasses. This is followed by the methodology used to manufacture the large single crystals used in this study, as well as the measurement techniques used. Next, a summary of the results of magnetic measurements on across the dilution series from pure terbium to pure yttrium is presented. This is followed by detailed measurements on particular dilutions which demonstrate spin glass behavior. Pure TbNi2Ge2 is an Ising antiferromagnet with a several distinct metamagnetic states below 17 K. As the terbium is alloyed with yttrium, these magnetic states are weakened in a consistent manner, as is seen in measurements of the transition temperatures and analysis of Curie-Weiss behavior at high temperature. At low concentrations of terbium, below 35%, long range order is no longer present and a spin-glass-like state emerges. This state is studied through various measurements, dc and ac susceptibility, resistivity, and specific heat. This magnetic behavior was then compared to that of other well characterized spin glasses. It is concluded that there is a region of

  5. Two-Dimensional Massless Dirac Fermions in Antiferromagnetic A Fe2As2 (A =Ba ,Sr )

    NASA Astrophysics Data System (ADS)

    Chen, Zhi-Guo; Wang, Luyang; Song, Yu; Lu, Xingye; Luo, Huiqian; Zhang, Chenglin; Dai, Pengcheng; Yin, Zhiping; Haule, Kristjan; Kotliar, Gabriel

    2017-09-01

    We report infrared studies of A Fe2As2 (A =Ba , Sr), two representative parent compounds of iron-arsenide superconductors, at magnetic fields (B ) up to 17.5 T. Optical transitions between Landau levels (LLs) were observed in the antiferromagnetic states of these two parent compounds. Our observation of a √{B } dependence of the LL transition energies, the zero-energy intercepts at B =0 T under the linear extrapolations of the transition energies and the energy ratio (˜2.4 ) between the observed LL transitions, combined with the linear band dispersions in two-dimensional (2D) momentum space obtained by theoretical calculations, demonstrates the existence of massless Dirac fermions in the antiferromagnet BaFe2 As2 . More importantly, the observed dominance of the zeroth-LL-related absorption features and the calculated bands with extremely weak dispersions along the momentum direction kz indicate that massless Dirac fermions in BaFe2 As2 are 2D. Furthermore, we find that the total substitution of the barium atoms in BaFe2 As2 by strontium atoms not only maintains 2D massless Dirac fermions in this system, but also enhances their Fermi velocity, which supports that the Dirac points in iron-arsenide parent compounds are topologically protected.

  6. Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe_{2}As_{2} (A=Ba,Sr).

    PubMed

    Chen, Zhi-Guo; Wang, Luyang; Song, Yu; Lu, Xingye; Luo, Huiqian; Zhang, Chenglin; Dai, Pengcheng; Yin, Zhiping; Haule, Kristjan; Kotliar, Gabriel

    2017-09-01

    We report infrared studies of AFe_{2}As_{2} (A=Ba, Sr), two representative parent compounds of iron-arsenide superconductors, at magnetic fields (B) up to 17.5 T. Optical transitions between Landau levels (LLs) were observed in the antiferromagnetic states of these two parent compounds. Our observation of a sqrt[B] dependence of the LL transition energies, the zero-energy intercepts at B=0  T under the linear extrapolations of the transition energies and the energy ratio (∼2.4) between the observed LL transitions, combined with the linear band dispersions in two-dimensional (2D) momentum space obtained by theoretical calculations, demonstrates the existence of massless Dirac fermions in the antiferromagnet BaFe_{2}As_{2}. More importantly, the observed dominance of the zeroth-LL-related absorption features and the calculated bands with extremely weak dispersions along the momentum direction k_{z} indicate that massless Dirac fermions in BaFe_{2}As_{2} are 2D. Furthermore, we find that the total substitution of the barium atoms in BaFe_{2}As_{2} by strontium atoms not only maintains 2D massless Dirac fermions in this system, but also enhances their Fermi velocity, which supports that the Dirac points in iron-arsenide parent compounds are topologically protected.

  7. Magnetic and structural properties of antiferromagnetic Mn2VSi alloy films grown at elevated temperatures

    NASA Astrophysics Data System (ADS)

    Wu, Haokaifeng; Vallejo-Fernandez, Gonzalo; Hirohata, Atsufumi

    2017-09-01

    Eighty nano meters thick polycrystalline Mn2VSi films have been deposited on silicon substrates with an 18 nm silver seed layer and a 3 nm aluminium capping layer using a sputtering system. The best quality film is obtained for 723 K growth. The Mn2VSi thin film is verified to be antiferromagnetic, where an exchange bias is found when a 3 nm ferromagnetic CoFe layer has been deposited on the top of the Mn2VSi layer. The exchange bias is measured to be 34 Oe at 100 K. The blocking and thermal activation temperature of Mn2VSi is estimated to be below 100 K and within a range between 100 K and 448 K, respectively. These properties can be improved by substituting the constituent atoms with the other elements (e.g. Co and Al), suggesting a potential of Mn2VSi to be used as an antiferromagnet in a spintronic device.

  8. Strong intermolecular antiferromagnetic verdazyl-verdazyl coupling in the solid state.

    PubMed

    Eusterwiemann, S; Doerenkamp, C; Dresselhaus, T; Janka, O; de Oliveira, M; Daniliuc, C G; Eckert, H; Neugebauer, J; Pöttgen, R; Studer, A

    2017-06-21

    Strong magnetic couplings are generally observed intramolecularly in organic diradicals or in systems in which they are promoted by crystal engineering strategies involving, for example, transition metal ligation. We herein present a strong intermolecularly coupling verdazyl radical in the solid state without the use of such design strategies. The crystal structure of an acetylene-substituted verdazyl radical shows a unique antiparallel face-to-face orientation of the neighboring verdazyl molecules along with verdazyl-acetylene interactions giving rise to an alternating antiferromagnetic Heisenberg chain. Single crystal structural data at 80, 100, 173, and 223 K show that one of the π-stacking distances depends on temperature, while heat capacity data indicate the absence of a phase transition. Based on this structural input, broken symmetry DFT calculations predict a change from an alternating linear Heisenberg chain with two comparable coupling constants J1 and J2 at higher temperatures towards dominant pair interactions at lower temperatures. The predicted antiferromagnetic coupling is confirmed experimentally by magnetic susceptibility, solid-state EPR and NMR spectroscopic results.

  9. Ba1-xKxMn2As2: An Antiferromagnetic Local-Moment Metal

    NASA Astrophysics Data System (ADS)

    Pandey, Abhishek; Dhaka, R. S.; Lamsal, J.; Lee, Y.; Anand, V. K.; Kreyssig, A.; Heitmann, T. W.; McQueeney, R. J.; Goldman, A. I.; Harmon, B. N.; Kaminski, A.; Johnston, D. C.

    2012-02-01

    The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature TN=625K and a large ordered moment μ=3.9μB/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large μ. Ba1-xKxMn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba1-xKxMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.

  10. Coalescence-driven magnetic order of the uncompensated antiferromagnetic Co doped ZnO

    NASA Astrophysics Data System (ADS)

    Ney, V.; Henne, B.; Lumetzberger, J.; Wilhelm, F.; Ollefs, K.; Rogalev, A.; Kovacs, A.; Kieschnick, M.; Ney, A.

    2016-12-01

    The evolution of the structural and magnetic properties of Co doped ZnO has been investigated over an unprecedented concentration range above the coalescence limit. ZnO films with Co concentrations from 20% to 60% of the cationic lattice have been grown by reactive magnetron sputtering. The wurtzite crystal structure was maintained even for these high dopant concentrations. By measuring the x-ray absorption at the near edge and the linear and circular dichroism of the films at the Zn and Co K edge, it could be shown that Co substitutes predominantly for Zn in the lattice. No indications of metallic Co have been found in the samples. At low Co concentrations, the films are paramagnetic, but with increasing Co content, the films become antiferromagnetically ordered with increasing order temperature. Uncompensated spins, coupled to the antiferromagnetic dopant configurations, lead to a vertical exchange-bias-like effect, which increases with increasing Co concentration. In parallel, the single-ion anisotropy is gradually lost.

  11. Bistable transmission of antiferromagnetic Fabry-Perot resonator

    NASA Astrophysics Data System (ADS)

    Zhao, Yan; Fu, Shu-Fang; Li, Hua; Wang, Xuan-Zhang

    2011-07-01

    We investigate the magnetically nonlinear optical transmission of the Fabry-Perot resonator filled with an antiferromagnetic medium. In a proper incident power range, we find very large nonlinear phase shifts so that the bistable switches appear even for a very thin medium film, such as of half-wavelength thickness. All results are based on antiferromagnetic MnF2 medium with far-infrared resonant frequencies.

  12. Preparation of Entangled and Antiferromagnetic States by Dissipative Rydberg Pumping

    NASA Astrophysics Data System (ADS)

    Carr, A. W.; Saffman, M.

    2013-07-01

    We propose and analyze an approach for preparation of high fidelity entanglement and antiferromagnetic states using Rydberg mediated interactions with dissipation. Using asymmetric Rydberg interactions the two-atom Bell singlet is a dark state of the Rydberg pumping process. Master equation simulations demonstrate Bell singlet preparation fidelity F=0.998. Antiferromagnetic states are generated on a four-spin plaquette in agreement with results found from diagonalization of the transverse field Ising Hamiltonian.

  13. Spin-transfer torque induced spin waves in antiferromagnetic insulators

    SciTech Connect

    Daniels, Matthew W.; Guo, Wei; Stocks, George Malcolm; Xiao, Di; Xiao, Jiang

    2015-01-01

    We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations.

  14. Spin-transfer torque induced spin waves in antiferromagnetic insulators

    DOE PAGES

    Daniels, Matthew W.; Guo, Wei; Stocks, George Malcolm; ...

    2015-01-01

    We explore the possibility of exciting spin waves in insulating antiferromagnetic films by injecting spin current at the surface. We analyze both magnetically compensated and uncompensated interfaces. We find that the spin current induced spin-transfer torque can excite spin waves in insulating antiferromagnetic materials and that the chirality of the excited spin wave is determined by the polarization of the injected spin current. Furthermore, the presence of magnetic surface anisotropy can greatly increase the accessibility of these excitations.

  15. Effect of Mn substitution on the transport properties of co-sputtered Fe{sub 3−x}Mn{sub x}Si epilayers

    SciTech Connect

    Tang, M.; Jin, C.; Bai, H. L.

    2014-11-07

    Motivated by the theoretical calculations that Fe{sub 3−x}Mn{sub x}Si can simultaneously exhibit a high spin polarization with a high Curie temperature to be applied in spintronic devices, and in order to further study the effect of Mn contents on the physical properties of Fe{sub 3−x}Mn{sub x}Si, we have investigated the effect of Mn substitution on the transport properties of epitaxial Fe{sub 3−x}Mn{sub x}Si (0≤x≤1) films systematically. The Fe{sub 3−x}Mn{sub x}Si films were epitaxially grown on MgO(001) plane with 45° rotation. The magnetization for various x shows enhanced irreversibility, implying the antiferromagnetic ordering induced by the substitution of Mn. A metal-semiconductor crossover was observed due to the enhanced disorders of interactions and the local lowering of symmetry induced by the substitution of Mn. The single-domain state in the Fe{sub 3−x}Mn{sub x}Si films leads to twofold symmetric curves of the anisotropic magnetoresistance and planar Hall resistivity.

  16. Thermally activated repolarization of antiferromagnetic particles: Monte Carlo dynamics

    NASA Astrophysics Data System (ADS)

    Soloviev, S. V.; Popkov, A. F.; Knizhnik, A. A.; Iskandarova, I. M.

    2017-02-01

    Based on the equation of motion of an antiferromagnetic moment, taking into account a random field of thermal fluctuations, we propose a Monte Carlo (MC) scheme for the numerical simulation of the evolutionary dynamics of an antiferromagnetic particle, corresponding to the Langevin dynamics in the Kramers theory for the two-well potential. Conditions for the selection of the sphere of fluctuations of random deviations of the antiferromagnetic vector at an MC time step are found. A good agreement with the theory of Kramers thermal relaxation is demonstrated for varying temperatures and heights of energy barrier over a wide range of integration time steps in an overdamped regime. Based on the developed scheme, we performed illustrative calculations of the temperature drift of the exchange bias under the fast annealing of a ferromagnet-antiferromagnet structure, taking into account the random variation of anisotropy directions in antiferromagnetic grains and their sizes. The proposed approach offers promise for modeling magnetic sensors and spintronic memory devices containing heterostructures with antiferromagnetic layers.

  17. Peculiarities of stochastic motion in antiferromagnetic nanoparticles

    NASA Astrophysics Data System (ADS)

    Gomonay, H.; Loktev, V.

    2013-01-01

    Antiferromagnetic (AFM) materials are widely used in spintronic devices as passive elements (for stabilization of ferromagnetic layers) and as active elements (for information coding). In both cases the switching between different AFM states, to a great extent depends on the environmental noise. In the present paper we derive stochastic Langevian equations for an AFM vector and a corresponding Fokker-Plank equation for a distribution function in the phase space of generalised coordinate and momentum. Thermal noise is modelled by a random delta-correlated magnetic field that interacts with the dynamic magnetisation of AFM particle. We scrupulously analyse a particular case of a collinear compensated AFM in the presence of spin-polarised current. The energy distribution function is found for normal modes in the vicinity of two equilibrium states (static and stationary) in sub- and super-critical regimes. It is shown that the noise-induced dynamics of AFM vector has some pecuilarities compared to the dynamics of magnetisation vector in ferromagnets.

  18. Quantum Phase Transitions in Antiferromagnets and Superfluids

    NASA Astrophysics Data System (ADS)

    Sachdev, Subir

    2000-03-01

    A general introduction to the non-zero temperature dynamic and transport properties of low-dimensional systems near a quantum phase transition shall be presented. Basic results will be reviewed in the context of experiments on the spin-ladder compounds. Recent large N computations (M. Vojta and S. Sachdev, Phys. Rev. Lett. 83), 3916 (1999) on an extended t-J model motivate a global scenario of the quantum phases and transitions in the high temperature superconductors, and connections will be made to numerous experiments. A universal theory (S. Sachdev, C. Buragohain, and M. Vojta, Science, in press M. Vojta, C. Buragohain, and S. Sachdev, cond- mat/9912020) of quantum impurities in spin-gap antiferromagnets near a magnetic ordering transition will be compared quantitatively to experiments on Zn doped Y Ba2 Cu3 O7 (Fong et al.), Phys. Rev. Lett. 82, 1939 (1999)

  19. Antiferromagnetic Skyrmion: Stability, Creation and Manipulation.

    PubMed

    Zhang, Xichao; Zhou, Yan; Ezawa, Motohiko

    2016-04-21

    Magnetic skyrmions are particle-like topological excitations in ferromagnets, which have the topo-logical number Q = ± 1, and hence show the skyrmion Hall effect (SkHE) due to the Magnus force effect originating from the topology. Here, we propose the counterpart of the magnetic skyrmion in the antiferromagnetic (AFM) system, that is, the AFM skyrmion, which is topologically protected but without showing the SkHE. Two approaches for creating the AFM skyrmion have been described based on micromagnetic lattice simulations: (i) by injecting a vertical spin-polarized current to a nanodisk with the AFM ground state; (ii) by converting an AFM domain-wall pair in a nanowire junction. It is demonstrated that the AFM skyrmion, driven by the spin-polarized current, can move straightly over long distance, benefiting from the absence of the SkHE. Our results will open a new strategy on designing the novel spintronic devices based on AFM materials.

  20. Thermalization of a dimerized antiferromagnetic spin chain.

    PubMed

    Konstantinidis, N P

    2016-01-20

    Thermalization is investigated for the one-dimensional anisotropic antiferromagnetic Heisenberg model with dimerized nearest-neighbor interactions that break integrability. For this purpose the time evolution of local operator expectation values after an interacting quench is calculated directly with the Chebyshev polynomial expansion, and the deviation of the diagonal from the canonical thermal ensemble value is calculated for increasing system size for these operators. The spatial and spin symmetries of the Hamiltonian are taken into account to divide it into symmetry subsectors. The rate of thermalization is found to weaken with the dimerization parameter as the Hamiltonian evolves between two integrable limits, the non-dimerized and the fully dimerized where the chain breaks up into isolated dimers. This conclusion is supported by the distribution of the local operator off-diagonal elements between the eigenstates of the Hamiltonian with respect to their energy difference, which determines the strength of temporal fluctuations. The off-diagonal elements have a low-energy peak for small dimerization which facilitates thermalization, and originates in the reduction of spatial symmetry with respect to the non-dimerized limit. For increasing dimerization their distribution changes and develops a single low-energy maximum that relates to the fully dimerized limit and slows down thermalization.

  1. Robust ferromagnetism carried by antiferromagnetic domain walls

    NASA Astrophysics Data System (ADS)

    Hirose, Hishiro T.; Yamaura, Jun-Ichi; Hiroi, Zenji

    2017-02-01

    Ferroic materials, such as ferromagnetic or ferroelectric materials, have been utilized as recording media for memory devices. A recent trend for downsizing, however, requires an alternative, because ferroic orders tend to become unstable for miniaturization. The domain wall nanoelectronics is a new developing direction for next-generation devices, in which atomic domain walls, rather than conventional, large domains themselves, are the active elements. Here we show that atomically thin magnetic domain walls generated in the antiferromagnetic insulator Cd2Os2O7 carry unusual ferromagnetic moments perpendicular to the wall as well as electron conductivity: the ferromagnetic moments are easily polarized even by a tiny field of 1 mT at high temperature, while, once cooled down, they are surprisingly robust even in an inverse magnetic field of 7 T. Thus, the magnetic domain walls could serve as a new-type of microscopic, switchable and electrically readable magnetic medium which is potentially important for future applications in the domain wall nanoelectronics.

  2. Robust ferromagnetism carried by antiferromagnetic domain walls

    PubMed Central

    Hirose, Hishiro T.; Yamaura, Jun-ichi; Hiroi, Zenji

    2017-01-01

    Ferroic materials, such as ferromagnetic or ferroelectric materials, have been utilized as recording media for memory devices. A recent trend for downsizing, however, requires an alternative, because ferroic orders tend to become unstable for miniaturization. The domain wall nanoelectronics is a new developing direction for next-generation devices, in which atomic domain walls, rather than conventional, large domains themselves, are the active elements. Here we show that atomically thin magnetic domain walls generated in the antiferromagnetic insulator Cd2Os2O7 carry unusual ferromagnetic moments perpendicular to the wall as well as electron conductivity: the ferromagnetic moments are easily polarized even by a tiny field of 1 mT at high temperature, while, once cooled down, they are surprisingly robust even in an inverse magnetic field of 7 T. Thus, the magnetic domain walls could serve as a new-type of microscopic, switchable and electrically readable magnetic medium which is potentially important for future applications in the domain wall nanoelectronics. PMID:28195565

  3. Study of magnetization switching in synthetic antiferromagnets

    NASA Astrophysics Data System (ADS)

    Radu, Cosmin; Cimpoesu, Dorin; Spinu, Leonard; Stancu, Alexandru

    2008-03-01

    Synthetic Antiferromagnet (SAF) structures are very important in designing modern spintronic devices. The theoretical studies of the toggle writing mode in MRAM^1 use the concepts of SAF critical curve, which is a generalization of the astroid from the coherent rotation model in the case of uniaxial anisotropy. Although extensively studied theoretically^2,3 there are no methods proposed to experimentally determine the critical curve of a SAF structure. We propose a way for determining the critical curve of the switching fields using reversible susceptibility experiments (RS) and we prove this to be more sensitive to the switching characteristics of SAF structures than a regular hysteresis loop. For certain coupling strengths the entire critical curve can't be determined using standard RS experiments and a strategy for revealing these hidden parts of the critical curve is proposed. 1. L. Savtchenko, B. N. Engel, N. D. Rizzo, M. F. Deherrera, and J. A. Janesky, US Patent 6,545,906 B1, (2003). 2. S. Y. Wang and H. Fujiwara, J. Magn. Magn. Mater. 286, 27-30 (2005). 3. H. Fujiwara, S. Y. Wang, and M. Sun, J. Appl. Phys. 97, 10P507-10P507-5 (2005). Work supported by DARPA grant HR0011-07-1-0031.

  4. Antiferromagnetic Skyrmion: Stability, Creation and Manipulation

    PubMed Central

    Zhang, Xichao; Zhou, Yan; Ezawa, Motohiko

    2016-01-01

    Magnetic skyrmions are particle-like topological excitations in ferromagnets, which have the topo-logical number Q = ± 1, and hence show the skyrmion Hall effect (SkHE) due to the Magnus force effect originating from the topology. Here, we propose the counterpart of the magnetic skyrmion in the antiferromagnetic (AFM) system, that is, the AFM skyrmion, which is topologically protected but without showing the SkHE. Two approaches for creating the AFM skyrmion have been described based on micromagnetic lattice simulations: (i) by injecting a vertical spin-polarized current to a nanodisk with the AFM ground state; (ii) by converting an AFM domain-wall pair in a nanowire junction. It is demonstrated that the AFM skyrmion, driven by the spin-polarized current, can move straightly over long distance, benefiting from the absence of the SkHE. Our results will open a new strategy on designing the novel spintronic devices based on AFM materials. PMID:27099125

  5. Nonequilibrium antiferromagnetic mixed-spin Ising model.

    PubMed

    Godoy, Mauricio; Figueiredo, Wagner

    2002-09-01

    We studied an antiferromagnetic mixed-spin Ising model on the square lattice subject to two competing stochastic processes. The model system consists of two interpenetrating sublattices of spins sigma=1/2 and S=1, and we take only nearest neighbor interactions between pairs of spins. The system is in contact with a heat bath at temperature T, and the exchange of energy with the heat bath occurs via one-spin flip (Glauber dynamics). Besides, the system interacts with an external agency of energy, which supplies energy to it whenever two nearest neighboring spins are simultaneously flipped. By employing Monte Carlo simulations and a dynamical pair approximation, we found the phase diagram for the stationary states of the model in the plane temperature T versus the competition parameter between one- and two-spin flips p. We observed the appearance of three distinct phases, that are separated by continuous transition lines. We also determined the static critical exponents along these lines and we showed that this nonequilibrium model belongs to the universality class of the two-dimensional equilibrium Ising model.

  6. Antiferromagnetic Skyrmion: Stability, Creation and Manipulation

    NASA Astrophysics Data System (ADS)

    Zhang, Xichao; Zhou, Yan; Ezawa, Motohiko

    2016-04-01

    Magnetic skyrmions are particle-like topological excitations in ferromagnets, which have the topo-logical number Q = ± 1, and hence show the skyrmion Hall effect (SkHE) due to the Magnus force effect originating from the topology. Here, we propose the counterpart of the magnetic skyrmion in the antiferromagnetic (AFM) system, that is, the AFM skyrmion, which is topologically protected but without showing the SkHE. Two approaches for creating the AFM skyrmion have been described based on micromagnetic lattice simulations: (i) by injecting a vertical spin-polarized current to a nanodisk with the AFM ground state; (ii) by converting an AFM domain-wall pair in a nanowire junction. It is demonstrated that the AFM skyrmion, driven by the spin-polarized current, can move straightly over long distance, benefiting from the absence of the SkHE. Our results will open a new strategy on designing the novel spintronic devices based on AFM materials.

  7. Robust ferromagnetism carried by antiferromagnetic domain walls.

    PubMed

    Hirose, Hishiro T; Yamaura, Jun-Ichi; Hiroi, Zenji

    2017-02-14

    Ferroic materials, such as ferromagnetic or ferroelectric materials, have been utilized as recording media for memory devices. A recent trend for downsizing, however, requires an alternative, because ferroic orders tend to become unstable for miniaturization. The domain wall nanoelectronics is a new developing direction for next-generation devices, in which atomic domain walls, rather than conventional, large domains themselves, are the active elements. Here we show that atomically thin magnetic domain walls generated in the antiferromagnetic insulator Cd2Os2O7 carry unusual ferromagnetic moments perpendicular to the wall as well as electron conductivity: the ferromagnetic moments are easily polarized even by a tiny field of 1 mT at high temperature, while, once cooled down, they are surprisingly robust even in an inverse magnetic field of 7 T. Thus, the magnetic domain walls could serve as a new-type of microscopic, switchable and electrically readable magnetic medium which is potentially important for future applications in the domain wall nanoelectronics.

  8. Effect of antiferromagnet on exchange bias in systems with antiferromagnetic interfacial coupling and inverted ferromagnetic-antiferromagnetic core-matrix morphology.

    PubMed

    Hu, Yong; Ma, Yan; Liu, Yan; Du, An

    2010-11-01

    A modified Monte Carlo Metropolis method is performed to simulate the effects of antiferromagnetic exchange interaction J(AF) and anisotropy K(AF) on exchange bias field H(E) and coercivity H(C) in the nanoparticle systems with antiferromagnetic interfacial coupling and inverted ferromagnetic-antiferromagnetic core-matrix morphology after cooling in weak and strong fields H(CF), respectively. The results show that the J(AF) dependence of H(E) is insensitive, except obvious changes occur at intermediate J(AF) for two H(CF). When the values of J(AF) are weak, the absolute values of H(E) may keep at a relatively large value. H(C) has a peak at approximately J(AF) = -0.6 with the increase of J(AF) for weak H(CF), while the opposite trend appears for the case of strong H(CF). H(E) is negative and its absolute value increases with the increase of K(AF) for weak H(CF), so does the trend of H(E) with K(AF) for strong H(CF) as K(AF) < or = 6. However, with further increase of K(AF) for strong H(CF), H(E) varies from the negative value to the positive value. Whereas H(C) for two values of H(CF) both decrease and finally level off with increasing K(AF). Variations of antiferromagnetic exchange interaction and anisotropy may alter the net magnetization and the pinning ability of spins on the surface of frustrated antiferromagnetic cores, resulting in the change of pinning configuration in the antiferromagnet during the magnetization reversal of ferromagnetic spins to influence the exchange bias.

  9. NMR investigation of antiferromagnetism and coherence in URu2Si2 -xPx

    NASA Astrophysics Data System (ADS)

    Shirer, K. R.; Lawson, M.; Kissikov, T.; Bush, B. T.; Gallagher, A.; Chen, K.-W.; Baumbach, R. E.; Curro, N. J.

    2017-01-01

    We report 31P and 29Si NMR in single crystals of URu2Si2 -xPx for x =0.09 and x =0.33 . The spectra in the x =0.33 sample are consistent with a homogenous commensurate antiferromagnetic phase below TN˜37 K. The Knight shift exhibits an anomaly at the coherence temperature T* that is slightly enhanced with P doping. Spin-lattice-relaxation rate data indicate that the density of states is suppressed for x =0.09 below 30 K, similar to the undoped compound, but there is no evidence of long-range order at this concentration. Our results suggest that Si substitution provides chemical pressure and electronic tuning mediated by filling of the s /p shells with minimal electronic inhomogeneity.

  10. Algebra Solutions of Antiferromagnet-Antiferromagnet-Ferromagnet Quantum Heisenberg Chains Related to Sp(6,R) Lie Algebra

    NASA Astrophysics Data System (ADS)

    Jin, Shuo; Xie, Bing-Hao

    2011-10-01

    Antiferromagnet-antiferromagnet-ferromagnet (AF-AF-F) quantum Heisenberg chains in linear spin-wave frame is shown explicitly to have an su(1,2) algebraic structure, and its algebra solutions related to the Sp(6,R) Lie algebra are derived by using an algebraic method. It is found that the energy spectrum of the system is determined by one-boson excitation energies built on a vector coherent state of Sp(6,R)⊃U(1,2).

  11. Segregation of antiferromagnetism and high-temperature superconductivity in Ca1-xLaxFe2As2

    NASA Astrophysics Data System (ADS)

    Saha, Shanta R.; Drye, T.; Goh, S. K.; Klintberg, L. E.; Silver, J. M.; Grosche, F. M.; Sutherland, M.; Munsie, T. J. S.; Luke, G. M.; Pratt, D. K.; Lynn, J. W.; Paglione, J.

    2014-04-01

    We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material Ca1-xLaxFe2As2. Using electrical transport, elastic neutron scattering, and resonant tunnel diode oscillator measurements on samples under both quasihydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) superconducting critical point. Our results indicate that the observed superconducting phase with a maximum transition temperature of Tc=47 K is intrinsic to these materials, appearing only upon suppression of magnetic order by pressure-tuning through the AFM critical point. Thus, the superconducting phase appears to exist exclusively in juxtaposition to the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. Unlike the lower-Tc systems, in which superconductivity and magnetism usually coexist, the tendency for the highest-Tc systems to show noncoexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family.

  12. Electric-field-induced spin disorder-to-order transition near a multiferroic triple phase point

    SciTech Connect

    Jang, Byung -Kweon; Lee, Jin Hong; Chu, Kanghyun; Sharma, Pankaj; Kim, Gi -Yeop; Ko, Kyung -Tae; Kim, Kwang -Eun; Kim, Yong -Jin; Kang, Kyungrok; Jang, Han -Byul; Jang, Hoyoung; Jung, Min Hwa; Song, Kyung; Koo, Tae Yeong; Choi, Si -Young; Seidel, Jan; Jeong, Yoon Hee; Ohldag, Hendrik; Lee, Jun -Sik; Yang, Chan -Ho

    2016-10-03

    Here, the emergence of a triple phase point in a two-dimensional parameter space (such as pressure and temperature) can offer unforeseen opportunities for the coupling of two seemingly independent order parameters. On the basis of this, we demonstrate the electric control of magnetic order by manipulating chemical pressure: lanthanum substitution in the antiferromagnetic ferroelectric BiFeO3. Our demonstration relies on the finding that a multiferroic triple phase point of a single spin-disordered phase and two spin-ordered phases emerges near room temperature in Bi0.9La0.1FeO3 ferroelectric thin films. By using spatially resolved X-ray absorption spectroscopy, we provide direct evidence that the electric poling of a particular region of the compound near the triple phase point results in an antiferromagnetic phase while adjacent unpoled regions remain magnetically disordered, opening a promising avenue for magnetoelectric applications at room temperature.

  13. Critical space-time networks and geometric phase transitions from frustrated edge antiferromagnetism.

    PubMed

    Trugenberger, Carlo A

    2015-12-01

    Recently I proposed a simple dynamical network model for discrete space-time that self-organizes as a graph with Hausdorff dimension d(H)=4. The model has a geometric quantum phase transition with disorder parameter (d(H)-d(s)), where d(s) is the spectral dimension of the dynamical graph. Self-organization in this network model is based on a competition between a ferromagnetic Ising model for vertices and an antiferromagnetic Ising model for edges. In this paper I solve a toy version of this model defined on a bipartite graph in the mean-field approximation. I show that the geometric phase transition corresponds exactly to the antiferromagnetic transition for edges, the dimensional disorder parameter of the former being mapped to the staggered magnetization order parameter of the latter. The model has a critical point with long-range correlations between edges, where a continuum random geometry can be defined, exactly as in Kazakov's famed 2D random lattice Ising model but now in any number of dimensions.

  14. Critical space-time networks and geometric phase transitions from frustrated edge antiferromagnetism

    NASA Astrophysics Data System (ADS)

    Trugenberger, Carlo A.

    2015-12-01

    Recently I proposed a simple dynamical network model for discrete space-time that self-organizes as a graph with Hausdorff dimension dH=4 . The model has a geometric quantum phase transition with disorder parameter (dH-ds) , where ds is the spectral dimension of the dynamical graph. Self-organization in this network model is based on a competition between a ferromagnetic Ising model for vertices and an antiferromagnetic Ising model for edges. In this paper I solve a toy version of this model defined on a bipartite graph in the mean-field approximation. I show that the geometric phase transition corresponds exactly to the antiferromagnetic transition for edges, the dimensional disorder parameter of the former being mapped to the staggered magnetization order parameter of the latter. The model has a critical point with long-range correlations between edges, where a continuum random geometry can be defined, exactly as in Kazakov's famed 2D random lattice Ising model but now in any number of dimensions.

  15. Antiferromagnet-induced perpendicular magnetic anisotropy in ferromagnetic/antiferromagnetic/ferromagnetic trilayers

    NASA Astrophysics Data System (ADS)

    Wang, Bo-Yao; Lin, Po-Han; Tsai, Ming-Shian; Shih, Chun-Wei; Lee, Meng-Ju; Huang, Chun-Wei; Jih, Nae-Yeou; Wei, Der-Hsin

    2016-08-01

    This study demonstrates the effect of antiferromagnet-induced perpendicular magnetic anisotropy (PMA) on ferromagnetic/antiferromagnetic/ferromagnetic (FM/AFM/FM) trilayers and reveals its interplay with a long-range interlayer coupling between separated FM layers. In epitaxially grown 12 monolayer (ML) Ni/Co/Mn/5 ML Co/Cu(001) films, magnetic hysteresis loops and element-resolved magnetic domain imaging showed that the magnetization direction of the top layers of 12 ML Ni/Co films could be changed from the in-plane direction to the perpendicular direction, when the thickness of the Mn films (tMn) was greater than a critical value close to the thickness threshold associated with the onset of AFM ordering (tMn=3.5 ML). The top FM layers exhibited a significantly enhanced PMA when tMn increased further, and this enhancement can be attributed to a strengthened AFM ordering of the volume moments of the Mn films, as evidenced by the presence of induced domain frustration. By contrast, the long-range interlayer coupling presented clear effects only when tMn was at a lower coverage.

  16. Structure and magnetism of antiferromagnetic FeMN thin films

    NASA Astrophysics Data System (ADS)

    Maat, Stefan

    2000-09-01

    The magnetic and electronic structure of bulk fcc Fe 50Mn50 was studied by first principles calculations using the layer-version of the Korringa- Kohn-Rostocker method with the local spin density approximation and the coherent potential approximation to treat chemical disorder. The self-consistent calculations suggest a non-collinear spin-structure for Fe50Mn50 as the magnetic ground state, which can be understood from the electronic structure. Possible spin-structures identified are the 3Q structure with spins pointing along the body diagonals and the 2Q structure with spins pointing along the face diagonals. The collinear 1Q structure with spins pointing along the cube edges was found to be energetically unstable. Epitaxial FeMn/Cu multilayers were grown epitaxially by sputtering onto a thick buffer layer of twinned Cu(111) on H-terminated Si(110). The growth was characterized in situ by Auger electron spectroscopy and electron diffraction techniques and ex situ by X-ray diffraction. Neutron diffraction experiments were performed on these multilayers to investigate the spin structure of thin Fe50Mn50 films. The measurements confirmed that Fe50Mn50 has either a 3Q structure or equally populated domains of 2Q or 1Q structures. Hysteresis loops of polycrystalline and epitaxial exchange-biased Ni 80Fe20/Fe50Mn50 films were measured as a function of Fe50Mn50 layer thickness with the longitudinal Kerr effect. The variation of the loop-shift and the coercivity were observed over a full 360° in plane rotation and Fourier analyzed. Coercivities and loop shifts show a strong dependence on the antiferromagnetic layer thickness. Examination of the angular dependent results within different models of exchange bias verified that the Fourier coefficients obey necessary conditions to achieve energetic stability together with spontaneous magnetization.

  17. Linear magnetoelectric effect as a signature of long-range collinear antiferromagnetic ordering in the frustrated spinel CoA l2O4

    NASA Astrophysics Data System (ADS)

    Ghara, Somnath; Ter-Oganessian, N. V.; Sundaresan, A.

    2017-03-01

    The ground state of the frustrated A -site magnetic spinel CoA l2O4 has been a controversial issue whether it is a collinear antiferromagnetic ordering or a spiral spin-liquid state, as the ratio of the two competing interactions J2/J1 lies close to the boundary between these two ground states. Here we address the magnetic ground state in CoA l2O4 with different amounts of C o2 +/A l3 + site disorder from the study of magnetoelectric effect and Monte Carlo simulations. CoA l2O4 with low site disorder exhibits a linear magnetoelectric effect below the magnetic ordering temperature. With increasing disorder, the magnetoelectric effect is suppressed and the sample with 14 % disorder exhibits a spin glass behavior without the magnetoelectric effect. Monte Carlo simulations support the experimental findings and suggest that the site disorder suppresses long-range antiferromagnetic order and induces a spin glass state. Since the linear magnetoelectric effect requires a long-range magnetic ordering, we suggest that the ground state of CoA l2O4 with low site disorder is a collinear antiferromagnet.

  18. Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn

    NASA Astrophysics Data System (ADS)

    Galceran, R.; Fina, I.; Cisneros-Fernández, J.; Bozzo, B.; Frontera, C.; López-Mir, L.; Deniz, H.; Park, K.-W.; Park, B.-G.; Balcells, Ll.; Martí, X.; Jungwirth, T.; Martínez, B.

    2016-10-01

    Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with Néel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit (Ir) combination. Isothermal magnetotransport measurements in an antiferromagnetic-metal(IrMn)/ferromagnetic-insulator thin film bilayer have been performed. The metal/insulator structure with magnetic coupling between both layers allows the measurement of the modulation of the transport properties exclusively in the antiferromagnetic layer. Anisotropic magnetoresistance as large as 0.15% has been found, which is much larger than that for a bare IrMn layer. Interestingly, it has been observed that anisotropic magnetoresistance is strongly influenced by the field cooling conditions, signaling the dependence of the found response on the formation of domains at the magnetic ordering temperature.

  19. Anisotropic resonance modes emerging in an antiferromagnetic superconducting state.

    PubMed

    Waßer, F; Lee, C H; Kihou, K; Steffens, P; Schmalzl, K; Qureshi, N; Braden, M

    2017-09-04

    Two strong arguments in favor of magnetically driven unconventional superconductivity arise from the coexistence and closeness of superconducting and magnetically ordered phases on the one hand, and from the emergence of magnetic spin-resonance modes at the superconducting transition on the other hand. Combining these two arguments one may ask about the nature of superconducting spin-resonance modes occurring in an antiferromagnetic state. This problem can be studied in underdoped BaFe2 As2, for which the local coexistence of large moment antiferromagnetism and superconductivity is well established by local probes. However, polarized neutron scattering experiments are required to identify the nature of the resonance modes. In the normal state of Co underdoped BaFe2 As2 the antiferromagnetic order results in broad magnetic gaps opening in all three spin directions that are reminiscent of the magnetic response in the parent compound. In the superconducting state two distinct anisotropic resonance excitations emerge, but in contrast to numerous studies on optimum and over-doped BaFe2 As2 there is no isotropic resonance excitation. The two anisotropic resonance modes appearing within the antiferromagnetic phase are attributed to a band selective superconducting state, in which longitudinal magnetic excitations are gapped by antiferromagnetic order with sizable moment.

  20. Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn.

    PubMed

    Galceran, R; Fina, I; Cisneros-Fernández, J; Bozzo, B; Frontera, C; López-Mir, L; Deniz, H; Park, K-W; Park, B-G; Balcells, Ll; Martí, X; Jungwirth, T; Martínez, B

    2016-10-20

    Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with Néel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit (Ir) combination. Isothermal magnetotransport measurements in an antiferromagnetic-metal(IrMn)/ferromagnetic-insulator thin film bilayer have been performed. The metal/insulator structure with magnetic coupling between both layers allows the measurement of the modulation of the transport properties exclusively in the antiferromagnetic layer. Anisotropic magnetoresistance as large as 0.15% has been found, which is much larger than that for a bare IrMn layer. Interestingly, it has been observed that anisotropic magnetoresistance is strongly influenced by the field cooling conditions, signaling the dependence of the found response on the formation of domains at the magnetic ordering temperature.

  1. Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn

    PubMed Central

    Galceran, R.; Fina, I.; Cisneros-Fernández, J.; Bozzo, B.; Frontera, C.; López-Mir, L.; Deniz, H.; Park, K.-W.; Park, B.-G.; Balcells, Ll.; Martí, X.; Jungwirth, T.; Martínez, B.

    2016-01-01

    Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with Néel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit (Ir) combination. Isothermal magnetotransport measurements in an antiferromagnetic-metal(IrMn)/ferromagnetic-insulator thin film bilayer have been performed. The metal/insulator structure with magnetic coupling between both layers allows the measurement of the modulation of the transport properties exclusively in the antiferromagnetic layer. Anisotropic magnetoresistance as large as 0.15% has been found, which is much larger than that for a bare IrMn layer. Interestingly, it has been observed that anisotropic magnetoresistance is strongly influenced by the field cooling conditions, signaling the dependence of the found response on the formation of domains at the magnetic ordering temperature. PMID:27762278

  2. Supersymmetry protected topological phases of isostatic lattices and kagome antiferromagnets

    NASA Astrophysics Data System (ADS)

    Lawler, Michael J.

    2016-10-01

    I generalize the theory of phonon topological band structures of isostatic lattices to frustrated antiferromagnets. I achieve this with a discovery of a many-body supersymmetry (SUSY) in the phonon problem of balls and springs and its connection to local constraints satisfied by ground states. The Witten index of the SUSY model demands the Maxwell-Calladine index of mechanical structures. "Spontaneous supersymmetry breaking" is identified as the need to gap all modes in the bulk to create the topological isostatic lattice state. Since ground states of magnetic systems also satisfy local constraint conditions (such as the vanishing of the total spin on a triangle), I identify a similar SUSY structure for many common models of antiferromagnets including the square, triangluar, kagome, pyrochlore nearest-neighbor antiferromagnets, and the J2=J1/2 square-lattice antiferromagnet. Remarkably, the kagome family of antiferromagnets is the analog of topological isostatic lattices among this collection of models. Thus, a solid-state realization of the theory of phonon topological band structure may be found in frustrated magnetic materials.

  3. Spin pumping and inverse spin Hall voltages from dynamical antiferromagnets

    NASA Astrophysics Data System (ADS)

    Johansen, Øyvind; Brataas, Arne

    2017-06-01

    Dynamical antiferromagnets can pump spins into adjacent conductors. The high antiferromagnetic resonance frequencies represent a challenge for experimental detection, but magnetic fields can reduce these resonance frequencies. We compute the ac and dc inverse spin Hall voltages resulting from dynamical spin excitations as a function of a magnetic field along the easy axis and the polarization of the driving ac magnetic field perpendicular to the easy axis. We consider the insulating antiferromagnets MnF2,FeF2, and NiO. Near the spin-flop transition, there is a significant enhancement of the dc spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets MnF2 and FeF2. In the uniaxial antiferromagnets it is also found that the ac spin pumping is independent of the external magnetic field when the driving field has the optimal circular polarization. In the biaxial NiO, the voltages are much weaker, and there is no spin-flop enhancement of the dc component.

  4. Intrinsic Properties of Fe-Substituted L1(0) Magnets

    SciTech Connect

    Manchanda, P; Kumar, P; Kashyap, A; Lucis, MJ; Shield, JE; Mubarok, A; Goldstein, JI; Constantinides, S; Barmak, K; Lewis, LH; Sellmyer, DJ; Skomski, R

    2013-10-01

    First-principle supercell calculations are used to determine how 3d elemental additions, especially Fe additions, modify the magnetization, exchange and anisotropy of L1(0)-ordered ferromagnets. Calculations are performed using the VASP code and partially involve configurational averaging over site disorder. Three isostructural systems are investigated: Fe-Co-Pt, Mn-Al-Fe, and transition metal-doped Fe-Ni. In all three systems the iron strongly influences the magnetic properties of these compounds, but the specific effect depends on the host. In CoPt(Fe) iron enhances the magnetization, with subtle changes in the magnetic moments that depend on the distribution of the Fe and Co atoms. The addition of Fe to MnAl is detrimental to the magnetization, because it creates antiferromagnetic exchange interactions, but it enhances the magnetic anisotropy. The replacement of 50% of Mn by Fe in MnFeAl2 enhances the anisotropy from 1.77 to 2.5 MJ/m(3). Further, the substitution of light 3d elements such as Ti, V, Cr into L1(0)-ordered FeNi is shown to substantially reduce the magnetization.

  5. Nucleophilic Aromatic Substitution.

    ERIC Educational Resources Information Center

    Avila, Walter B.; And Others

    1990-01-01

    Described is a microscale organic chemistry experiment which demonstrates one feasible route in preparing ortho-substituted benzoic acids and provides an example of nucleophilic aromatic substitution chemistry. Experimental procedures and instructor notes for this activity are provided. (CW)

  6. Nucleophilic Aromatic Substitution.

    ERIC Educational Resources Information Center

    Avila, Walter B.; And Others

    1990-01-01

    Described is a microscale organic chemistry experiment which demonstrates one feasible route in preparing ortho-substituted benzoic acids and provides an example of nucleophilic aromatic substitution chemistry. Experimental procedures and instructor notes for this activity are provided. (CW)

  7. Effects of the antiferromagnetic spin structure on antiferromagnetically induced perpendicular magnetic anisotropy

    NASA Astrophysics Data System (ADS)

    Wang, Bo-Yao; Tsai, Ming-Shian; Huang, Chun-Wei; Shih, Chun-Wei; Chen, Chia-Ju; Lin, Kai; Li, Jin-Jhuan; Jih, Nae-Yeou; Lu, Chun-I.; Chuang, Tzu-Hung; Wei, Der-Hsin

    2017-09-01

    Antiferromagnetic (AFM) thin films are promising materials for inducing perpendicular magnetic anisotropy (PMA) in adjacent ferromagnetic (FM) films. This study demonstrates that in a selected AFM spin structure with out-of-plane uncompensated moments, the magnitude of the induced PMA in its neighboring Co/Ni film could be significantly enhanced by the establishment of a collinearlike exchange interaction between the volume moments of the AFM film and the perpendicular magnetic FM film. Detailed magnetic hysteresis loops and x-ray analysis revealed a quench of perpendicular surface anisotropy in a monolayered Fe50Mn50/Co /Ni film due to the formation of antiparallel-like coupled Fe and Mn moments. By contrast, the establishment of a three-dimensional quadratic-type AFM spin structure of an Fe50Mn50 film triggered parallel-like out-of-plane uncompensated Fe and Mn moments at the interface and reinforced the PMA induced in the Co/Ni film.

  8. Emergence of soliton chirality in a quantum antiferromagnet

    NASA Astrophysics Data System (ADS)

    Braun, Hans-Benjamin; Kulda, Jiri; Roessli, Bertrand; Visser, Dirk; Krämer, Karl W.; Güdel, Hans-Ulrich; Böni, Peter

    2005-12-01

    Left- and right-handed chiral matter is present at every scale ranging from seashells to molecules to elementary particles. In magnetism, chirality may be inherited from the asymmetry of the underlying crystal structure, or it may emerge spontaneously. In particular, there has been a long-standing search for chiral spin states that emerge spontaneously with the disappearance of antiferromagnetic long-range order. Here we identify a generic system supporting such a behaviour and report on experimental evidence for chirality associated with the quantum dynamics of solitons in antiferromagnetic spin chains. The soliton chirality observed by polarized neutron scattering is in agreement with theoretical predictions and is a manifestation of a Berry phase. Our observations provide the first example of the emergence of spin currents and hidden chiral order that accompany the disappearance of antiferromagnetic order, a scheme believed to lie at the heart of the enigmatic normal state of cuprate superconductors.

  9. Revealing the properties of Mn2Au for antiferromagnetic spintronics.

    PubMed

    Barthem, V M T S; Colin, C V; Mayaffre, H; Julien, M-H; Givord, D

    2013-01-01

    The continuous reduction in size of spintronic devices requires the development of structures, which are insensitive to parasitic external magnetic fields, while preserving the magnetoresistive signals of existing systems based on giant or tunnel magnetoresistance. This could be obtained in tunnel anisotropic magnetoresistance structures incorporating an antiferromagnetic, instead of a ferromagnetic, material. To turn this promising concept into real devices, new magnetic materials with large spin-orbit effects must be identified. Here we demonstrate that Mn2Au is not a Pauli paramagnet as hitherto believed but an antiferromagnet with Mn moments of ~4 μB. The particularly large strength of the exchange interactions leads to an extrapolated Néel temperature well above 1,000 K, so that ground-state magnetic properties are essentially preserved up to room temperature and above. Combined with the existence of a significant in-plane anisotropy, this makes Mn2Au the most promising material for antiferromagnetic spintronics identified so far.

  10. Spin dynamics simulations for a nanoscale Heisenberg antiferromagnet

    NASA Astrophysics Data System (ADS)

    Hou, Zhuofei; Landau, D. P.; Brown, G.; Stocks, G. M.

    2010-03-01

    Thermoinduced magnetization(TiM) is a novel response which was predicted to occur in nanoscale antiferromagnetic materials. Extensive Monte Carlo simulations footnotetextG. Brown, A. Janotti, M. Eisenbach, and G. M. Stocks, Phys.Rev.B 72, 140405(2005) have shown that TiM is an intrinsic property of the antiferromagnetic classical Heisenberg model below the Neel temperature. To obtain a fundamental understanding of TiM, spin dynamics(SD) simulations are performed to study the spin wave behavior, which seems to be the cause of TiM. A classical Heisenberg model with an antiferromagnetic nearest-neighbor exchange interaction and uniaxial single-site anisotropy is studied. Simple-cubic lattices with free boundary conditions are used. We employed the fast spin dynamics algorithms with fourth-order Suzuki-Trotter decompositions of the exponential operator. Additional small excitation peaks due to surface effects are found in transverse S(q,w).

  11. Kondo Screening and Fermi Surface in the Antiferromagnetic Metal Phase

    NASA Astrophysics Data System (ADS)

    Yamamoto, Seiji; Si, Qimiao

    2006-03-01

    We address the Kondo effect deep inside the antiferromagnetic metal phase of a Kondo lattice Hamiltonian with SU(2) invariance. The local- moment component is described in terms of a non-linear sigma model. The Fermi surface of the conduction electron component is taken to be sufficiently small, so that it is not spanned by the antiferromagnetic wavevector. The effective low energy form of the Kondo coupling simplifies drastically, corresponding to the uniform component of the magnetization that forward-scatters the conduction electrons on their own Fermi surface. We use a combined bosonic and fermionic (Shankar) renormalization group procedure to analyze this effective theory and study the Kondo screening and Fermi surface in the antiferromagnetic phase. The implications for the global magnetic phase diagram, as well as quantum critical points, of heavy fermion metals are discussed.

  12. Antiferromagnetic model of aggregation of a magnetic fluid

    SciTech Connect

    Morozov, K.I.

    1987-07-01

    Reversible aggregation of ferromagnetic particles of a magnetic fluid (MF) in an external magnetic field has been observed in a number of experimental studies. In this work the aggregation of MF is interpreted as a separation into paramagnetic and antiferromagnetic phases. A comparative analysis of the thermodynamic stability of antiferromagnetic (droplike) and ferromagnetic (needlelike) agglomerates is carried out. It is shown that in the absence of an applied field the former are thermodynamically more advantageous than the latter; the critical field of the antiferromagnetic-ferromagnetic phase transition is found, the critical values of the concentration of the MF, the temperature, and the osmotic pressure, determining the onset of the separation of the MF into regions with high and low particle concentration, are calculated.

  13. Antiferromagnetic phase transition in a nonequilibrium lattice of Rydberg atoms

    SciTech Connect

    Lee, Tony E.; Cross, M. C.; Haeffner, H.

    2011-09-15

    We study a driven-dissipative system of atoms in the presence of laser excitation to a Rydberg state and spontaneous emission. The atoms interact via the blockade effect, whereby an atom in the Rydberg state shifts the Rydberg level of neighboring atoms. We use mean-field theory to study how the Rydberg population varies in space. As the laser frequency changes, there is a continuous transition between the uniform and antiferromagnetic phases. The nonequilibrium nature also leads to a novel oscillatory phase and bistability between the uniform and antiferromagnetic phases.

  14. Spin transport through the metallic antiferromagnet FeMn

    NASA Astrophysics Data System (ADS)

    Saglam, H.; Zhang, W.; Jungfleisch, M. B.; Sklenar, J.; Pearson, J. E.; Ketterson, J. B.; Hoffmann, A.

    2016-10-01

    We investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in N i80F e20/FeMn /W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. Using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents, respectively. The latter can extend to relatively large distances (≈9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

  15. Quantification of quantum discord in a antiferromagnetic Heisenberg compound

    SciTech Connect

    Singh, H. Chakraborty, T. Mitra, C.

    2014-04-24

    An experimental quantification of concurrence and quantum discord from heat capacity (C{sub p}) measurement performed over a solid state system has been reported. In this work, thermodynamic measurements were performed on copper nitrate (CN, Cu(NO{sub 3}){sub 2}⋅2.5H{sub 2}O) single crystals which is an alternating antiferromagnet Heisenberg spin 1/2 system. CN being a weak dimerized antiferromagnet is an ideal system to investigate correlations between spins. The theoretical expressions were used to obtain concurrence and quantum discord curves as a function of temperature from heat capacity data of a real macroscopic system, CN.

  16. Spin excitations in the antiferromagnet NaNiO2

    NASA Astrophysics Data System (ADS)

    de Brion, S.; Darie, C.; Holzapfel, M.; Talbayev, D.; Mihály, L.; Simon, F.; Jánossy, A.; Chouteau, G.

    2007-03-01

    In NaNiO2 , Ni3+ ions form a quasi-two-dimensional triangular lattice of S=1/2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferromagnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20cm-1 , in magnetic fields up to 14T . The bulk of the results are interpreted in terms of a phenomenological model involving biaxial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane.

  17. Spin Excitations in the Antiferromagnet NaNiO2

    SciTech Connect

    de Brion,S.; Darie, C.; Holzapfel, M.; Talbayev, D.; Mihaly, L.; Simon, F.; Janossy, A.; Chouteau, G.

    2007-01-01

    In NaNiO2 , Ni3+ ions form a quasi-two-dimensional triangular lattice of S=1/2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferromagnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1 20cm-1 , in magnetic fields up to 14T . The bulk of the results are interpreted in terms of a phenomenological model involving biaxial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane

  18. Identification of a Griffiths singularity in a geometrically frustrated antiferromagnet

    NASA Astrophysics Data System (ADS)

    Kumar, Jitender; Panja, Soumendra Nath; Dengre, Shanu; Nair, Sunil

    2017-02-01

    We report the observation of a Griffiths phase in the geometrically frustrated antiferromagnet DyBaCo 4O 7 +δ . Its onset is identified using measurements of the thermoremanent magnetization, which appears to be superior to conventional in-field measurement protocols for the characterization of the Griffiths phase. Within this phase, the temporal relaxation of magnetization exhibits a functional form which is expected for Heisenberg systems, reflecting the nature of spin interactions in this class of materials. Interestingly, the effective Co 2 + /Co 3 + ratio tailored by varying the oxygen nonstoichiometry δ is only seen to influence the antiferromagnetic ordering temperature (T N ), leaving the Griffiths temperature (T G ) invariant.

  19. Uniaxial pressure dependence of the antiferromagnetic order in UPt3

    NASA Astrophysics Data System (ADS)

    van Dijk, N. H.; Rodière, P.; Fåk, B.; Huxley, A.; Flouquet, J.; Fernández-Díaz, M. T.; Yakhou, F.

    The weak antiferromagnetic order of the heavy-fermion superconductor UPt3 has been investigated by elastic neutron-scattering measurements under applied uniaxial pressure up to 6 kbar along the a and c axes of the hexagonal crystal structure. For p||c the small antiferromagnetically ordered moment of 0.02μB/U-atom shows a non-linear decrease for increasing pressures and is still not completely suppressed at the maximum applied pressure of 6 kbar. For p||a a significant increase in the magnetic Bragg peak intensity is observed, which suggests an incomplete domain repopulation and confirms the presence of a single-k structure.

  20. Magnetic relaxation in a suspension of antiferromagnetic nanoparticles

    NASA Astrophysics Data System (ADS)

    Raikher, Yu. L.; Stepanov, V. I.

    2008-09-01

    A kinetic model is proposed to describe the low-frequency magnetodynamics of antiferromagnetic nanoparticles suspended in a fluid. Because of their small size, apart from an anisotropic magnetic susceptibility typical of antiferromagnets, these particles also have a constant magnetic moment caused by sublattice decompensation. An orientational crossover takes place in such a nanosuspension (colloid) when magnetized by a constant field: the axes of easy particle magnetization that were initially aligned along the field become oriented perpendicularly. This effect changes significantly the characteristics of the system’s magnetic response: the dynamic susceptibility spectrum and the relaxation time in a pulsed field.

  1. Magnetization damping in noncollinear spin valves with antiferromagnetic interlayer couplings

    NASA Astrophysics Data System (ADS)

    Chiba, Takahiro; Bauer, Gerrit E. W.; Takahashi, Saburo

    2015-08-01

    We study the magnetic damping in the simplest of synthetic antiferromagnets, i.e., antiferromagnetically exchange-coupled spin valves, in the presence of applied magnetic fields that enforce noncolliear magnetic configurations. We formulate the dynamic exchange of spin currents in a noncollinear texture based on the spin-diffusion theory with quantum mechanical boundary conditions at the ferrromagnet/normal-metal interfaces and derive the Landau-Lifshitz-Gilbert equations coupled by the interlayer static and dynamic exchange interactions. We predict noncollinearity-induced additional damping that is modulated by an applied magnetic field. We compare theoretical results with published experiments.

  2. Multicritical point in a diluted bilayer Heisenberg quantum antiferromagnet.

    PubMed

    Sandvik, Anders W

    2002-10-21

    The S=1/2 Heisenberg bilayer antiferromagnet with randomly removed interlayer dimers is studied using quantum Monte Carlo simulations. A zero-temperature multicritical point (p(*),g(*)) at the classical percolation density p=p(*) and interlayer coupling g(*) approximately equal 0.16 is demonstrated. The quantum critical exponents of the percolating cluster are determined using finite-size scaling. It is argued that the associated finite-temperature quantum critical regime extends to zero interlayer coupling and could be relevant for antiferromagnetic cuprates doped with nonmagnetic impurities.

  3. Spin Nernst Effect of Magnons in Collinear Antiferromagnets

    NASA Astrophysics Data System (ADS)

    Cheng, Ran; Okamoto, Satoshi; Xiao, Di

    2016-11-01

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the Γ point and the K point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. Possible material candidates are discussed.

  4. Gauge fields and related forces in antiferromagnetic soliton physics

    NASA Astrophysics Data System (ADS)

    Dasgupta, Sayak; Kim, Se Kwon; Tchernyshyov, Oleg

    2017-06-01

    We derive equations of motion for topological solitons in antiferromagnets under the combined action of perturbations such as an external magnetic field and torque-generating electrical current. Aside from conservative forces, such perturbations generate an effective "magnetic field" exerting a gyrotropic force on the soliton and an induced "electric field" if the perturbation is time-dependent. We apply the general formalism to the cases of a domain wall and of a vortex. An antiferromagnetic vortex can be effectively moved by combined applications of a magnetic field and an electric current.

  5. High-throughput screening for antiferromagnetic Heusler compounds using density functional theory

    NASA Astrophysics Data System (ADS)

    Balluff, Jan; Diekmann, Kevin; Reiss, Günter; Meinert, Markus

    2017-08-01

    Commonly used antiferromagnets contain expensive precious metals, which limits their applicability. Novel materials that are made of abundant elements are thus required for a large scale application, e.g., in spintronic devices. We propose a combinatorial, high-throughput approach based on density functional theory calculations to search for such new antiferromagnets. The power of the method is demonstrated by screening the ternary Heusler compounds for antiferromagnetic phases. We utilize the AFLOWLib, a computational materials database that contains over one million ternary phases. Among these we identify 291 potentially stable magnetic Heusler compounds. By explicitly checking for antiferromagnetic configurations we identify 70 antiferromagnetic Heusler compounds. Comparison with available experimental data shows that the method has excellent selectivity: all known antiferromagnetic Heusler compounds are correctly identified and no material is erroneously assigned an antiferromagnetic ground state. By calculating the Néel temperatures we predict 21 antiferromagnetic Heusler compounds with a Néel temperature above room temperature.

  6. Quantum critical behavior of low-dimensional spin 1/2 Heisenberg antiferromagnets

    NASA Astrophysics Data System (ADS)

    Stone, Matthew Brandon

    In this dissertation, experiments on four different insulating antiferromagnetic spin 1/2 Heisenberg systems are presented and described. Copper pyrazine dinitrate is a linear chain spin 1/2 (S = 1/2) Heisenberg antiferromagnet. In an applied magnetic field, the continuum splits into multiple continua including incommensurate gapless excitations. The inelastic neutron scattering measurements presented represent the first complete experimental study of the S = 1/2 linear chain excitation spectrum in an applied magnetic field. Copper nitrate is a S = 1/2 alternating chain Heisenberg antiferromagnet. This system is near the isolated dimer limit, such that perturbation theory based on weakly coupled spin pairs accurately describes the excitation spectrum. Inelastic neutron scattering measurements were performed as a function of applied magnetic field. The data presented here represent the first such measure in all portions of the magnetic phase diagram of a gapped quantum magnet. Piperazinium hexachlorodicuprate is a two-dimensional S = 1/2 Heisenberg antiferromagnet. It is shown in this work that the structure consists of a collection of coupled spins in the crystalline ac plane. Multiple spin-spin interactions are important in this material. This has consequences for the nature of the dominant interactions and causes there to be significant spin frustration in this system. The spectrum consists of coherent dispersive singlet-triplet excitations describable in terms of multiple significant exchange interactions with geometrical frustration. Thermodynamic and inelastic neutron scattering measurements are presented which characterize the magnetic excitations as a function of temperature and applied magnetic field. In addition, the full magnetic phase diagram including a gapless disordered phase and a reentrant phase transition is presented. Cu2(1,4-diazacycloheptane)2Cl4 was widely believed to be a S = 1/2 Heisenberg spin-ladder material. Neutron scattering measurements

  7. Higgs mode and its decay in a two-dimensional antiferromagnet

    NASA Astrophysics Data System (ADS)

    Jain, A.; Krautloher, M.; Porras, J.; Ryu, G. H.; Chen, D. P.; Abernathy, D. L.; Park, J. T.; Ivanov, A.; Chaloupka, J.; Khaliullin, G.; Keimer, B.; Kim, B. J.

    2017-07-01

    Condensed-matter analogues of the Higgs boson in particle physics allow insights into its behaviour in different symmetries and dimensionalities. Evidence for the Higgs mode has been reported in a number of different settings, including ultracold atomic gases, disordered superconductors, and dimerized quantum magnets. However, decay processes of the Higgs mode (which are eminently important in particle physics) have not yet been studied in condensed matter due to the lack of a suitable material system coupled to a direct experimental probe. A quantitative understanding of these processes is particularly important for low-dimensional systems, where the Higgs mode decays rapidly and has remained elusive to most experimental probes. Here, we discover and study the Higgs mode in a two-dimensional antiferromagnet using spin-polarized inelastic neutron scattering. Our spin-wave spectra of Ca2RuO4 directly reveal a well-defined, dispersive Higgs mode, which quickly decays into transverse Goldstone modes at the antiferromagnetic ordering wavevector. Through a complete mapping of the transverse modes in the reciprocal space, we uniquely specify the minimal model Hamiltonian and describe the decay process. We thus establish a novel condensed-matter platform for research on the dynamics of the Higgs mode.

  8. Size dependence of the magnetic properties of antiferromagnetic Cr2O3 nanoparticles

    NASA Astrophysics Data System (ADS)

    Tobia, D.; Winkler, E.; Zysler, R. D.; Granada, M.; Troiani, H. E.

    2008-09-01

    Magnetic properties of antiferromagnetic (AFM) Cr2O3 nanoparticles have been studied as a function of the nanoparticle size. The synthesized nanoparticles present an ellipsoidal shape with the major axis of approximately 170 nm and the minor axis that increases with the synthesis temperature from 30 to 70 nm. By magnetization and electron paramagnetic resonance experiments, we have obtained the parameters that characterize the AFM nanoparticles system. We have found that the Néel temperature, TN , and the spin-flop field, HSF , increase with the particle size from TN=288K and HSF(5K)=10kOe for the smaller nanoparticles and approach the bulk values [ TN=308K and HSF(5K)=60kOe ] for the larger particles. From the experimental results and the molecular-field theory applied to AFM coupled sublattices, we estimated the magnetic anisotropy, K , and the molecular-field constant, λ , as a function of the Cr2O3 nanoparticle size. When the size is reduced, λ only diminishes ˜8% with respect to its bulk value (4.9×104Oe2g/erg) ; instead, K decreases more than an order of magnitude from K=3.8×104 to 8.7×102erg/g . We analyzed the results on the basis of a core shell model where the nanoparticle internal order consists of an antiferromagnetically ordered core and a disordered surface shell, which presents a frustrated magnetic state.

  9. Multiplicative logarithmic corrections to quantum criticality in three-dimensional dimerized antiferromagnets

    NASA Astrophysics Data System (ADS)

    Qin, Yanqi; Normand, Bruce; Sandvik, Anders; Meng, Zi Yang

    We investigate the quantum phase transition in an S=1/2 dimerized Heisenberg antiferromagnet in three spatial dimensions. By means of quantum Monte Carlo simulations and finite-size scaling analyses, we get high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the ordered Neel phase. This transition breaks O(N) symmetry with N=3 in D=3+1 dimensions. This is the upper critical dimension, where multiplicative logarithmic corrections to the leading mean-field critical properties are expected; we extract these corrections, establishing their precise forms for both the zero-temperature staggered magnetization, ms, and the Neel temperature, TN. We present a scaling ansatz for TN, including logarithmic corrections, which agrees with our data and indicates exact linearity with ms, implying a complete decoupling of quantum and thermal fluctuation effects close to the quantum critical point. These logarithmic scaling forms have not previously identified or verified by unbiased numerical methods and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as TlCuCl3. Ref.: arXiv:1506.06073

  10. Static and high-frequency magnetic properties of Fe and Cr substituted lanthanum manganites

    NASA Astrophysics Data System (ADS)

    Srinivasan, G.; Hanna, D.; Suryanarayanan, R.; Berthon, J.

    2000-03-01

    Studies aimed at the nature of magnetic interactions in Fe and Cr substituted lanthanum deficient manganites, La 0.88Mn 1- xFe xO 3 ( x=0-0.15) and La 0.88Mn 1- yCr yO 3 ( y=0-0.175) are reported. The oxides order ferromagnetically for the entire range of x- and y-values. A decrease in the Curie temperature Tc with increasing substitution is observed for both compounds, but the decrease in Tc is large for Fe substitution. A large decrease was also seen in the low-temperature magnetic moment of Fe substituted samples. The data indicate possible ferromagnetic ordering of Cr and antiferromagnetic ordering of Fe moments in the oxides. Ferromagnetic resonance studies indicate (i) strong spin-lattice effects in the paramagnetic state, (ii) valence-exchange effects in the ferromagnetic state, and (iii) possible microscopic inhomogeneities in Cr-substituted samples

  11. Part i: Numerical Investigation of the Rkky Interaction in a BCS Superconductor; Part II: Dynamical Analysis of Leed from the (110) Surfaces of Substitutionally Disordered GALLIUM(X)ALUMINUM(1-X)ARSENIDE.

    NASA Astrophysics Data System (ADS)

    Richardson, Steven Leslie

    In Part I the spatial spin susceptibility (chi)(r) of a BCS superconductor is calculated using finite temperature Green's functions. A numerical study of the results reveals that the RKKY interaction is only modified by about 2 - 3% from its normal value for distances less than 10 p(,o)(' -1) where p(,o) is the Fermi momentum, and that the change in the RKKY interaction is exponentially damped out at further distances. In Part II we discuss how low-energy electron diffraction (LEED), which has been successful in determining the surface atomic geometries of compound semiconductors, can be extended to surface structure studies of substitutionally disordered ternary semiconductor interfaces. We use an average T-matrix approximation (ATA) to compute the intensities of 30 - 240 eV low-energy electrons diffracted from the (110) surfaces of Ga(,x)Al(,1-x)As(110), for values of x = 0.00, 0.25, 0.32, 0.50, 1.00. We find that our computed diffracted intensities correctly mimic the trends in the experimental data and attempt a structure search for Ga(,0.50)Al(,0.50)As.

  12. DMRG Study of the S >= 1 quantum Heisenberg Antiferromagnet on a Kagome-like lattice without loops

    NASA Astrophysics Data System (ADS)

    Lamberty, R. Zach; Changlani, Hitesh J.; Henley, Christopher L.

    2013-03-01

    The Kagome quantum Heisenberg antiferromagnet, for spin up to S = 1 and perhaps S = 3 / 2 , is a prime candidate to realize a quantum spin liquid or valence bond crystal state, but theoretical or computational studies for S > 1 / 2 are difficult and few. We consider instead the same interactions and S >= 1 on the Husimi Cactus, a graph of corner sharing triangles whose centers are vertices of a Bethe lattice, using a DMRG procedure tailored for tree graphs. Since both lattices are locally identical, properties of the Kagome antiferromagnet dominated by nearest-neighbor spin correlations should also be exhibited on the Cactus, whereas loop-dependent effects will be absent on the loopless Cactus. Our study focuses on the possible transition(s) that must occur with increasing S for the Cactus antiferromagnet. (It has a disordered valence bond state at S = 1 / 2 but a 3-sublattice coplanar ordered state in the large S limit). We also investigate the phase diagram of the S = 1 quantum XXZ model with on-site anisotropy, which we expect to have three-sublattice and valence-bond-crystal phases similar to the kagome case. This work is supported by the National Science Foundation through a Graduate Research Fellowship to R. Zach Lamberty, as well as grant DMR-

  13. Studies of the magnetic structure at the ferromagnet - antiferromagnet interface

    SciTech Connect

    Scholl, A.; Nolting, F.; Stohr, J.; Luning, J.; Seo, J.W.; Locquet, J.-P.; Anders, S.; Ohldag, H.; Padmore, H.A.

    2001-01-02

    Antiferromagnetic layers are a scientifically challenging component in magneto-electronic devices such as magnetic sensors in hard disk heads, or magnetic RAM elements. In this paper we show that photo-electron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized soft x-rays from a synchrotron source, give rise to a magnetic image contrast. Images, acquired with the PEEM2 experiment at the Advanced Light Source, show magnetic contrast for antiferromagnetic LaFeO{sub 3}, microscopically resolving the magnetic domain structure in an antiferromagnetically ordered thin film for the first time. Magnetic coupling between LaFeO{sub 3} and an adjacent Co layer results in a complete correlation of their magnetic domain structures. From field dependent measurements a unidirectional anisotropy resulting in a local exchange bias of up to 30 Oe in single domains could be deduced. The elemental specificity and the quantitative magnetic sensitivity render PEEM a perfect tool to study magnetic coupling effects in multi-layered thin film samples.

  14. ZnFe2O4 antiferromagnetic structure redetermination

    NASA Astrophysics Data System (ADS)

    Kremenović, Aleksandar; Antić, Bratislav; Vulić, Predrag; Blanuša, Jovan; Tomic, Aleksandra

    2017-03-01

    Magnetic structure of ZnFe2O4 normal spinel is re-examined. Antiferromagnetic structure non-collinear model is established within Ca2 space group having four different crystallographic/magnetic sites for 32 Fe3+ spins within magnetic unit cell.

  15. Coupling the valley degree of freedom to antiferromagnetic order.

    PubMed

    Li, Xiao; Cao, Ting; Niu, Qian; Shi, Junren; Feng, Ji

    2013-03-05

    Conventional electronics are based invariably on the intrinsic degrees of freedom of an electron, namely its charge and spin. The exploration of novel electronic degrees of freedom has important implications in both basic quantum physics and advanced information technology. Valley, as a new electronic degree of freedom, has received considerable attention in recent years. In this paper, we develop the theory of spin and valley physics of an antiferromagnetic honeycomb lattice. We show that by coupling the valley degree of freedom to antiferromagnetic order, there is an emergent electronic degree of freedom characterized by the product of spin and valley indices, which leads to spin-valley-dependent optical selection rule and Berry curvature-induced topological quantum transport. These properties will enable optical polarization in the spin-valley space, and electrical detection/manipulation through the induced spin, valley, and charge fluxes. The domain walls of an antiferromagnetic honeycomb lattice harbors valley-protected edge states that support spin-dependent transport. Finally, we use first-principles calculations to show that the proposed optoelectronic properties may be realized in antiferromagnetic manganese chalcogenophosphates (MnPX3, X = S, Se) in monolayer form.

  16. Coupling the valley degree of freedom to antiferromagnetic order

    PubMed Central

    Li, Xiao; Cao, Ting; Niu, Qian; Shi, Junren; Feng, Ji

    2013-01-01

    Conventional electronics are based invariably on the intrinsic degrees of freedom of an electron, namely its charge and spin. The exploration of novel electronic degrees of freedom has important implications in both basic quantum physics and advanced information technology. Valley, as a new electronic degree of freedom, has received considerable attention in recent years. In this paper, we develop the theory of spin and valley physics of an antiferromagnetic honeycomb lattice. We show that by coupling the valley degree of freedom to antiferromagnetic order, there is an emergent electronic degree of freedom characterized by the product of spin and valley indices, which leads to spin–valley-dependent optical selection rule and Berry curvature–induced topological quantum transport. These properties will enable optical polarization in the spin–valley space, and electrical detection/manipulation through the induced spin, valley, and charge fluxes. The domain walls of an antiferromagnetic honeycomb lattice harbors valley-protected edge states that support spin-dependent transport. Finally, we use first-principles calculations to show that the proposed optoelectronic properties may be realized in antiferromagnetic manganese chalcogenophosphates (MnPX3, X = S, Se) in monolayer form. PMID:23435746

  17. Symmetry breaking in low-dimensional SU(N) antiferromagnets

    SciTech Connect

    Kolezhuk, Alexei

    2008-10-01

    Consequences of explicit symmetry breaking in a physically motivated model of SU(N) antiferromagnet in spatial dimensions one and two are studied. It is shown that the case N=3, which can be realized in spin-1 cold atom systems, displays special properties distinctly different from those for N{>=}4. Qualitative form of the phase diagram depending on the model parameters is given.

  18. Correlation between antiferromagnetic interface coupling and positive exchange bias

    SciTech Connect

    Nogues, J.; Leighton, C.; Schuller, Ivan K.

    2000-01-01

    The induced moment in antiferromagnetic (AFM)-ferromagnetic (FM) (FeF{sub 2}-Fe and MnF{sub 2}-Fe) bilayers has been studied from the shift along the magnetization axis of the exchange-biased hysteresis loops. The magnetization shift depends strongly on the cooling field and microstructure of the AFM layer. The shift for small cooling fields can be opposite to the cooling field, indicating that, in some cases, the presence of the FM layer induces an antiferromagnetic coupling at the interface. Samples with negative magnetization shifts (antiferromagnetic coupling) exhibit large changes in exchange bias H{sub E} as a function of cooling field and positive exchange bias. Samples with positive magnetization shifts (ferromagnetic coupling) show almost no change in H{sub E} with cooling field and the exchange bias field remains always negative. These results confirm the theoretical assumption that an antiferromagnetic interface coupling is necessary to observe positive exchange bias. (c) 2000 The American Physical Society.

  19. Spiraling spin structure in an exchange-coupled antiferromagnetic layer

    PubMed

    Yang; Chien

    2000-09-18

    Using trilayers of permalloy/FeMn/Co with various thicknesses t(AF) of the antiferromagnetic FeMn, we have observed evidence of a spiraling spin structure within FeMn. For t(AF)<90 A, the turn angle straight theta of the spiral varies as straight theta = (1.76 degrees /A)t(AF).

  20. A cold-atom Fermi-Hubbard antiferromagnet.

    PubMed

    Mazurenko, Anton; Chiu, Christie S; Ji, Geoffrey; Parsons, Maxwell F; Kanász-Nagy, Márton; Schmidt, Richard; Grusdt, Fabian; Demler, Eugene; Greif, Daniel; Greiner, Markus

    2017-05-24

    Exotic phenomena in systems with strongly correlated electrons emerge from the interplay between spin and motional degrees of freedom. For example, doping an antiferromagnet is expected to give rise to pseudogap states and high-temperature superconductors. Quantum simulation using ultracold fermions in optical lattices could help to answer open questions about the doped Hubbard Hamiltonian, and has recently been advanced by quantum gas microscopy. Here we report the realization of an antiferromagnet in a repulsively interacting Fermi gas on a two-dimensional square lattice of about 80 sites at a temperature of 0.25 times the tunnelling energy. The antiferromagnetic long-range order manifests through the divergence of the correlation length, which reaches the size of the system, the development of a peak in the spin structure factor and a staggered magnetization that is close to the ground-state value. We hole-dope the system away from half-filling, towards a regime in which complex many-body states are expected, and find that strong magnetic correlations persist at the antiferromagnetic ordering vector up to dopings of about 15 per cent. In this regime, numerical simulations are challenging and so experiments provide a valuable benchmark. Our results demonstrate that microscopy of cold atoms in optical lattices can help us to understand the low-temperature Fermi-Hubbard model.

  1. Holographic model for the paramagnetism/antiferromagnetism phase transition

    NASA Astrophysics Data System (ADS)

    Cai, Rong-Gen; Yang, Run-Qiu

    2015-04-01

    In this paper we build a holographic model of paramagnetism/antiferromagnetism phase transition, which is realized by introducing two real antisymmetric tensor fields coupling to the background gauge field strength and interacting with each other in a dyonic black brane background. In the case without an external magnetic field and in low temperatures, the magnetic moments condense spontaneously in an antiparallel manner with the same magnitude and the time reversal symmetry is also broken spontaneously (if the boundary spatial dimension is more than 2, spatial rotational symmetry is broken spontaneously as well), which leads to an antiferromagnetic phase. In the case with the weak external magnetic field, the magnetic susceptibility density has a peak at the critical temperature and satisfies the Curie-Weiss law in the paramagnetic phase of antiferromagnetism. In the strong external magnetic field case, there is a critical magnetic field Bc in the antiferromagnetic phase: when the magnetic field reaches Bc, the system will return into the paramagnetic phase by a second order phase transition.

  2. NMR Investigation of an Itinerant Weakly Antiferromagnetism in Metallic Thiospinels CoCo 2S 4 and (Co 1- xCu x)Co 2S 4

    NASA Astrophysics Data System (ADS)

    Sugita, Hiroshi; Wada, Shinji; Yamada, Yoshihiro; Miyatani, Kazuo; Tanaka, Toshiro

    1998-04-01

    Nuclear magnetic resonance of 59Co in a metallic thiospinel compound CoCo2S4 was carried out at 75 MHz in a temperature range between 1.4 and 260 K to study the properties of the low frequency spin dynamics at the tetrahedral A and the octahedral B sites. From the combination of the 59Co Knight shift, spin-lattice relaxation rate and susceptibility measurements, it is concluded that the stoichiometric CoCo2S4 belongs to a group of weakly antiferromagnetic metals with the Néel temperature of ≃55 K. The magnetic properties of Cu substituted compounds (Co1-xCux)Co2S4 were also studied. The results exhibited that the light substitution lowers the Néel temperature and the heavy substitution leads the compound to a nearly Pauli paramagnetic metal.

  3. Strain correlated effect on structural, magnetic, and dielectric properties in Ti4+ substituted Bi0.8Ba0.2Fe1-xTixO3

    NASA Astrophysics Data System (ADS)

    Paul Blessington Selvadurai, A.; Pazhanivelu, V.; Murugaraj, R.

    2015-08-01

    Ti4+ substituted Bi0.8Ba0.2Fe1-xTixO3 for x = 0.0, 0.1 and 0.2 are prepared by modified solid state reaction method. The prepared samples sintered at 850 °C for 1 h show a single phase nature. A structural change was observed on Ti4+ substitutions are confirmed through X-ray Diffraction, Fourier Transform Infrared spectroscopy and Raman spectra. An anomalous phase transition is observed in Bi0.8Ba0.2FeO3 at 1173 K. The absence of ferroelectric transition and enhancement of decomposition temperature is observed in the substituted samples from the thermal analysis. A dielectric spectroscopic measurement shows that on Ti4+ substitutions, the magnitude of dielectric constant and loss tangent (tan δ) value is decreased. Vibrating Sample Magnetometer (VSM) study shows both antiferromagnetic and ferromagnetic phases coexist in the M-H curve. On Ti4+ substitutions in Bi0.8Ba0.2FeO3, the antiferromagnetism dominates over the ferromagnetic phase. In corroboration to magnetisation process, ZFC-FC measurement confirms it that on Ti4+ substitution, the antiferromagnetic behaviour gets dominated. The report suggests that the interplay of strain upon Ti4+ substitution causes the structural and magnetic phase transition.

  4. NMR Characterization of Sulphur Substitution Effects in the KxFe2−ySe2−zSz High-Tc Superconductor

    SciTech Connect

    Petrovic C.; Torchetti, D.A.; Imai, T.; Lei, H.C.

    2012-04-17

    We present a {sup 77}Se NMR study of the effect of S substitution in the high-T{sub c} superconductor K{sub x}Fe{sub 2-y}Se{sub 2-z}S{sub z} in a temperature range up to 250 K. We examine two S concentrations, with z = 0.8 (T{sub c} {approx} 26 K) and z = 1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the K{sub x}Fe{sub 2}Se{sub 2} sample due to local disorder in the Se environment. Our Knight shift {sup 77}K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None of the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near T{sub c} in 1/T{sub 1}T, as seen in FeSe.

  5. Competing Antiferromagnetic and Spin-Glass phases in a hollandite structure

    NASA Astrophysics Data System (ADS)

    Crespo Hernandez, Yanier; Andreanov, Alexei; Seriani, Nicola

    2013-03-01

    We introduce a simple model to explain recent experimental results on spin freezing in a hollandite-type structure. We argue that geometrical frustration of the lattice with antiferromagnetic (AFM) interactions is responsible for the appearance of a spin-glass phase in presence of disorder. We check our predictions numerically using parallel tempering on a model that considers Ising spins and nearest-neighbor AFM interactions. The proposed model presents a rich phenomenology: in absence of disorder two ground states are possible, depending on the strength of the interactions, namely an AFM or a geometrically frustrated phase. Remarkably for any set of AFM couplings having an AFM ground state in the clean system, there exist a critical value of the disorder for which the ground state is replaced by a spin-glass one while maintaining all couplings AFM. To the best of our knowledge in the literature there is not a model that presents this kind of transition considering just short-range AFM interactions. Therefore we argue that this model would be useful to understand the relation between AFM coupling, disorder and the appearance of spin glasses phase.

  6. Impact of anisotropy on antiferromagnet rotation in Heusler-type ferromagnet/antiferromagnet epitaxial bilayers

    NASA Astrophysics Data System (ADS)

    Hajiri, T.; Matsushita, M.; Ni, Y. Z.; Asano, H.

    2017-04-01

    We report the magnetotransport properties of ferromagnet (FM)/antiferromagnet (AFM) Fe2CrSi /Ru2MnGe epitaxial bilayers using current-in-plane configurations. Above the critical thickness of the Ru2MnGe layer to induce exchange bias, symmetric and asymmetric curves were observed in response to the direction of FM magnetocrystalline anisotropy. Because each magnetoresistance curve showed full and partial AFM rotation, the magnetoresistance curves imply the impact of the Fe2CrSi magnetocrystalline anisotropy to govern the AFM rotation. The maximum magnitude of the angular-dependent resistance-change ratio of the bilayers is more than an order of magnitude larger than that of single-layer Fe2CrSi films, resulting from the reorientation of AFM spins via the FM rotation. These results highlight the essential role of controlling the AFM rotation and reveal a facile approach to detect the AFM moment even in current-in-plane configurations in FM/AFM bilayers.

  7. Reentrant spin glass behavior in antiferromagnetic single crystalline Ba 6Mn 24O 48 nanoribbons

    NASA Astrophysics Data System (ADS)

    Zhang, Xianke; Tang, Shaolong; Li, Yulong; Du, Youwei

    2010-04-01

    Single crystalline Ba 6Mn 24O 48 nanoribbons with diameters ranging from one hundred nanometers to a few hundred nanometers and length up to tens of microns are synthesized via a facile molten salt method. These nanoribbons are characterized by a range of methods including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The magnetic properties of Ba 6Mn 24O 48 nanoribbons are investigated by the zero-field-cooled (ZFC), field-cooled (FC) magnetization, and ac susceptibility. Upon cooling, we find the reentrant spin glass (RSG) behavior in these nanoribbons, i.e., paramagnetic (PM), antiferromagnetic (AFM), and spin glass (SG). The RSG behavior might be due to the surface spin disorder, geometrical frustration and Mn 3+/Mn 4+ mixture in Ba 6Mn 24O 48 nanoribbons.

  8. Antiferromagnetic order in the pyrochlores R2Ge2O7 (R = Er, Yb)

    NASA Astrophysics Data System (ADS)

    Dun, Zhiling; Li, Xiang; Freitas, Rafael; Arrighi, Everton; Cruz, Clarina; Lee, Minseong; Choi, Eun Sang; Cao, Huibo; Silverstein, Harlyn; Wiebe, Chris; Chen, Jinguang; Zhou, Haidong

    Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er2Ge2O7 and Yb2Ge2O7 show that both systems are antiferromagnetically ordered in the Γ5 manifold. The ground state is a ψ3 phase for the Er sample and a ψ2 or ψ3 phase for the Yb sample, which suggests ``Order by Disorder''(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R2X2O7 (R = Er, Yb, X = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction J+/- under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].

  9. Antiferromagnetic order in the pyrochlores R2Ge2O7 (R =Er ,Yb )

    NASA Astrophysics Data System (ADS)

    Dun, Z. L.; Li, X.; Freitas, R. S.; Arrighi, E.; Dela Cruz, C. R.; Lee, M.; Choi, E. S.; Cao, H. B.; Silverstein, H. J.; Wiebe, C. R.; Cheng, J. G.; Zhou, H. D.

    2015-10-01

    Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er2Ge2O7 and Yb2Ge2O7 show that both systems are antiferromagnetically ordered in the Γ5 manifold. The ground state is a ψ3 phase for the Er sample and a ψ2 or ψ3 phase for the Yb sample, which suggests "Order by Disorder" physics. Furthermore, we unify the various magnetic ground states of all known R2X2O7 (R =Er , Yb; X =Sn , Ti, Ge) compounds through the enlarged X Y -type exchange interaction J± under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study by Wong et al. [Phys. Rev. B 88, 144402 (2013), 10.1103/PhysRevB.88.144402].

  10. Anomalous curie response of impurities in quantum-critical spin-1/2 Heisenberg antiferromagnets.

    PubMed

    Höglund, Kaj H; Sandvik, Anders W

    2007-07-13

    We consider a magnetic impurity in two different S=1/2 Heisenberg bilayer antiferromagnets at their respective critical interlayer couplings separating Néel and disordered ground states. We calculate the impurity susceptibility using a quantum Monte Carlo method. With intralayer couplings in only one of the layers (Kondo lattice), we observe an anomalous Curie constant C*, as predicted on the basis of field-theoretical work [S. Sachdev, Science 286, 2479 (1999)10.1126/science.286.5449.2479]. The value C* = 0.262 +/- 0.002 is larger than the normal Curie constant C=S(S+1)/3. Our low-temperature results for a symmetric bilayer are consistent with a universal C*.

  11. μ SR insight into the impurity problem in quantum kagome antiferromagnets

    NASA Astrophysics Data System (ADS)

    Gomilšek, M.; Klanjšek, M.; Pregelj, M.; Luetkens, H.; Li, Y.; Zhang, Q. M.; Zorko, A.

    2016-07-01

    Impurities, which are unavoidable in real materials, may play an important role in the magnetism of frustrated spin systems with a spin-liquid ground state. We address the impurity issue in quantum kagome antiferromagnets by investigating ZnCu3(OH) 6SO4 (Zn-brochantite) by means of muon spin spectroscopy. We show that muons dominantly couple to impurities, originating from Cu-Zn intersite disorder, and that the impurity spins are highly correlated with the kagome spins, allowing us to probe the host kagome physics via a Kondo-like effect. The low-temperature plateau in the impurity susceptibility suggests that the kagome spin-liquid ground state is gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and a nontrivial field dependence.

  12. Simplex SU(3) quantum antiferromagnets on the kagome and hyperkagome lattices

    NASA Astrophysics Data System (ADS)

    Kiselev, Yury Yu.; Parameswaran, Siddharth A.; Arovas, Daniel P.

    2013-03-01

    We investigate SU(3) ``simplex solid'' antiferromagnets on the kagome and hyperkagome lattices. The ground states of these systems are annihilated by certain local projectors acting on triples of sites, and are analogous to the valence bond solid wavefunctions constructed by Affleck, Kennedy, Lieb, and Tasaki. Using a coherent state representation, we map to a classical model of CP2 spins with 3-spin interactions, which we analyze via single-spin Monte Carlo simulations and a cluster algorithm for the three-body interactions. We compute the static structure factor and short-range correlations encoded by the simplex solid wavefunction and rationalize the results in terms of the ``order by disorder'' mechanism. We acknowledge support from NSF grant DMR-1007028 (YYK and DPA) and from the Simons Foundation (SAP).

  13. [Sugar substitute products impact on oral fluid biochemical properties].

    PubMed

    Tsapok, P I; Imbriakov, K V; Chuchkova, M R

    2012-01-01

    Sugar substitute products impact on oral fluid protein and carbohydrate content, as well as oxidative balance were studied in 60 medical school students in compare with conventional sugar. Sugar intake proved to cause cariesogenic carbohydrate metabolism disorders in oral fluid, intensification of lipoperoxidation and decrease in antioxidation activity. Sugar substitute products help to prevent dental decay.

  14. Resonant spin tunneling in small antiferromagnetic particles

    NASA Astrophysics Data System (ADS)

    Luis, F.; del Barco, E.; Hernández, J. M.; Remiro, E.; Bartolomé, J.; Tejada, J.

    1999-05-01

    The paper reports a detailed experimental study on magnetic relaxation of natural horse-spleen ferritin. ac susceptibility measurements performed on three samples of different concentration show that dipole-dipole interactions between uncompensated moments play no significant role. Furthermore, the distribution of relaxation times in these samples has been obtained from a scaling of experimental χ'' data, obtained at different frequencies. The average uncompensated magnetic moment per protein is compatible with a disordered arrangement of atomic spins throughout the core, rather than with surface disorder. The observed field dependence of the blocking temperature suggests that magnetic relaxation is faster at zero field than at intermediate field values. This is confirmed by the fact that the magnetic viscosity peaks at zero field, too. Using the distribution of relaxation times obtained independently, we show that these results cannot be explained in terms of classical relaxation theory. The most plausible explanation of these results is the existence, near zero field, of resonant magnetic tunneling between magnetic states of opposite orientation, which are thermally populated.

  15. Proximity-induced magnetism in transition-metal substituted graphene

    SciTech Connect

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian -Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-03

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.

  16. Proximity-induced magnetism in transition-metal substituted graphene

    NASA Astrophysics Data System (ADS)

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.

  17. Understanding Magnetic Trimer Interactions in (Cr,Mn)-Substituted Graphene

    NASA Astrophysics Data System (ADS)

    Haraldsen, Jason T.; Crook, Charles B.; Houchins, Gregory; Zhu, Jian-Xin; Constantin, Costel; Balatsky, Alexander V.

    We investigate the magnetic interactions within a graphene superlattice produced by three directly substituted transition-metal atoms (specifically chromium and manganese). Using a first principles approach, we calculate the electronic and magnetic properties for this system assuming an equilateral trimer configuration with varying atomic separations. Through an examination of the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, we find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are dependent on the spatial and magnetic characteristic between the magnetic atoms and the carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, we use magnetization mapping to show that the substituted atoms induce an overall magnetic moment in the graphene lattice, which may help guide the discussion on spintronic graphene. JTH, CBC, GH, and AVB acknowledge support from the Institute for Materials Science via the United States Basic Energy Sciences (E304).

  18. Proximity-induced magnetism in transition-metal substituted graphene

    PubMed Central

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-01-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction. PMID:26235646

  19. Control of antiferromagnetic domain distribution via polarization-dependent optical annealing

    PubMed Central

    Higuchi, Takuya; Kuwata-Gonokami, Makoto

    2016-01-01

    The absence of net magnetization inside antiferromagnetic domains has made the control of their spatial distribution quite challenging. Here we experimentally demonstrate an optical method for controlling antiferromagnetic domain distributions in MnF2. Reduced crystalline symmetry can couple an order parameter with non-conjugate external stimuli. In the case of MnF2, time-reversal symmetry is macroscopically broken reflecting the different orientations of the two magnetic sublattices. Thus, it exhibits different absorption coefficients between two orthogonal linear polarizations below its antiferromagnetic transition temperature under an external magnetic field. Illumination with linearly polarized laser light under this condition selectively destructs the formation of a particular antiferromagnetic order via heating. As a result, the other antiferromagnetic order is favoured inside the laser spot, achieving spatially localized selection of an antiferromagnetic order. Applications to control of interface states at antiferromagnetic domain boundaries, exchange bias and control of spin currents are expected. PMID:26911337

  20. Control of antiferromagnetic domain distribution via polarization-dependent optical annealing.

    PubMed

    Higuchi, Takuya; Kuwata-Gonokami, Makoto

    2016-02-25

    The absence of net magnetization inside antiferromagnetic domains has made the control of their spatial distribution quite challenging. Here we experimentally demonstrate an optical method for controlling antiferromagnetic domain distributions in MnF2. Reduced crystalline symmetry can couple an order parameter with non-conjugate external stimuli. In the case of MnF2, time-reversal symmetry is macroscopically broken reflecting the different orientations of the two magnetic sublattices. Thus, it exhibits different absorption coefficients between two orthogonal linear polarizations below its antiferromagnetic transition temperature under an external magnetic field. Illumination with linearly polarized laser light under this condition selectively destructs the formation of a particular antiferromagnetic order via heating. As a result, the other antiferromagnetic order is favoured inside the laser spot, achieving spatially localized selection of an antiferromagnetic order. Applications to control of interface states at antiferromagnetic domain boundaries, exchange bias and control of spin currents are expected.

  1. Control of antiferromagnetic domain distribution via polarization-dependent optical annealing

    NASA Astrophysics Data System (ADS)

    Higuchi, Takuya; Kuwata-Gonokami, Makoto

    2016-02-01

    The absence of net magnetization inside antiferromagnetic domains has made the control of their spatial distribution quite challenging. Here we experimentally demonstrate an optical method for controlling antiferromagnetic domain distributions in MnF2. Reduced crystalline symmetry can couple an order parameter with non-conjugate external stimuli. In the case of MnF2, time-reversal symmetry is macroscopically broken reflecting the different orientations of the two magnetic sublattices. Thus, it exhibits different absorption coefficients between two orthogonal linear polarizations below its antiferromagnetic transition temperature under an external magnetic field. Illumination with linearly polarized laser light under this condition selectively destructs the formation of a particular antiferromagnetic order via heating. As a result, the other antiferromagnetic order is favoured inside the laser spot, achieving spatially localized selection of an antiferromagnetic order. Applications to control of interface states at antiferromagnetic domain boundaries, exchange bias and control of spin currents are expected.

  2. Relationship between Superconductivity and Antiferromagnetism in LaFe(As1-xPx)O Revealed by 31P-NMR

    NASA Astrophysics Data System (ADS)

    Kitagawa, Shunsaku; Iye, Tetsuya; Nakai, Yusuke; Ishida, Kenji; Wang, Cao; Cao, Guang-Han; Xu, Zhu-An

    2014-02-01

    We performed 31P-NMR measurements on LaFe(As1-xPx)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature TN and the moment µord are continuously suppressed with increasing P content x and disappear at x = 0.3 where bulk superconductivity appears. At this superconducting x = 0.3, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As1-xPx)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe2(As1-xPx)2 and SrFe2(As1-xPx)2, with the "122" structure. Moreover, the AFM order reappears with further P substitution (x > 0.4). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3d orbitals around the Fermi level.

  3. Neutron scattering studies of a bond-disordered S = 1 quantum spin liquid

    NASA Astrophysics Data System (ADS)

    Povarov, Kirill; Wulf, Erik; Hüvonen, Dan; Gvasaliya, Severian; Ressouche, Eric; Ollivier, Jacques; Paduan-Filho, Armando; Zheludev, Andrey

    2015-03-01

    We report the results of the neutron scattering studies of a bond-disordered modification of a well-known gapped S = 1 antiferromagnetic quantum system NiCl2 . 4SC(NH2)2 (commonly referred to as DTN). The focus of the study is a zero-field spin-liquid phase of a compound with 6% of Cl to Br substitution. Inelastic neutron time-of-flight measurements at a temperature of 60 mK were employed to map the magnetic excitation spectrum over the whole Brillouin zone with a high resolution. In addition, we have also investigated the critical properties of the field-induced phase transition in DTN specimen with various concentration of Br by means of neutron diffraction. We compare these experimental results to the bulk measurements on DTN with similar levels of Cl/Br substitution and recent theoretical predictions for disordered quantum magnets. This work was partially supported by the Swiss National Fund.

  4. A Hand to Substitutes

    ERIC Educational Resources Information Center

    Javernick, Ellen

    2005-01-01

    In the days before computers, passing on information to substitutes was difficult. Now teachers can individualize their lesson plans to guarantee that substitute teachers and the students will both have a good day. This article provides some plan-ahead procedures that have worked for the author.

  5. Working with Substitute Teachers.

    ERIC Educational Resources Information Center

    Wilson, Karen

    1999-01-01

    Resolving the perennial problem of substitute teachers' high turnover and low availability is feasible, if principals welcome, orient, and assign them properly. Regular teachers should prepare a folder containing class lists, daily schedules, seating charts, expectations of students, and meaningful lesson plans. Substitutes should be treated as…

  6. Florida's Substitute Teachers.

    ERIC Educational Resources Information Center

    Odutola, Adeniji A.; Etemadi, Judy N.

    2002-01-01

    Describes the statutory duties of the Florida Education Standards Commission, highlighting a study of the working conditions of Florida's substitute teachers. Researchers collected data on school board policies regarding substitutes' educational levels required, initial training and staff development opportunities required, salary schedules, and…

  7. Sustainability and substitutability.

    PubMed

    Fenichel, Eli P; Zhao, Jinhua

    2015-02-01

    Developing a quantitative science of sustainability requires bridging mathematical concepts from fields contributing to sustainability science. The concept of substitutability is central to sustainability but is defined differently by different fields. Specifically, economics tends to define substitutability as a marginal concept while fields such as ecology tend to focus on limiting behaviors. We explain how to reconcile these different views. We develop a model where investments can be made in knowledge to increase the elasticity of substitution. We explore the set of sustainable and optimal trajectories for natural capital extraction and built and knowledge capital accumulation. Investments in substitutability through knowledge stock accumulation affect the value of natural capital. Results suggest that investing in the knowledge stock, which can enhance substitutability, is critical to desirable sustainable outcomes. This result is robust even when natural capital is not managed optimally. This leads us to conclude that investments in the knowledge stock are of first order importance for sustainability.

  8. Spin Nernst effect of magnons in collinear antiferromagnets

    DOE PAGES

    Cheng, Ran; Okamoto, Satoshi; Xiao, Di

    2016-11-15

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around themore » $$\\Gamma$$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. As a result, possible material candidates are discussed.« less

  9. Currentless reversal of Néel vector in antiferromagnets

    NASA Astrophysics Data System (ADS)

    Semenov, Yuriy G.; Li, Xi-Lai; Kim, Ki Wook

    2017-01-01

    The possibility of magnetization reversal via a bias-mediated perpendicular magnetic anisotropy is examined theoretically in an antiferromagnet. The numerical analyses based on a Néel vector formulation as well as the micromagnetic Landau-Lifshitz-Gilbert simulation reveal that the desired switching can be achieved through dynamical responses that are significantly different from their ferromagnetic counterparts. Instead of the usual precessional trajectories around the applied effective magnetic field, their motions are rather pendulum-like due to the layered magnetic sublattices with a strong antiparallel exchange interaction, where the inertial behavior plays a crucial role. The absence of spiral damping can also lead to faster relaxation by orders of magnitude. With no reliance on the current driven processes, the investigated mechanism is predicted with a low energy requirement of only a few aJ per switching operation in the antiferromagnets.

  10. Layer Anti-Ferromagnetism on Bilayer Honeycomb Lattice

    PubMed Central

    Tao, Hong-Shuai; Chen, Yao-Hua; Lin, Heng-Fu; Liu, Hai-Di; Liu, Wu-Ming

    2014-01-01

    Bilayer honeycomb lattice, with inter-layer tunneling energy, has a parabolic dispersion relation, and the inter-layer hopping can cause the charge imbalance between two sublattices. Here, we investigate the metal-insulator and magnetic phase transitions on the strongly correlated bilayer honeycomb lattice by cellular dynamical mean-field theory combined with continuous time quantum Monte Carlo method. The procedures of magnetic spontaneous symmetry breaking on dimer and non-dimer sites are different, causing a novel phase transition between normal anti-ferromagnet and layer anti-ferromagnet. The whole phase diagrams about the magnetism, temperature, interaction and inter-layer hopping are obtained. Finally, we propose an experimental protocol to observe these phenomena in future optical lattice experiments. PMID:24947369

  11. Nonthermal antiferromagnetic order and nonequilibrium criticality in the Hubbard model.

    PubMed

    Tsuji, Naoto; Eckstein, Martin; Werner, Philipp

    2013-03-29

    We study dynamical phase transitions from antiferromagnetic to paramagnetic states driven by an interaction quench in the fermionic Hubbard model using the nonequilibrium dynamical mean-field theory. We identify two dynamical transition points where the relaxation behavior qualitatively changes: one corresponds to the thermal phase transition at which the order parameter decays critically slowly in a power law ∝t(-1/2), and the other is connected to the existence of nonthermal antiferromagnetic order in systems with effective temperature above the thermal critical temperature. The frequency of the amplitude mode extrapolates to zero as one approaches the nonthermal (quasi)critical point, and thermalization is significantly delayed by the trapping in the nonthermal state. A slow relaxation of the nonthermal order is followed by a faster thermalization process.

  12. Antiferromagnetic Stabilization in the Ti8O12 Cluster.

    PubMed

    Yu, Xiaohu; Oganov, Artem R; Popov, Ivan A; Qian, Guangrui; Boldyrev, Alexander I

    2016-01-26

    Using the evolutionary algorithm USPEX and DFT+U calculations, we predicted a high-symmetry geometric structure of the bare Ti8 O12 cluster composed of 8 Ti atoms forming a cube, in which O atoms are at midpoints of all of its edges, in excellent agreement with experimental results. Using natural bond orbital analysis, adaptive natural density partitioning algorithm, electron localization function, and partial charge plots, we find the origin of the particular stability of bare Ti8 O12 cluster: unique chemical bonding where eight electrons of Ti atoms interacting with each other in antiferromagnetic fashion to lower the total energy of the system. The bare Ti8 O12 is thus an unusual molecule stabilized by d-orbital antiferromagnetic coupling. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  13. Ferro- and antiferro-magnetism in (Np, Pu)BC

    SciTech Connect

    Klimczuk, T.; Kozub, A. L.; Griveau, J.-C.; Colineau, E.; Wastin, F.; Falmbigl, M.; Rogl, P.

    2015-04-01

    Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of (Np,Pu)BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below T{sub N} = 44 K, whereas ferromagnetic ordering was found for NpBC below T{sub C} = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC.

  14. Effects of random fields in an antiferromagnetic Ising spin glass

    PubMed

    Vieira; Nobre; Yokoi

    2000-05-01

    The effects of random fields on the two-sublattice infinite-ranged Ising spin-glass model are investigated. This model is expected to be appropriate as a mean-field description of antiferromagnetic spin glasses such as FexMn1-xTiO3. Within replica-symmetric calculations, we study the influence of Gaussian and bimodal random fields on the phase transitions and phase diagrams. It is shown that, in the presence of random fields, the first-order transitions are weakened and may become continuous. Also, the antiferromagnetic phases are always destroyed by sufficiently strong random fields. A qualitative comparison with existing experimental results and the limitations of the present calculations are discussed.

  15. Jamming behavior of domains in a spiral antiferromagnetic system.

    PubMed

    Chen, S-W; Guo, H; Seu, K A; Dumesnil, K; Roy, S; Sinha, S K

    2013-05-24

    Using resonant magnetic x-ray photon correlation spectroscopy, we show that the domains of a spiral antiferromagnet enter a jammed state at the onset of long-range order. We find that the slow thermal fluctuations of the domain walls exhibit a compressed exponential relaxation with an exponent of 1.5 found in a wide variety of solidlike jammed systems and can be qualitatively explained in terms of stress release in a stressed network. As the temperature decreases, the energy barrier for fluctuations becomes large enough to arrest further domain wall fluctuations, and the domains freeze into a spatial configuration within 10 K of the Néel temperature. The relaxation times can be fitted with the Vogel-Fulcher law as observed in polymers, glasses, and colloids, thereby indicating that the dynamics of domain walls in an ordered antiferromagnet exhibit some of the universal features associated with jamming behavior.

  16. Role of the antiferromagnetic bulk spins in exchange bias

    NASA Astrophysics Data System (ADS)

    Schuller, Ivan K.; Morales, Rafael; Batlle, Xavier; Nowak, Ulrich; Güntherodt, Gernot

    2016-10-01

    This "Critical Focused Issue" presents a brief review of experiments and models which describe the origin of exchange bias in epitaxial or textured ferromagnetic/antiferromagnetic bilayers. Evidence is presented which clearly indicates that inner, uncompensated, pinned moments in the bulk of the antiferromagnet (AFM) play a very important role in setting the magnitude of the exchange bias. A critical evaluation of the extensive literature in the field indicates that it is useful to think of this bulk, pinned uncompensated moments as a new type of a ferromagnet which has a low total moment, an ordering temperature given by the AFM Néel temperature, with parallel aligned moments randomly distributed on the regular AFM lattice.

  17. Ferromagnetic and antiferromagnetic order in bacterial vortex lattices

    NASA Astrophysics Data System (ADS)

    Wioland, Hugo; Woodhouse, Francis G.; Dunkel, Jörn; Goldstein, Raymond E.; Goldstein Lab Team

    2013-11-01

    In conventional electronic materials, spins can organize into ordered phases that give rise to ferromagnetic or antiferromagnetic behavior. Here, we report similar observations in a completely different system: a suspension of swimming bacteria. When a dense Bacillus subtilis suspension is confined to a small circular chamber, it can spontaneously form a stable vortex (``spin'') state that can persist for several minutes. By coupling up to 100 such chambers in microfluidic devices, we are able to realize bacterial spin lattices of different geometries. Depending on that geometry and the effective coupling strength between neighboring vortices, we observe the formation of stable ``antiferromagnetic'' and ``ferromagnetic'' bacterial vortex states, that appear to be controlled by the subtle competition between bacterial boundary layer flows and bulk dynamics.

  18. Spin transport through the metallic antiferromagnet FeMn

    SciTech Connect

    Saglam, H.; Zhang, W.; Jungfleisch, M. B.; Sklenar, J.; Pearson, J. E.; Ketterson, J. B.; Hoffmann, A.

    2016-10-19

    Here, we investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. By using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents, respectively. Furthermore, the latter can extend to relatively large distances (approximate to 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

  19. Spin transport through the metallic antiferromagnet FeMn

    DOE PAGES

    Saglam, H.; Zhang, W.; Jungfleisch, M. B.; ...

    2016-10-19

    Here, we investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. By using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents, respectively. Furthermore, the latter can extend to relatively large distances (approximate to 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close tomore » the measurement temperature.« less

  20. Spin Nernst effect of magnons in collinear antiferromagnets

    SciTech Connect

    Cheng, Ran; Okamoto, Satoshi; Xiao, Di

    2016-11-15

    In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the $\\Gamma$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. As a result, possible material candidates are discussed.

  1. Remarkably robust and correlated coherence and antiferromagnetism in (Ce1-xLax)Cu2Ge2

    DOE PAGES

    Hodovanets, H.; Bud’ko, S. L.; Straszheim, W. E.; ...

    2015-06-08

    We present magnetic susceptibility, resistivity, specific heat, and thermoelectric power measurements on (Ce1-xLax)Cu2Ge2 single crystals (0 ≤ x ≤ 1). With La-substitution, the antiferromagnetic temperature TN is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of our measurements for x > 0.8. Surprisingly, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below Tcoh up to ~0.9 of La, indicating a small percolation limit ~9% of Ce. Tcoh as a function of magnetic field was found to have different behavior for x < 0.9 and x > 0.9. Remarkably, (Tcoh)2more » at H = 0 was found to be linearly proportional to TN. In conclusion, the jump in the magnetic specific heat δCm at TN as a function of TK/TN for (Ce1-xLax)Cu2Ge2 follows the theoretical prediction based on the molecular field calculation for the S = 1/2 resonant level model.« less

  2. Magnetic-resonance investigation of the ordering in randomly-site-diluted antiferromagnets: KMnF3: Mg

    NASA Astrophysics Data System (ADS)

    D'Ariano, G.; Borsa, F.

    1982-12-01

    Measurements of 19F NMR in a series of polycrystalline mixed paramagnetic compounds KMnxMg1-xF3 with x varying over the whole concentration range have been performed as a function of temperature. The paramagnetic shift and the linewidth and relaxation time of the three NMR lines corresponding to those distinct classes of 19F nuclei that have different numbers of nearest-neighbor magnetic ions were measured as the temperature approaches the transition temperature. The paramagnetic shifts of fluorine nuclei having one and two nearest-neighbor Mn ions compare well with the predictions of a simple random-molecular-field approximation (RMFA). It is shown that the substitution of Mn with Mg modifies locally the nuclear-electron transferred hfs interaction by about 20%. The linewidth of the two lines increases drastically as TN is approached. The attempt to explain the data by utilizing Moriya's theory rescaled in the RMFA framework fails completely, indicating that the dynamic effects should be treated in real space. The antiferromagnetic ordering was monitored by measuring the rf susceptibility in zero external magnetic field. It is found that the antiferromagnetic transition is broadened by the concentration gradients present in the samples. The average transition temperature was determined as a function of the average concentration, and the results are compared with the theoretical predictions of recent calculations. Good agreement is found if one takes into account the concentration dependence of the exchange coupling constant.

  3. Physical properties in the cluster-based magnetic-diluted triangular lattice antiferromagnets Li2Sc1-xSnxMo3O8

    NASA Astrophysics Data System (ADS)

    Haraguchi, Yuya; Michioka, Chishiro; Ueda, Hiroaki; Matsuo, Akira; Kindo, Koichi; Yoshimura, Kazuyoshi

    2017-04-01

    We have investigated the physical properties of the magnetic diluted triangular lattice antiferromagnetic system Li2Sc1-xSnxMo3O8. For all compounds, no mangetic ordering has been observed. On the other hand, the partial spin disappearing behavior is found in all Sn-substituted compounds except x = 0, which has been also observed in the similar magnetic system LiZn2Mo3O8. Considering the relationship between the crystal structure and the magnetism, the partial spin disappearance is properly explained by a formation of the valence bond glass derived from the randomness effect

  4. Elemental analysis and magnetism of hydronium jarosites--model kagome antiferromagnets and topological spin glasses.

    PubMed

    Wills, A S; Bisson, W G

    2011-04-27

    The jarosites are the most studied examples of kagome antiferromagnets. Research into them has inspired new directions in magnetism, such as the role of the Dzyaloshinsky-Moriya interaction in symmetry breaking, kagome spin ice, and whether spin glass-like phases can exist in the disorder-free limit. This last point is based around the observation of unconventional thermodynamic and kinetic responses in hydronium jarosite, H(3)OFe(3)(SO(4))(2)(OH)(6), that have led to its classification as a 'topological' spin glass, reflecting the defining role that the underlying geometry of the kagome lattice plays in the formation of the spin glass state. In this paper we explore one of the fundamental questions concerning the frustrated magnetism in hydronium jarosite: whether the spin glass phase is the result of chemical disorder and concomitant randomness in the exchange interactions. Confirming previous crystallographic studies, we use elemental analysis to show that the nature of the low temperature magnetic state is not a simple function of chemical disorder and provide evidence to support the hypothesis that anisotropies drive the spin glass transition.

  5. Singular field response and singular screening of vacancies in antiferromagnets.

    PubMed

    Wollny, Alexander; Andrade, Eric C; Vojta, Matthias

    2012-10-26

    For isolated vacancies in ordered local-moment antiferromagnets we show that the magnetic-field linear-response limit is generically singular: The magnetic moment associated with a vacancy in zero field is different from that in a finite field h in the limit h→0(+). The origin is a universal and singular screening cloud, which moreover leads to perfect screening as h→0(+) for magnets which display spin-flop bulk states in the weak-field limit.

  6. Academic Meeting Scheduling Using an Antiferromagnetic Potts Model

    NASA Astrophysics Data System (ADS)

    Kudo, Kazue

    2017-07-01

    Scheduling parallel sessions of an academic meeting is a complicated task. If each presentation is assigned to an appropriate session, an antiferromagnetic Potts model can be used for semi-automatic timetabling. The timetabling method proposed here is based on graph coloring and includes additional constraints to be considered in a practical situation. We examine the feasibility of semi-automatic timetabling in some practical examples.

  7. Effects of random fields in an antiferromagnetic Ising bilayer film

    NASA Astrophysics Data System (ADS)

    Kaneyoshi, T.

    2017-10-01

    The magnetic properties (phase diagrams and magnetizations) of an antiferromagnetic Ising bilayer film with random fields are investigated by the use of the effective field theory with correlations. It is examined how an uncompensated magnetization can be realized in the system, due to the effects of random fields in the two layers. They show the tricritical, compensation point and reentrant phenomena, depending on these parameters.

  8. Cooling by corralling: a route to antiferromagnetism in optical lattices

    NASA Astrophysics Data System (ADS)

    Loh, Yen Lee

    2012-02-01

    Cold atoms in optical lattices have emerged as a promising tool for emulating condensed matter Hamiltonians. Current experiments have observed ``Mott insulating'' behavior in the Fermi-Hubbard model at an average entropy S/N 1 kB/atom. Our quantum Monte Carlo simulations [1], in agreement with other methods, show that S/N 0.65 kB/atom is low enough to produce antiferromagnetism (AF) at the center of a harmonic trap. However, further progress in the field requires even lower entropies that are beyond the reach of traditional cooling techniques. I have proposed a way to attain very low temperatures and entropies (S/N < 0.03 kB/atom) by trapping fermions in a corral formed from another species of atoms [2]. This Fermi system can then be evolved into an antiferromagnet by morphing the lattice into a set of double wells, quasi-adiabatically. Quantum dynamics simulations have, so far, given promising results.[4pt] [1] Thereza Paiva, Yen Lee Loh, Mohit Randeria, Richard T. Scalettar, and Nandini Trivedi, ``Fermions in 3D optical lattices: Cooling protocol to obtain antiferromagnetism,'' PRL 107, 086401 (2011).[0pt] [2] Yen Lee Loh, ``Proposal for achieving very low entropies in optical lattice systems,'' arxiv:1108.0628.

  9. Electrical manipulation of a ferromagnet by an antiferromagnet

    NASA Astrophysics Data System (ADS)

    Tshitoyan, V.; Ciccarelli, C.; Mihai, A. P.; Ali, M.; Irvine, A. C.; Moore, T. A.; Jungwirth, T.; Ferguson, A. J.

    Several recent studies of antiferromagnetic (AFM) spintronics have focused on transmission and detection of spin-currents in AFMs. Efficient spin transmission through AFMs was inferred from experiments in FM/AFM/NM (normal metal) structures. Measurements in FM/AFM bilayers have demonstrated that a metallic AFM can also act as an efficient ISHE detector of the spin-current, with spin-Hall angles comparable to heavy NMs. Here we demonstrate that an antiferromagnet can be employed for a highly efficient electrical manipulation of a ferromagnet. We use an all-electrical excitation and detection technique of ferromagnetic resonance in a NiFe/IrMn bilayer. We observe antidamping-like spin torque acting on the NiFe generated by the in-plane current driven through the IrMn antiferromagnet. A large enhancement of the torque, characterized by an effective spin-Hall angle exceeding most heavy transition metals, correlates with the presence of the exchange-bias field at the NiFe/IrMn interface. It highlights that, in addition to strong spin-orbit coupling, the AFM order in IrMn governs the observed phenomenon.

  10. Fractional excitations in the square-lattice quantum antiferromagnet

    NASA Astrophysics Data System (ADS)

    Dalla Piazza, B.; Mourigal, M.; Christensen, N. B.; Nilsen, G. J.; Tregenna-Piggott, P.; Perring, T. G.; Enderle, M.; McMorrow, D. F.; Ivanov, D. A.; Rønnow, H. M.

    2015-01-01

    Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spin-1/2 particles is far from complete. The quantum square-lattice Heisenberg antiferromagnet, for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wavevector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound Cu(DCOO)2.4D2O, a known realization of the quantum square-lattice Heisenberg antiferromagnet model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially extended pairs of fractional S = 1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wavevector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.

  11. Antiferromagnetic domain wall as spin wave polarizer and retarder.

    PubMed

    Lan, Jin; Yu, Weichao; Xiao, Jiang

    2017-08-02

    As a collective quasiparticle excitation of the magnetic order in magnetic materials, spin wave, or magnon when quantized, can propagate in both conducting and insulating materials. Like the manipulation of its optical counterpart, the ability to manipulate spin wave polarization is not only important but also fundamental for magnonics. With only one type of magnetic lattice, ferromagnets can only accommodate the right-handed circularly polarized spin wave modes, which leaves no freedom for polarization manipulation. In contrast, antiferromagnets, with two opposite magnetic sublattices, have both left and right-circular polarizations, and all linear and elliptical polarizations. Here we demonstrate theoretically and confirm by micromagnetic simulations that, in the presence of Dzyaloshinskii-Moriya interaction, an antiferromagnetic domain wall acts naturally as a spin wave polarizer or a spin wave retarder (waveplate). Our findings provide extremely simple yet flexible routes toward magnonic information processing by harnessing the polarization degree of freedom of spin wave.Spin waves are promising candidates as carriers for energy-efficient information processing, but they have not yet been fully explored application wise. Here the authors theoretically demonstrate that antiferromagnetic domain walls are naturally spin wave polarizers and retarders, two key components of magnonic devices.

  12. Enhanced Spin Conductance of a Thin-Film Insulating Antiferromagnet

    NASA Astrophysics Data System (ADS)

    Bender, Scott A.; Skarsvâg, Hans; Brataas, Arne; Duine, Rembert A.

    2017-08-01

    We investigate spin transport by thermally excited spin waves in an antiferromagnetic insulator. Starting from a stochastic Landau-Lifshitz-Gilbert phenomenology, we obtain the out-of-equilibrium spin-wave properties. In linear response to spin biasing and a temperature gradient, we compute the spin transport through a normal-metal-antiferromagnet-normal-metal heterostructure. We show that the spin conductance diverges as one approaches the spin-flop transition; this enhancement of the conductance should be readily observable by sweeping the magnetic field across the spin-flop transition. The results from such experiments may, on the one hand, enhance our understanding of spin transport near a phase transition, and on the other be useful for applications that require a large degree of tunability of spin currents. In contrast, the spin Seebeck coefficient does not diverge at the spin-flop transition. Furthermore, the spin Seebeck coefficient is finite even at zero magnetic field, provided that the normal metal contacts break the symmetry between the antiferromagnetic sublattices.

  13. Large anomalous Hall effect in a half-Heusler antiferromagnet

    NASA Astrophysics Data System (ADS)

    Suzuki, T.; Chisnell, R.; Devarakonda, A.; Liu, Y.-T.; Feng, W.; Xiao, D.; Lynn, J. W.; Checkelsky, J. G.

    2016-12-01

    The quantum mechanical (Berry) phase of the electronic wavefunction plays a critical role in the anomalous and spin Hall effects, including their quantized limits. While progress has been made in understanding these effects in ferromagnets, less is known in antiferromagnetic systems. Here we present a study of antiferromagnet GdPtBi, whose electronic structure is similar to that of the topologically non-trivial HgTe (refs ,,), and where the Gd ions offer the possibility to tune the Berry phase via control of the spin texture. We show that this system supports an anomalous Hall angle ΘAH > 0.1, comparable to the largest observed in bulk ferromagnets and significantly larger than in other antiferromagnets. Neutron scattering measurements and electronic structure calculations suggest that this effect originates from avoided crossing or Weyl points that develop near the Fermi level due to a breaking of combined time-reversal and lattice symmetries. Berry phase effects associated with such symmetry breaking have recently been explored in kagome networks; our results extend this to half-Heusler systems with non-trivial band topology. The magnetic textures indicated here may also provide pathways towards realizing the topological insulating and semimetallic states predicted in this material class.

  14. Magnetic susceptibilities of rectangular Heisenberg S=1/2 antiferromagnets

    NASA Astrophysics Data System (ADS)

    Valleau, Tom; Butcher, Rob; Keith, Brian; Landee, Christopher; Turnbull, Mark; Sandvik, Anders

    2008-03-01

    Rectangular antiferromagnets are two-dimensional systems with inequivalent exchange strengths (J', J) along the two principle axes with J' ≡ αJ, α <1. They have an intermediate dimensionality that can vary continuously from 1D (α = 0 ) to square 2D (α = 1). There exist a number of physical realizations of rectangular antiferromagnets (CuPzBr2, CuPzCl2, CuPz(N3)2 where Pz = pyrazine) but there has been no previous mechanism for interpreting their susceptibilities in terms of two exchange parameters. We have simulated the susceptibility of the rectangular S=1/2 Heisenberg antiferromagnet using the stochastic series expansion quantum Monte Carlo method [1] and used the results to interpret our experimental data. For example, copper pyrazine diazide, CuPz(N3)2, has a primary exchange of 15.5 K and an anisotropy parameter α = 0.4. The stronger exchange is due to the superexchange pathway through the pyrazine molecule and the weaker corresponds to the azide bridges. [1] A. Sandvik, PRB 59, R14157 (1999).

  15. Antiferromagnetic resonance in alkali-metal clusters in sodalite

    NASA Astrophysics Data System (ADS)

    Nakano, Takehito; Tsugeno, Hajime; Hanazawa, Atsufumi; Kashiwagi, Takanari; Nozue, Yasuo; Hagiwara, Masayuki

    2013-11-01

    We have performed electron spin resonance (ESR) studies of K43+ and (K3Rb)3+ nanoclusters incorporated in powder specimens of aluminosilicate sodalite at several microwave frequencies between 9 and 34 GHz. The K43+ and (K3Rb)3+ clusters are arrayed in a bcc structure and are known to show antiferromagnetic ordering below the Néel temperatures of TN ≃72 and ≃80 K, respectively, due to the exchange coupling between s electrons confined in the clusters. We have found sudden broadenings of ESR spectra in both samples below TN. The line shape of the spectra below TN is analyzed by powder pattern simulations of antiferromagnetic resonance (AFMR) spectra. The calculated line shapes well reproduce the experimental ones at all the frequencies by assuming a biaxial magnetic anisotropy. We have evaluated extremely small anisotropy fields of approximately 1 Oe indicating that these materials are ideal Heisenberg antiferromagnets. We have also found that the magnetic anisotropy changes from easy-plane type to uniaxial type by changing into a heavier alkali-metal cluster and that the g value shifts to a large value beyond two below TN for K43+ and (K3Rb)3+ nanoclusters. These novel features of K43+ and (K3Rb)3+ nanoclusters incorporated in sodalite are discussed.

  16. Field Evolution of Antiferromagnetic Domains and Domain Walls

    NASA Astrophysics Data System (ADS)

    Fullerton, Eric E.; Hellwig, Olav; Berger, Andreas K.

    2003-03-01

    We have used magnetron sputtered [Co(4Å)Pt(7Å)]X Co(4Å)Ru(9Å)N multiplayer films to create artificially layered antiferromagnets. In contrast to atomic antiferromagnets our model system has an antiferromagnetic (AF) exchange energy comparable to the Zeemann energy in moderate fields and allows to fine tune the relative magnitude of the different magnetic energy terms by varying the parameters X and N. With increasing X and N we observe a transition from traditionally observed sharp AF domain walls towards AF domain walls with a finite width which consist of ferromagnetic stripes, i.e. the AF domains have zero net moment whereas the domain walls carry a finite magnetic moment. Such AF domain walls have not been observed before and are a direct consequence of balancing out exchange and Zeeman energy. We also show that such domain walls are expected from theoretical energy calculations. In this contribution we study the nature and field evolution of the AF stripe domain walls by Magnetic Force Microscopy (MFM). The surface sensitivity of MFM and the finite moment of the AF domain walls allow us to image AF domains as well as domain walls. We are showing first experiments to study the AF domain wall evolution in real space while applying an external field. O.H. was supported by the Deutsche Forschungsgemeinschaft via a Forschungsstipendium under the contract number HE 3286/1-1.

  17. Cubic structure and canted antiferromagnetism of CaMn7O12 doped with trivalent cations (Fe, Al, Cr)

    NASA Astrophysics Data System (ADS)

    Motin Seikh, Md.; Caignaert, V.; Lebedev, O. I.; Raveau, B.

    2014-02-01

    In this study, we show the dramatic effect of the doping of the octahedral sites with M3+ cations (Fe3+, Al3+ and Cr3+) upon the structure and magnetism of the rhombohedral double perovskite CaMn7O12. In the oxides CaMn7-xMxO12, charge ordering between Mn3+ and Mn4+ octahedral sites is destroyed leading to the cubic structure (Im-3), whereas the initial magnetic properties (TN~90 K) have disappeared leading to canted antiferromagnetism (TN≈50-70 K) for small x values (x ~0.2-1). A spin glass like behaviour is also observed for larger values (x~1) in the case of Fe substitution.

  18. Model of ultrafast demagnetization driven by spin-orbit coupling in a photoexcited antiferromagnetic insulator Cr2O3.

    PubMed

    Guo, Feng; Zhang, Na; Jin, Wei; Chang, Jun

    2017-06-28

    We theoretically study the dynamic time evolution following laser pulse pumping in an antiferromagnetic insulator Cr2O3. From the photoexcited high-spin quartet states to the long-lived low-spin doublet states, the ultrafast demagnetization processes are investigated by solving the dissipative Schrödinger equation. We find that the demagnetization times are of the order of hundreds of femtoseconds, in good agreement with recent experiments. The switching times could be strongly reduced by properly tuning the energy gaps between the multiplet energy levels of Cr(3+). Furthermore, the relaxation times also depend on the hybridization of atomic orbitals in the first photoexcited state. Our results suggest that the selective manipulation of the electronic structure by engineering stress-strain or chemical substitution allows effective control of the magnetic state switching in photoexcited insulating transition-metal oxides.

  19. Model of ultrafast demagnetization driven by spin-orbit coupling in a photoexcited antiferromagnetic insulator Cr2O3

    NASA Astrophysics Data System (ADS)

    Guo, Feng; Zhang, Na; Jin, Wei; Chang, Jun

    2017-06-01

    We theoretically study the dynamic time evolution following laser pulse pumping in an antiferromagnetic insulator Cr2O3. From the photoexcited high-spin quartet states to the long-lived low-spin doublet states, the ultrafast demagnetization processes are investigated by solving the dissipative Schrödinger equation. We find that the demagnetization times are of the order of hundreds of femtoseconds, in good agreement with recent experiments. The switching times could be strongly reduced by properly tuning the energy gaps between the multiplet energy levels of Cr3+. Furthermore, the relaxation times also depend on the hybridization of atomic orbitals in the first photoexcited state. Our results suggest that the selective manipulation of the electronic structure by engineering stress-strain or chemical substitution allows effective control of the magnetic state switching in photoexcited insulating transition-metal oxides.

  20. 75As NMR study of antiferromagnetic fluctuations in Ba(Fe1-xRux)2As2

    NASA Astrophysics Data System (ADS)

    Dey, Tusharkanti; Khuntia, P.; Mahajan, A. V.; Sharma, Shilpam; Bharathi, A.

    2011-11-01

    The evolution of 75As NMR parameters with composition and temperature was probed in the Ba(Fe1-xRux)2As2 system where Fe is replaced by isovalent Ru. While the Ru end member was found to be a conventional Fermi liquid, the composition (x = 0.5) corresponding to the highest Tc (20 K) in this system shows an upturn in the 75As \\frac{1}{{T}_{1}T} below about 80 K, evidencing the presence of antiferromagnetic (AFM) fluctuations. These results are similar to those obtained in another system with isovalent substitution, BaFe2(As1-xPx)2 (Nakai et al 2010 Phys. Rev. Lett. 105 107003) and point to a possible role of AFM fluctuations in driving superconductivity.

  1. 40 CFR 721.981 - Substituted naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex.

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ... naphthalenyl-substituted azonaphthol chromium complex. 721.981 Section 721.981 Protection of Environment...-substituted naphthalenyl-substituted azonaphthol chromium complex. (a) Chemical substance and significant new... naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex (PMN P-93-1631) is subject...

  2. 40 CFR 721.981 - Substituted naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex.

    Code of Federal Regulations, 2013 CFR

    2013-07-01

    ... naphthalenyl-substituted azonaphthol chromium complex. 721.981 Section 721.981 Protection of Environment...-substituted naphthalenyl-substituted azonaphthol chromium complex. (a) Chemical substance and significant new... naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex (PMN P-93-1631) is subject...

  3. 40 CFR 721.981 - Substituted naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex.

    Code of Federal Regulations, 2011 CFR

    2011-07-01

    ... naphthalenyl-substituted azonaphthol chromium complex. 721.981 Section 721.981 Protection of Environment...-substituted naphthalenyl-substituted azonaphthol chromium complex. (a) Chemical substance and significant new... naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex (PMN P-93-1631) is subject...

  4. 40 CFR 721.981 - Substituted naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... naphthalenyl-substituted azonaphthol chromium complex. 721.981 Section 721.981 Protection of Environment...-substituted naphthalenyl-substituted azonaphthol chromium complex. (a) Chemical substance and significant new... naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex (PMN P-93-1631) is subject...

  5. 40 CFR 721.981 - Substituted naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... naphthalenyl-substituted azonaphthol chromium complex. 721.981 Section 721.981 Protection of Environment...-substituted naphthalenyl-substituted azonaphthol chromium complex. (a) Chemical substance and significant new... naphtholoazo-substituted naphthalenyl-substituted azonaphthol chromium complex (PMN P-93-1631) is subject...

  6. Partial Ferrimagnetism in S = 1/2 Heisenberg Ladders with a Ferromagnetic Leg, an Antiferromagnetic Leg, and Antiferromagnetic Rungs

    NASA Astrophysics Data System (ADS)

    Sekiguchi, Kazutaka; Hida, Kazuo

    2017-08-01

    Ground-state and finite-temperature properties of S = 1/2 Heisenberg ladders with a ferromagnetic leg, an antiferromagnetic leg, and antiferromagnetic rungs are studied. It is shown that a partial ferrimagnetic phase extends over a wide parameter range in the ground state. The numerical results are supported by an analytical calculation based on a mapping onto the nonlinear σ model and a perturbation calculation from the strong-rung limit. It is shown that the partial ferrimagnetic state is a spontaneously magnetized Tomonaga-Luttinger liquid with incommensurate magnetic correlation, which is confirmed by a DMRG calculation. The finite-temperature magnetic susceptibility is calculated using the thermal pure quantum state method. It is suggested that the susceptibility diverges as T-2 in the ferrimagnetic phases as in the case of ferromagnetic Heisenberg chains.

  7. Antiferromagnetic CsCrF5 and canted antiferromagnetism in RbCrF5 and KCrF5

    NASA Astrophysics Data System (ADS)

    Jagličić, Zvonko; Mazej, Zoran

    2017-07-01

    In ACrF5 (A = Cs, Rb, K), Cr(IV) ions are coordinated by six fluoride ligands where the resulting CrF6 octahedra share cis vertexes to form infinite chains of ([CrIVF5]-)n. The geometry of the latter in Cs compound differs from that in K and Rb compounds. The results of investigations of the magnetic behaviour of these compounds have shown that an antiferromagnetic superexchange interaction is present within the chains with JCs = -10.2 cm-1, JRb = -13.3 cm-1, and JK = -13.1 cm-1. Additional ferromagnetic-like long-range ordering has been observed in KCrF5 and RbCrF5 below 6 K which can be explained, in a correlation with their crystal structures, as canted antiferromagnetism.

  8. Sugar substitutes during pregnancy

    PubMed Central

    Pope, Eliza; Koren, Gideon; Bozzo, Pina

    2014-01-01

    Abstract Question I have a pregnant patient who regularly consumes sugar substitutes and she asked me if continuing their use would affect her pregnancy or child. What should I tell her, and are there certain options that are better for use during pregnancy? Answer Although more research is required to fully determine the effects of in utero exposure to sugar substitutes, the available data do not suggest adverse effects in pregnancy. However, it is recommended that sugar substitutes be consumed in moderate amounts, adhering to the acceptable daily intake standards set by regulatory agencies. PMID:25392440

  9. Sugar substitutes during pregnancy.

    PubMed

    Pope, Eliza; Koren, Gideon; Bozzo, Pina

    2014-11-01

    I have a pregnant patient who regularly consumes sugar substitutes and she asked me if continuing their use would affect her pregnancy or child. What should I tell her, and are there certain options that are better for use during pregnancy? Although more research is required to fully determine the effects of in utero exposure to sugar substitutes, the available data do not suggest adverse effects in pregnancy. However, it is recommended that sugar substitutes be consumed in moderate amounts, adhering to the acceptable daily intake standards set by regulatory agencies. Copyright© the College of Family Physicians of Canada.

  10. From antiferromagnetism to superconductivity in Fe1+yTe1-xSex (0≤x≤0.20) : Neutron powder diffraction analysis

    NASA Astrophysics Data System (ADS)

    Martinelli, A.; Palenzona, A.; Tropeano, M.; Ferdeghini, C.; Putti, M.; Cimberle, M. R.; Nguyen, T. D.; Affronte, M.; Ritter, C.

    2010-03-01

    The nuclear and magnetic structures of Fe1+yTe1-xSex (0≤x≤0.20) compounds was analyzed between 2 and 300 K by means of Rietveld refinement of neutron powder diffraction data. Samples with x≤0.075 undergo a tetragonal to monoclinic phase transition at low temperature, whose critical temperature decreases with increasing Se content; this structural transition is strictly coupled to a long-range antiferromagnetic ordering at the Fe site. Both the transition to a monoclinic phase and the long-range antiferromagnetism are suppressed for 0.10≤x≤0.20 . The onset of the structural and of the magnetic transition remains coincident with the increase in Se substitution. The low-temperature monoclinic crystal structure has been revised. Superconductivity arises for x≥0.05 , therefore a significant region where superconductivity and long-range antiferromagnetism coexist is present in the pseudobinary FeTe-FeSe phase diagram.

  11. Specific features of magnetoresistance during the antiferromagnet-paramagnet transition in Tm{sub 1-x}Yb{sub x}B{sub 12}

    SciTech Connect

    Sluchanko, N. E. Azarevich, A. N.; Bogach, A. V.; Glushkov, V. V.; Demishev, S. V.; Levchenko, A. V.; Filippov, V. B.; Shitsevalova, N. Yu.

    2013-05-15

    The transverse magnetoresistance {Delta}{rho}/{rho}(H, T) of Tm{sub 1-x}Yb{sub x}B{sub 12} single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm{sub 1-x}Yb{sub x}B{sub 12} solid solutions with x {<=} 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -{Delta}{rho}/{rho} {proportional_to} H{sub 2}, the magnetically ordered phase of these compounds is found to have component {Delta}{rho}/{rho} {proportional_to} H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm{sub 1-x}Yb{sub x}B{sub 12} is analyzed in terms of the Yosida model for a local magnetic susceptibility.

  12. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    PubMed Central

    Frandsen, Benjamin A.; Liu, Lian; Cheung, Sky C.; Guguchia, Zurab; Khasanov, Rustem; Morenzoni, Elvezio; Munsie, Timothy J. S.; Hallas, Alannah M.; Wilson, Murray N.; Cai, Yipeng; Luke, Graeme M.; Chen, Bijuan; Li, Wenmin; Jin, Changqing; Ding, Cui; Guo, Shengli; Ning, Fanlong; Ito, Takashi U.; Higemoto, Wataru; Billinge, Simon J. L.; Sakamoto, Shoya; Fujimori, Atsushi; Murakami, Taito; Kageyama, Hiroshi; Alonso, Jose Antonio; Kotliar, Gabriel; Imada, Masatoshi; Uemura, Yasutomo J.

    2016-01-01

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition. PMID:27531192

  13. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    SciTech Connect

    B. A. Frandsen; Liu, L.; Cheung, S. C.; Guguchia, Z.; Khasanov, R.; Morenzoni, E.; Munsie, T. J.S.; Hallas, A. M.; Wilson, M. N.; Cai, Y.; Luke, G. M.; Chen, B.; Li, W.; Jin, C.; Ding, C; Guo, S.; Ning, F.; Ito, T. U.; Higemoto, W.; Billinge, S. J.L.; Sakamoto, S.; Fujimori, A.; Murakami, T.; Kageyama, H.; Alonso, J. A.; Kotliar, G.; Imada, M.; Uemura, Y. J.

    2016-08-17

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.

  14. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    NASA Astrophysics Data System (ADS)

    Frandsen, Benjamin A.; Liu, Lian; Cheung, Sky C.; Guguchia, Zurab; Khasanov, Rustem; Morenzoni, Elvezio; Munsie, Timothy J. S.; Hallas, Alannah M.; Wilson, Murray N.; Cai, Yipeng; Luke, Graeme M.; Chen, Bijuan; Li, Wenmin; Jin, Changqing; Ding, Cui; Guo, Shengli; Ning, Fanlong; Ito, Takashi U.; Higemoto, Wataru; Billinge, Simon J. L.; Sakamoto, Shoya; Fujimori, Atsushi; Murakami, Taito; Kageyama, Hiroshi; Alonso, Jose Antonio; Kotliar, Gabriel; Imada, Masatoshi; Uemura, Yasutomo J.

    2016-08-01

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.

  15. Single-crystal study on the heavy-fermion antiferromagnet UZn12

    NASA Astrophysics Data System (ADS)

    Gonçalves, A. P.; Estrela, P.; de Visser, A.; Lopes, E. B.; Catarino, I.; Bonfait, G.; Godinho, M.; Almeida, M.; Gnida, D.; Kaczorowski, D.

    2011-02-01

    Millimetre size UZn12 single crystals were grown by the high temperature solution growth method using zinc as the solvent. Single-crystal x-ray diffraction data confirm that this compound crystallizes in the hexagonal high temperature form of SmZn12 (S.G. P6/mmm) and points to a U1.01(1)Zn11.7(1) stoichiometry for the crystals, with ~ 4% of the U atoms being located at the 2c site due to the partial substitution of 4h Zn pairs. UZn12 orders antiferromagnetically at TN = 5.0(2) K, and the magnetization and resistivity measurements suggest that the magnetic moments are confined within the a-b plane. The Sommerfeld coefficient, derived from the paramagnetic region by the standard method, is γp≈200 mJ (mol K2) - 1, which definitely classifies UZn12 as a moderate heavy-fermion system. The heavy-fermion character of UZn12 is also manifested in the overall shape of temperature-dependent electrical resistivity that is dominated by a single-ion Kondo effect at high temperatures and coherent Kondo scattering at low temperatures. The paramagnetic magnetoresistivity isotherms can be fairly well superimposed onto each other using Schlottmann's scaling for the single-ion Kondo model, as expected for a Kondo system.

  16. Local moments and suppression of antiferromagnetism in correlated Zr4Fe4Si7

    SciTech Connect

    Simonson, Jack; Pezzoli, M; Garlea, Vasile O; Aronson, M.

    2013-01-01

    We report magnetic, transport, and neutron diffraction measurements as well as a doping study of the V-phase compound Zr4Fe4Si7. This compound exhibits collinear antiferromagnetic order below TN = 98 1 K with a staggered moment of 0.57(3) B/Fe as T 0. The magnetic order can be quenched with Co substitution to the Fe site, but even then a 1.5 B/Fe paramagnetic moment remains. The resistivity and heat capacity of Zr4Fe4Si7 are Fermi-liquid-like below 16 and 7 K, respectively, and reveal correlations on the scale of those observed in superconducting Fe pnictides and chalcogenides. Electronic structure calculations overestimate the ordered moment, suggesting the importance of dynamical effects. The existence of magnetic order, electronic correlations, and spin fluctuations make Zr4Fe4Si7 distinct from the majority of Fe-Si compounds, fostering comparison instead with the parent compounds of Fe-based superconductors.

  17. Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

    DOE PAGES

    B. A. Frandsen; Liu, L.; Cheung, S. C.; ...

    2016-08-17

    RENiO3 (RE=rare-earth element) and V2O3 are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO3) or pressure (V2O3), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO3 and V2O3 is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phasemore » separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.« less

  18. Ligand control of magnetic ordering temperature in copper-pyrazine square lattice antiferromagnets

    NASA Astrophysics Data System (ADS)

    Singleton, John; Goddard, P.; Franke, I.; Moeller, J.; Blundell, S.; Steele, A.; Topping, C.; Lancaster, T.; Baines, C.; Bendix, J.; McDonald, R.; Brambleby, J.; Lees, M.; Lapidus, S.; Stephens, P.; Tramley, B.; Funk, K.; Conner, M.; Corbey, J.; Tran, H.; Schlueter, J.; Manson, J.

    Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional (Q2D) antiferromagnets: [Cu(HF2)(pyz)2]ClO4 [pyz = pyrazine], [CuL2(pyz)2](ClO4)2 [ L = pyO = pyridine-N-oxide and 4-phpyO = 4-phenylpyridine-N-oxide). These possess equivalent 2D [Cu(pyz)2]2+ nearly square layers, but show interlayer spacings from 6.57 Åto 16.78 Å, dictated by the axial ligands. Structural and magnetic properties are derived from x-ray diffraction, ESR, pulsed-field magnetometry and muon-spin rotation, and compared to those of the prototypical 2D magnetic polymer Cu(ClO4)2(pyz)2. We find that the 2D exchange coupling remains largely unaffected by the axial ligand substitution, while the magnetic ordering temperature decreases slowly with increasing layer separation. Experimental data are compared to theory, including DFT. Supported by NSF, DoE, the State of Florida and EPSRC (UK).

  19. Single-crystal study on the heavy-fermion antiferromagnet UZn₁₂.

    PubMed

    Gonçalves, A P; Estrela, P; de Visser, A; Lopes, E B; Catarino, I; Bonfait, G; Godinho, M; Almeida, M; Gnida, D; Kaczorowski, D

    2011-02-02

    Millimetre size UZn(12) single crystals were grown by the high temperature solution growth method using zinc as the solvent. Single-crystal x-ray diffraction data confirm that this compound crystallizes in the hexagonal high temperature form of SmZn(12) (S.G. P6/mmm) and points to a U(1.01(1))Zn(11.7(1)) stoichiometry for the crystals, with ∼ 4% of the U atoms being located at the 2c site due to the partial substitution of 4h Zn pairs. UZn(12) orders antiferromagnetically at T(N) = 5.0(2) K, and the magnetization and resistivity measurements suggest that the magnetic moments are confined within the a-b plane. The Sommerfeld coefficient, derived from the paramagnetic region by the standard method, is γ(p)≈200 mJ (mol K(2))( - 1), which definitely classifies UZn(12) as a moderate heavy-fermion system. The heavy-fermion character of UZn(12) is also manifested in the overall shape of temperature-dependent electrical resistivity that is dominated by a single-ion Kondo effect at high temperatures and coherent Kondo scattering at low temperatures. The paramagnetic magnetoresistivity isotherms can be fairly well superimposed onto each other using Schlottmann's scaling for the single-ion Kondo model, as expected for a Kondo system.

  20. High-Tc Superconductivity and Antiferromagnetism in Multilayered Copper Oxides ---A New Paradigm of Superconducting Mechanism---

    NASA Astrophysics Data System (ADS)

    Mukuda, Hidekazu; Shimizu, Sunao; Iyo, Akira; Kitaoka, Yoshio

    2012-01-01

    High-temperature superconductivity (HTSC) in copper oxides emerges on a layered CuO2 plane when an antiferromagnetic Mott insulator is doped with mobile hole carriers. We review extensive studies of multilayered copper oxides by site-selective nuclear magnetic resonance (NMR), which have uncovered the intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a disorder-free CuO2 plane with hole carriers. We present our experimental findings such as the existence of the AFM metallic state in doped Mott insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of d-wave SC with a maximum Tc just outside a critical carrier density, at which the AFM moment on a CuO2 plane disappears. These results can be accounted for by the Mott physics based on the t--J model. The superexchange interaction Jin among spins plays a vital role as a glue for Cooper pairs or mobile spin-singlet pairs, in contrast to the phonon-mediated attractive interaction among electrons established in the Bardeen--Cooper--Schrieffer (BCS) theory. We remark that the attractive interaction for raising the Tc of HTSC up to temperatures as high as 160 K is the large Jin (˜ 0.12 eV), which binds electrons of opposite spins to be on neighboring sites, and that there are no bosonic glues. It is the Coulomb repulsive interaction U (> 6 eV) among Cu-3d electrons that plays a central role in the physics behind high-Tc phenomena. A new paradigm of the SC mechanism opens to strongly correlated electron matter.

  1. Hedgehog Excitations and their Superconducting Cores in the Antiferromagnetic State of SO(5) Materials

    NASA Astrophysics Data System (ADS)

    Goldbart, Paul M.

    1998-03-01

    Zhang's SO(5) approach to the physics of high-temperature superconducting materials(S.-C. Zhang, Science 275), 1089 (1997). contains the possibility that the antiferromagnetic state should support novel excitations that resemble antiferromagnetic hedgehogs at large distances but are predominantly superconducting inside a core region(P. M. Goldbart, Antiferromagnetic hedgehogs with superconducting cores); cond- mat/9711088 (UIUC Preprint P-97-10-030-iii).. Neither singular nor topologically stable, in contrast with their hedgehog cousins in pure antiferromagnetism, these excitations are what hedgehogs become when antiferromagnetic order is permitted to `` escape'' toward superconductivity---a central element in Zhang's approach. We describe the structure of antiferromagnetic hedgehog excitations with superconducting cores within the context of Zhang's approach to high-temperature superconducting materials, and touch upon a number of the experimental implications that these excitations engender.

  2. Electrical measurement of antiferromagnetic moments in exchange-coupled IrMn/NiFe stacks.

    PubMed

    Martí, X; Park, B G; Wunderlich, J; Reichlová, H; Kurosaki, Y; Yamada, M; Yamamoto, H; Nishide, A; Hayakawa, J; Takahashi, H; Jungwirth, T

    2012-01-06

    We employ antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift, which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals an electrically measurable memory effect in an antiferromagnet.

  3. Magnetic field induced switching of the antiferromagnetic order parameter in thin films of magnetoelectric chromia

    NASA Astrophysics Data System (ADS)

    Fallarino, Lorenzo; Berger, Andreas; Binek, Christian

    2015-02-01

    A Landau-theoretical approach is utilized to model the magnetic field induced reversal of the antiferromagnetic order parameter in thin films of magnetoelectric antiferromagnets. A key ingredient of this peculiar switching phenomenon is the presence of a robust spin polarized state at the surface of the antiferromagnetic films. Surface or boundary magnetization is symmetry allowed in magnetoelectric antiferromagnets and experimentally established for chromia thin films. It couples rigidly to the antiferromagnetic order parameter and its Zeeman energy creates a pathway to switch the antiferromagnet via magnetic field application. In the framework of a minimalist Landau free energy expansion, the temperature dependence of the switching field and the field dependence of the transition width are derived. Least-squares fits to magnetometry data of (0001 ) textured chromia thin films strongly support this model of the magnetic reversal mechanism.

  4. Synthesis of substituted pyrazines

    SciTech Connect

    Pagoria, Philip F.; Zhang, Mao Xi

    2016-10-04

    A method for synthesizing a pyrazine-containing material according to one embodiment includes contacting an iminodiacetonitrile derivative with a base and a reagent selected from a group consisting of hydroxylamine, a hydroxylamine salt, an aliphatic primary amine, a secondary amine, an aryl-substituted alkylamine a heteroaryl-substituted alkyl amine, an alcohol, an alkanolamine and an aryl alcoholamine. Additional methods and several reaction products are presented. ##STR00001##

  5. Tuning the Curie temperature of L10 ordered FePt thin films through site-specific substitution of Rh

    NASA Astrophysics Data System (ADS)

    Xu, Dongbin; Sun, Cheng-Jun; Chen, Jing-Sheng; Zhou, Tie-Jun; Heald, Steve M.; Bergman, Anders; Sanyal, Biplab; Chow, Gan Moog

    2014-10-01

    In structurally ordered magnetic thin films, the Curie temperature (TC) of ferromagnetic films depends on the exchange integral of the short range ordered neighboring atoms. The exchange integral may be adjusted by controlling the elemental substitutional concentration at the lattice site of interest. We show how to control the TC in high anisotropy L10 Fe50Pt50 magnetic thin films by substituting Rh into the Pt site. Rh substitution in L10 FePt modified the local atomic environment and the corresponding electronic properties, while retaining the ordered L10 phase. The analysis of extended x-ray Absorption Fine Structure spectra shows that Rh uniformly substitutes for Pt in L10 FePt. A model of antiferromagnetic defects caused by controlled Rh substitution of the Pt site, reducing the TC, is proposed to interpret this phenomenon and its validity is further examined by ab initio density functional calculations.

  6. Substitution of anticonvulsant drugs

    PubMed Central

    Steinhoff, Bernhard J; Runge, Uwe; Witte, Otto W; Stefan, Hermann; Hufnagel, Andreas; Mayer, Thomas; Krämer, Günter

    2009-01-01

    Changing from branded drugs to generic alternatives, or between different generic formulations, is common practice aiming at reducing health care costs. It has been suggested that antiepileptic drugs (AEDs) should be exempt from substitution because of the potential negative consequences of adverse events and breakthrough seizures. Controlled data are lacking on the risk of substitution. However, retrospective data from large medical claims databases suggest that switching might be associated with increased use of AED and non-AED medications, and health care resources (including hospitalization). In addition, some anecdotal evidence from patients and health care providers’ surveys suggest a potentially negative impact of substitution. Well-controlled data are needed to assess the real risk associated with substitution, allowing health care professionals involved in the care of patients with epilepsy to make informed decisions. This paper reviews currently available literature, based on which the authors suggest that the decision to substitute should be made on an individual basis by the physician and an informed patient. Unendorsed or undisclosed substitution at the pharmacy level should be discouraged. PMID:19707254

  7. Effects of size, shape, and frequency on the antiferromagnetic resonance linewidth of MnF

    NASA Technical Reports Server (NTRS)

    Obrien, K. C.

    1973-01-01

    The research concerning the properties and application of solid state materials at submillimeter frequencies is summarized. Work reported includes: far infrared Fourier spectroscopy; studies of the antiferromagnetic resonance line in MnF2 at millimeter wavelengths; numerical solution of the equations of motion of a general two-sublattice antiferromagnet; study of antiferromagnetic resonance line in NiO powder; and resonance investigations of several indium thisospinels at millimeter wavelengths.

  8. Mn2Au: body-centered-tetragonal bimetallic antiferromagnets grown by molecular beam epitaxy.

    PubMed

    Wu, Han-Chun; Liao, Zhi-Min; Sofin, R G Sumesh; Feng, Gen; Ma, Xiu-Mei; Shick, Alexander B; Mryasov, Oleg N; Shvets, Igor V

    2012-12-11

    Mn(2)Au, a layered bimetal, is successfully grown using molecular beam epitaxy (MBE). The experiments and theoretical calculations presented suggest that Mn(2)Au film is antiferromagnetic with a very low critical temperature. The antiferromagnetic nature is demonstrated by measuring the exchange-bias effect of Mn(2)Au/Fe bilayers. This study establishes a primary basis for further research of this new antiferromagnet in spin-electronic device applications.

  9. Magnetoelectric Force Microscopy on Antiferromagnetic 180(∘) Domains in Cr₂O₃.

    PubMed

    Schoenherr, Peggy; Giraldo, L Marcela; Lilienblum, Martin; Trassin, Morgan; Meier, Dennis; Fiebig, Manfred

    2017-09-07

    Magnetoelectric force microscopy (MeFM) is characterized as methodical tool for the investigation of antiferromagnetic domain states, in particular of the 180 ∘ variety. As reference compound for this investigation we use Cr 2 O 3 . Access to the antiferromagnetic order is provided by the linear magnetoelectric effect. We resolve the opposite antiferromagnetic 180 ∘ domain states of Cr 2 O 3 and estimate the sensitivity of the MeFM approach, its inherent advantages in comparison to alternative techniques and its general feasibility for probing antiferromagnetic order.

  10. Cyclic period-3 window in antiferromagnetic potts and Ising models on recursive lattices

    NASA Astrophysics Data System (ADS)

    Ananikian, N. S.; Ananikyan, L. N.; Chakhmakhchyan, L. A.

    2011-09-01

    The magnetic properties of the antiferromagnetic Potts model with two-site interaction and the antiferromagnetic Ising model with three-site interaction on recursive lattices have been studied. A cyclic period-3 window has been revealed by the recurrence relation method in the antiferromagnetic Q-state Potts model on the Bethe lattice (at Q < 2) and in the antiferromagnetic Ising model with three-site interaction on the Husimi cactus. The Lyapunov exponents have been calculated, modulated phases and a chaotic regime in the cyclic period-3 window have been found for one-dimensional rational mappings determined the properties of these systems.

  11. The antiferromagnetic structures of IrMn3 and their influence on exchange-bias

    PubMed Central

    Kohn, A.; Kovács, A.; Fan, R.; McIntyre, G. J.; Ward, R. C. C.; Goff, J. P.

    2013-01-01

    We have determined the magnetic structures of single-crystal thin-films of IrMn3 for the crystallographic phases of chemically-ordered L12, and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L12 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90° domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L12 triangular structure. PMID:23934541

  12. Two-dimensional-lattice spin models with long-range antiferromagnetic interactions

    NASA Astrophysics Data System (ADS)

    Romano, S.

    1991-10-01

    We consider a classical system, consisting of m-component unit vectors (m=2,3), associated with a two-dimensional lattice \\{uk||k∈openZ2\\} and interacting via translationally and rotationally invariant antiferromagnetic pair potentials of the long-range form W=Wjk=ɛ||xj-xk||-puj.uk, p>2, where ɛ is a positive quantity, setting energy and temperature scales (i.e., T*=kBT/ɛ), and xk are the coordinates of the lattice sites. A spin-wave approach predicts orientational disorder (in the thermodynamic limit) at all finite temperatures and for all p>2 this agrees with available rigorous results for p>=4, whereas no such theorems are known in the literature when 2disordered at all finite temperatures, for all p>2.

  13. Phase transitions and critical properties in the antiferromagnetic Ising model on a layered triangular lattice with allowance for intralayer next-nearest-neighbor interactions

    SciTech Connect

    Badiev, M. K. Murtazaev, A. K.; Ramazanov, M. K.

    2016-10-15

    The phase transitions (PTs) and critical properties of the antiferromagnetic Ising model on a layered (stacked) triangular lattice have been studied by the Monte Carlo method using a replica algorithm with allowance for the next-nearest-neighbor interactions. The character of PTs is analyzed using the histogram technique and the method of Binder cumulants. It is established that the transition from the disordered to paramagnetic phase in the adopted model is a second-order PT. Static critical exponents of the heat capacity (α), susceptibility (γ), order parameter (β), and correlation radius (ν) and the Fischer exponent η are calculated using the finite-size scaling theory. It is shown that (i) the antiferromagnetic Ising model on a layered triangular lattice belongs to the XY universality class of critical behavior and (ii) allowance for the intralayer interactions of next-nearest neighbors in the adopted model leads to a change in the universality class of critical behavior.

  14. How to move domain walls in an antiferromagnet

    NASA Astrophysics Data System (ADS)

    Kim, Se Kwon

    Domain walls (DWs) in an easy-axis antiferromagnet can be driven by several stimuli: a charge current (in conducting antiferromagnets), a magnon current, and a temperature gradient. In this talk, we discuss the dynamics of a DW induced by two latter external perturbations, which are applicable in both metallic and insulating antiferromagnets. First of all, we study the Brownian dynamics of a DW subjected to a temperature gradient. To this end, we derive the Langevin equation for the DW's center of mass with the aid of the fluctuation-dissipation theorem. A DW behaves as a classical massive particle immersed in a viscous medium. By considering a thermodynamic ensemble of DWs, we obtain the Fokker-Planck equation, from which we extract the average drift velocity of a DW. We briefly address other mechanisms of thermally driven DW motion. Secondly, we analyze the dynamics of a DW driven by circularly polarized magnons. Magnons passing through a DW reverse their spin upon transmission, thereby transferring two quanta of angular momentum to the DW and causing it to precess. A precessing DW partially reflects magnons back to the source. The reflection of magnons creates a previously identified reactive force. We point out a second mechanism of propulsion of the DW, which we term redshift: magnons passing through a precessing DW reduce their linear momentum and transfer the decrease to the DW. We solve the equations of motion for magnons in the background of a uniformly precessing DW with the aid of supersymmetric quantum mechanics and compute the net force and torque applied by magnons to the DW. The theory agrees well with micromagnetic simulations. This work has been supported in part by the ARO, the U.S. DOE-BES, and the U.S. NSF grants.

  15. Spin injection and absorption in antiferromagnets (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Frangou, Lamprini; Merodio, Pablo; Ghosh, Abhijit; Oyarzun, Simon; Auffret, Stephane; Ebels, Ursula; Chshiev, Mair; Bea, Helene; Vila, Laurent; Bailey, William E.; Gambarelli, Serge; Baltz, Vincent

    2016-10-01

    The antiferromagnetic order is expected to have a high potential in next-generation spintronic applications. It is resistant to perturbation by magnetic fields, produces no stray fields, displays ultrafast dynamics and may generate large magneto-transport effects. In spintronic materials, spin currents are key to unravelling spin dependent transport phenomena. Here, spin pumping results from the non-equilibrium magnetization dynamics of a ferromagnetic spin injector, which pumps a spin current into an adjacent spin sink. This spin sink absorbs the current to an extent which depends on its spin-dependent properties. The properties of the spin sink can be recorded either through the changes induced in ferromagnetic damping or through direct electrical means, such as by measuring the inverse spin Hall voltage. In this talk, we will deal with the injection of a spin current in thin antiferromagnetic sinks. Measurements of the spin penetration depths and absorption mechanisms were obtained for polycrystalline Ir20Mn80 and Fe50Mn50 films (Appl. Phys. Lett. 104, 032406 (2014)). More interestingly, spins propagate more efficiently in layers where the magnetic order is fluctuating rather than static. The experimental data were compared to some of the recently developed theories and converted into interfacial spin mixing conductance enhancements. These findings help us progress towards the development of more efficient spin sources, while also providing an alternative method to probe magnetic phase transitions (Phys. Rev. Lett. in press (2016)). This type of alternative method is particularly needed to deal with the case of thin materials with no net magnetic moments, such as thin antiferromagnets.

  16. Control of the third dimension in copper-based square-lattice antiferromagnets

    DOE PAGES

    Goddard, Paul A.; Singleton, John; Franke, Isabel; ...

    2016-03-25

    Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF2)(pyz)2]ClO4 [pyz = pyrazine], [CuL2(pyz)2](ClO4)2 [L = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)2]2+ nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)2(ClO4)2. We find that, within the limits of themore » experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)2(ClO4)2, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. Here, we discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S = 1/2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.« less

  17. Control of the third dimension in copper-based square-lattice antiferromagnets

    SciTech Connect

    Goddard, Paul A.; Singleton, John; Franke, Isabel; Moller, Johannes S.; Lancaster, Tom; Steele, Andrew J.; Topping, Craig V.; Blundell, Stephen J.; Pratt, Francis L.; Baines, Chris; Bendix, Jesper; McDonald, Ross David; Brambleby, Jamie; Lees, Martin R.; Lapidus, Saul H.; Stephens, Peter W.; Twamley, Brendan W.; Conner, Marianne M.; Funk, Kylee; Corbey, Jordan F.; Tran, Hope E.; Schlueter, John A.; Manson, Jamie L.

    2016-03-25

    Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF2)(pyz)2]ClO4 [pyz = pyrazine], [CuL2(pyz)2](ClO4)2 [L = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)2]2+ nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)2(ClO4)2. We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)2(ClO4)2, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. Here, we discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S = 1/2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled

  18. Control of the third dimension in copper-based square-lattice antiferromagnets

    NASA Astrophysics Data System (ADS)

    Goddard, Paul A.; Singleton, John; Franke, Isabel; Möller, Johannes S.; Lancaster, Tom; Steele, Andrew J.; Topping, Craig V.; Blundell, Stephen J.; Pratt, Francis L.; Baines, C.; Bendix, Jesper; McDonald, Ross D.; Brambleby, Jamie; Lees, Martin R.; Lapidus, Saul H.; Stephens, Peter W.; Twamley, Brendan W.; Conner, Marianne M.; Funk, Kylee; Corbey, Jordan F.; Tran, Hope E.; Schlueter, J. A.; Manson, Jamie L.

    2016-03-01

    Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu (HF2) (pyz) 2] ClO4 [pyz = pyrazine], [Cu L2(pyz) 2] (ClO4)2 [L = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)2] 2 + nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz) 2(ClO4)2 . We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz) 2(ClO4)2 , the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S =1 /2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.

  19. Control of the third dimension in copper-based square-lattice antiferromagnets

    SciTech Connect

    Goddard, Paul A.; Singleton, John; Franke, Isabel; Möller, Johannes S.; Lancaster, Tom; Steele, Andrew J.; Topping, Craig V.; Blundell, Stephen J.; Pratt, Francis L.; Baines, C.; Bendix, Jesper; McDonald, Ross D.; Brambleby, Jamie; Lees, Martin R.; Lapidus, Saul H.; Stephens, Peter W.; Twamley, Brendan W.; Conner, Marianne M.; Funk, Kylee; Corbey, Jordan F.; Tran, Hope E.; Schlueter, J. A.; Manson, Jamie L.

    2016-03-01

    Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF2)(pyz)(2)]ClO4 [pyz = pyrazine], [CuL2(pyz)(2)](ClO4)(2) [L = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)(2)](2+) nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 angstrom, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed-and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)(2)(ClO4)(2). We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)(2)(ClO4)(2), the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S = 1/2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.

  20. Z2 antiferromagnetic topological insulators with broken C4 symmetry

    NASA Astrophysics Data System (ADS)

    Bègue, Frédéric; Pujol, Pierre; Ramazashvili, Revaz

    2017-04-01

    A two-dimensional topological insulator may arise in a centrosymmetric commensurate Néel antiferromagnet (AF), where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. Fang et al. [6] proposed an expression for a Z2 topological invariant to characterize such systems. Here, we show that this expression does not allow to detect all the existing phases if a certain lattice symmetry is lacking. We implement numerical techniques to diagnose topological phases of a toy Hamiltonian, and verify our results by computing the Chern numbers of degenerate bands, and also by explicitly constructing the edge states, thus illustrating the efficiency of the method.

  1. Emergent transition for superconducting fluctuations in antiferromagnetic ruthenocuprates

    NASA Astrophysics Data System (ADS)

    Mclaughlin, A. C.; Attfield, J. P.

    2014-12-01

    The emergence of carrier pairing from the electronically inhomogeneous phase of lightly hole-doped copper oxides has been investigated through magnetoresistance measurements on 1222-type ruthenocuprates RuSr2(R,Ce ) 2Cu2O10 -δ , principally with R =Gd , Sm, Nd. A well-defined transition at which superconducting fluctuations emerge is discovered at a remarkably low critical doping, pc=0.0084 , deep within the antiferromagnetic phase. Systematic variations of the low-temperature fluctuation density with doping and cell volume demonstrate the intrinsic nature of the electronic inhomogeneity and provide new support for bosonic models of the superconducting mechanism.

  2. Antiferromagnetic structure in tetragonal CuMnAs thin films

    PubMed Central

    Wadley, P.; Hills, V.; Shahedkhah, M. R.; Edmonds, K. W.; Campion, R. P.; Novák, V.; Ouladdiaf, B.; Khalyavin, D.; Langridge, S.; Saidl, V.; Nemec, P.; Rushforth, A. W.; Gallagher, B. L.; Dhesi, S. S.; Maccherozzi, F.; Železný, J.; Jungwirth, T.

    2015-01-01

    Tetragonal CuMnAs is an antiferromagnetic material with favourable properties for applications in spintronics. Using a combination of neutron diffraction and x-ray magnetic linear dichroism, we determine the spin axis and magnetic structure in tetragonal CuMnAs, and reveal the presence of an interfacial uniaxial magnetic anisotropy. From the temperature-dependence of the neutron diffraction intensities, the Néel temperature is shown to be (480 ± 5) K. Ab initio calculations indicate a weak anisotropy in the (ab) plane for bulk crystals, with a large anisotropy energy barrier between in-plane and perpendicular-to-plane directions. PMID:26602978

  3. Excitations in a four-leg antiferromagnetic Heisenberg spin tube

    SciTech Connect

    Garlea, Vasile O; Zheludev, Andrey I; Regnault, L.-P.; Chung, J.-H.; Qiu, Y.; Boehm, Martin; Habicht, Klaus; Meissner, Michael

    2008-01-01

    Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system Cu$_2$Cl$_{4}\\cdot$ D$_8$C$_4$SO$_2$. Contrary to previously conjectured models that relied on bond-alternating nearest neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a $S=1/2$ 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.

  4. Excitations in a Four-Leg Antiferromagnetic Heisenberg Spin Tube,

    SciTech Connect

    Garlea, Vasile O; Zheludev, Andrey I; Regnault, L.-P.; Chung, J.-H.; Qiu, Y.; Boehm, Martin; Habicht, Klaus; Meissner, Michael; Fernandez-Baca, Jaime A

    2008-01-01

    Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system Cu2Cl4 D8C4SO2. Contrary to previously conjectured models that relied on bond-alternating nearest-neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a S 1=2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.

  5. Excitations in a four-leg antiferromagnetic Heisenberg spin tube.

    PubMed

    Garlea, V O; Zheludev, A; Regnault, L-P; Chung, J-H; Qiu, Y; Boehm, M; Habicht, K; Meissner, M

    2008-01-25

    Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system Cu(2)Cl(4).D(8)C(4)SO(2). Contrary to previously conjectured models that relied on bond-alternating nearest-neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a S=1/2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.

  6. Symmetry Reduction in the Quantum Kagome Antiferromagnet Herbertsmithite

    NASA Astrophysics Data System (ADS)

    Zorko, A.; Herak, M.; Gomilšek, M.; van Tol, J.; Velázquez, M.; Khuntia, P.; Bert, F.; Mendels, P.

    2017-01-01

    Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the threefold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state.

  7. Competing interactions in ferromagnetic/antiferromagnetic perovskite superlattices

    SciTech Connect

    Takamura, Y.; Biegalski, M.B.; Christen, H.M.

    2009-10-22

    Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La{sub 0.7}Sr{sub 0.3}FeO{sub 3}(LSFO)/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}(LSMO) perovskite oxide superlattices leads to unexpected functional properties. The antiferromagnetic order parameter in LSFO and ferromagnetic order parameter in LSMO show a dissimilar dependence on sublayer thickness and temperature, illustrating the high degree of tunability in these artificially layered materials.

  8. Antiferromagnetic Mn 50Fe 50 wire with large magnetostriction

    NASA Astrophysics Data System (ADS)

    He, Aina; Ma, Tianyu; Zhang, Jingjing; Luo, Wei; Yan, Mi

    2009-11-01

    This work presents a study on the relation between the fiber texture and the magnetostrictive performance in an antiferromagnetic Mn 50Fe 50 alloy wire, which was prepared through the combining process of hot rolling and cold drawing. The face-centered cubic (fcc) crystal structure can be retained during the plastic deformation process. Mixed fiber textures consisting of both <1 1 0> and <1 0 0> components were formed along the drawing direction (DD) in the wire. A large magnetostriction of 750 ppm was obtained along DD under 1.2 T, which can be ascribed to the single γ phase and the formation of preferred crystal orientation.

  9. Structural origin of magnetic birefringence in rutile-type antiferromagnets

    NASA Astrophysics Data System (ADS)

    Jauch, W.

    1991-10-01

    The microscopic origin of magnetic birefringence in the rutile-type antiferromagnets XF2 (X=Mn, Fe, Co, or Ni) is analyzed on the basis of the theory of structural birefringence developed by Ewald and Born. The general principles of the Ewald-Born theory are reviewed. The magnetic birefringence can be explained by a small exchange-induced internal displacement of the fluorine atoms. Predictions from theory are compared with accurate crystal-structure analyses based on γ-ray-diffraction data. The agreement found between theory and experiment is excellent.

  10. Antiferromagnetic structure in tetragonal CuMnAs thin films.

    PubMed

    Wadley, P; Hills, V; Shahedkhah, M R; Edmonds, K W; Campion, R P; Novák, V; Ouladdiaf, B; Khalyavin, D; Langridge, S; Saidl, V; Nemec, P; Rushforth, A W; Gallagher, B L; Dhesi, S S; Maccherozzi, F; Železný, J; Jungwirth, T

    2015-11-25

    Tetragonal CuMnAs is an antiferromagnetic material with favourable properties for applications in spintronics. Using a combination of neutron diffraction and x-ray magnetic linear dichroism, we determine the spin axis and magnetic structure in tetragonal CuMnAs, and reveal the presence of an interfacial uniaxial magnetic anisotropy. From the temperature-dependence of the neutron diffraction intensities, the Néel temperature is shown to be (480 ± 5) K. Ab initio calculations indicate a weak anisotropy in the (ab) plane for bulk crystals, with a large anisotropy energy barrier between in-plane and perpendicular-to-plane directions.

  11. Phase transitions in antiferromagnets with a NaCl structure

    NASA Astrophysics Data System (ADS)

    Kassan-Ogly, F. A.; Filippov, B. N.

    2006-05-01

    A revised derivation scheme of possible magnetic structures in an FCC lattice with the nearest- and next-nearest-neighbor interactions taken into account is proposed. A model of simultaneous magnetic and structural phase transitions of the first order is developed for antiferromagnets with a NaCl structure and with a strong cubic magnetic anisotropy on the base of synthesis of magnetic modified 6-state Potts model and theoretical models of structural phase transitions in cubic crystals. It is shown that the high-temperature diffuse magnetic scattering of neutrons transforms into magnetic Bragg reflections below Néel point.

  12. Symmetry Reduction in the Quantum Kagome Antiferromagnet Herbertsmithite.

    PubMed

    Zorko, A; Herak, M; Gomilšek, M; van Tol, J; Velázquez, M; Khuntia, P; Bert, F; Mendels, P

    2017-01-06

    Employing complementary torque magnetometry and electron spin resonance on single crystals of herbertsmithite, the closest realization to date of a quantum kagome antiferromagnet featuring a spin-liquid ground state, we provide novel insight into different contributions to its magnetism. At low temperatures, two distinct types of defects with different magnetic couplings to the kagome spins are found. Surprisingly, their magnetic response contradicts the threefold symmetry of the ideal kagome lattice, suggesting the presence of a global structural distortion that may be related to the establishment of the spin-liquid ground state.

  13. The substitutability of reinforcers

    PubMed Central

    Green, Leonard; Freed, Debra E.

    1993-01-01

    Substitutability is a construct borrowed from microeconomics that describes a continuum of possible interactions among the reinforcers in a given situation. Highly substitutable reinforcers, which occupy one end of the continuum, are readily traded for each other due to their functional similarity. Complementary reinforcers, at the other end of the continuum, tend to be consumed jointly in fairly rigid proportion, and therefore cannot be traded for one another except to achieve that proportion. At the center of the continuum are reinforcers that are independent with respect to each other; consumption of one has no influence on consumption of another. Psychological research and analyses in terms of substitutability employ standard operant conditioning paradigms in which humans and nonhumans choose between alternative reinforcers. The range of reinforcer interactions found in these studies is more readily accommodated and predicted when behavior-analytic models of choice consider issues of substitutability. New insights are gained into such areas as eating and drinking, electrical brain stimulation, temporal separation of choice alternatives, behavior therapy, drug use, and addictions. Moreover, the generalized matching law (Baum, 1974) gains greater explanatory power and comprehensiveness when measures of substitutability are included. PMID:16812696

  14. NMR characterization of sulphur substitution effects in the KxFe2-ySe2-zSz high-Tc superconductor

    SciTech Connect

    Torchetti, D. A.; Imai, T.; Lei, H. C.; Petrovic, C.

    2012-04-17

    We present a⁷⁷ Se NMR study of the effect of S substitution in the high-Tc superconductor KxFe2-ySe2-zSz in a temperature range up to 250 K. We examine two S concentrations, with z=0.8 (Tc~ 26 K) and z=1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the KxFe₂Se₂ sample due to local disorder in the Se environment. Our Knight shift ⁷⁷K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None of the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near Tc in 1/T₁T, as seen in FeSe.

  15. NMR characterization of sulphur substitution effects in the KxFe2-ySe2-xSz high-Tc superconductor

    SciTech Connect

    Torchetti, D. A.; Imai, T.; Lei, H. C.; Petrovic, C.

    2012-04-17

    We present a 77Se NMR study of the effect of S substitution in the high-Tc superconductor KxFe2-ySe2-zSz in a temperature range up to 250 K. We examine two S concentrations, with z=0.8 (Tc~ 26 K) and z=1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the KxFe2Se2 sample due to local disorder in the Se environment. Our Knight shift 77K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None of the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near Tc in 1/T1T, as seen in FeSe.

  16. Antiferromagnetism in EuNiGe 3

    SciTech Connect

    Goetsch, R. J.; Anand, V. K.; Johnston, D. C.

    2013-02-01

    The synthesis and crystallographic and physical properties of polycrystalline EuNiGe3 are reported. EuNiGe3 crystallizes in the noncentrosymmetric body-centered tetragonal BaNiSn3-type structure (space group I4mm), in agreement with previous reports, with the Eu atoms at the corners and body center of the unit cell. The physical property data consistently demonstrate that this is a metallic system in which Eu spins S = 7/2 order antiferromagnetically at a temperature TN = 13.6 K.Magnetic susceptibility χ data forT >TN indicate that the Eu atoms have spin 7/2 with g = 2, that the Ni atoms are nonmagnetic, and that the dominant interactions between the Eu spins are ferromagnetic. Thus we propose that EuNiGe3 has a collinear A-type antiferromagnetic structure, with the Eu ordered moments in the ab plane aligned ferromagnetically and with the moments in adjacent planes along the c axis aligned antiferromagnetically. A fit of χ(T TN) by our molecular field theory is consistent with a collinear magnetic structure. Electrical resistivity ρ data from TN to 350 K are fitted by the Bloch-Gr¨uneisen model for electron-phonon scattering, yielding a Debye temperature of 265(2) K.Astrong decrease in ρ occurs belowTN due to loss of spin-disorder scattering. Heat capacity data at 25 K T 300Kare fitted by the Debye model, yielding the same Debye temperature 268(2) K as found from ρ(T ). The extracted magnetic heat capacity is consistent with S = 7/2 and shows that significant short-range dynamical spin correlations occur above TN. The magnetic entropy at TN = 13.6 K is 83% of the expected asymptotic high-T value, with the remainder recovered by 30 K.

  17. Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7.

    PubMed

    Gaultois, Michael W; Barton, Phillip T; Birkel, Christina S; Misch, Lauren M; Rodriguez, Efrain E; Stucky, Galen D; Seshadri, Ram

    2013-05-08

    Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a combination of structural, magnetic, and electrical and thermal transport studies. Analysis of synchrotron x-ray and neutron diffraction patterns suggests some site disorder on the A-site in the pyrochlore sublattice: Ru substitutes on the Nd-site up to 7.0(3)%, regardless of the different preparative conditions explored. Intrinsic magnetic and electrical transport properties have been measured. Ru 4d spins order antiferromagnetically at 143 K, as seen both in the susceptibility and in the specific heat, and there is a corresponding change in the electrical resistivity. The onset of a second antiferromagnetic ordering transition seen below 5 K is attributed to ordering of Nd 4f spins. Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be independent of the Ru-antisite disorder on the Nd-site. The electrical properties of Nd2Ru2O7 are presented in the light of data published on all A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi(3+) ions on the A-site play in driving metallic behaviour. High-temperature thermoelectric properties have also been measured. When considered in the context of known thermoelectric materials with useful figures-of-merit, it is clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents it from being an effective thermoelectric. A method for screening candidate thermoelectrics is suggested.

  18. Aryl substitution of pentacenes

    PubMed Central

    Waterloo, Andreas R; Sale, Anna-Chiara; Lehnherr, Dan; Hampel, Frank

    2014-01-01

    Summary A series of 11 new pentacene derivatives has been synthesized, with unsymmetrical substitution based on a trialkylsilylethynyl group at the 6-position and various aryl groups appended to the 13-position. The electronic and physical properties of the new pentacene chromophores have been analyzed by UV–vis spectroscopy (solution and thin films), thermoanalytical methods (DSC and TGA), cyclic voltammetry, as well as X-ray crystallography (for 8 derivatives). X-ray crystallography has been specifically used to study the influence of unsymmetrical substitution on the solid-state packing of the pentacene derivatives. The obtained results add to our ability to better predict substitution patterns that might be helpful for designing new semiconductors for use in solid-state devices. PMID:25161729

  19. Cluster glass behaviour in Co-substituted double perovskite Ca{sub 2}FeMoO{sub 6}

    SciTech Connect

    Poddar, Asok; Mazumdar, Chandan

    2011-05-15

    Research highlights: {yields} We have presented the crystallographic, electrical transport and dc and ac magnetic measurements on double perovskite Ca{sub 2}Fe{sub 1-x}Co{sub x}MoO{sub 6} (0.1 {<=} x {<=} 0.4) system. {yields} Co exists in divalent state in these compounds. {yields} The substituted system exhibits cluster-glass like behaviour, close to that observed earlier in Sr{sub 2}Fe{sub 1-x}Co{sub x}MoO{sub 6} compatible with an electronic phase segregation scenario (Phys. Rev. B 73 (2006) 104417). -- Abstract: The transport and magnetic properties of the double perovskite compounds Ca{sub 2}Fe{sub 1-x}Co{sub x}MoO{sub 6} (0.1 {<=} x {<=} 0.4) have been explored through resistivity [{rho}(T)], dc magnetisation [M(H, T)] and ac susceptibility [{chi}(T, f)] measurements. Introduction of Co increases the lattice volume implying the divalent nature of cobalt in this system. For all the samples, {rho}(T) behaviour over the temperature range (25-273 K) can be adequately described by considering possible disorder-enhanced electron-electron interaction effect as well as spin-wave contribution. Our results reveal that with the increase of Co concentration, the ferromagnetic Curie temperatures (T{sub C}) gradually reduced due to the incorporation of antiferromagnetic Co{sup 2+}-Mo{sup 6+} pairs replacing ferromagnetic Fe{sup 3+}-Mo{sup 5+} pairs. A cluster-glass like behaviour is also observed in the system due to the presence of highly spin-disordered regions.

  20. Thermally stable magnetic media based on antiferromagnetically coupled layers

    NASA Astrophysics Data System (ADS)

    Fullerton, Eric E.

    2001-03-01

    The combination of signal-to-noise requirements, write field limitations, and thermal activation of small particles is thought to limit the potential areal density of longitudinal media and is commonly referred to as the "superparamagnetic limit". Recording media composed of antiferromagnetically coupled (AFC) magnetic recording layers is a promising approach to extend areal densities of longitudinal media beyond these perceived limits [1,2]. The recording medium is made up of two ferromagnetic recording layer separated by a nonmagnetic layer whose thickness is tuned to couple the layers antiferromagnetically. For such a structure, the effective areal moment density (Mrt) of the composite structure is given by the difference between the ferromagnetic layers allowing the effective magnetic thickness to scale independently of the physical thickness of the media. The resulting media appears magnetically thin while being physically thick and, thus, allows AFC media to maintain thermal stability even for low Mrt values. Experimental realization of this concept using CoPtCrB alloy layers that demonstrates thermally stable low-Mrt media suitable for high-density recording will be discussed. This work is done in collaboration with D. T. Margulies, M. E. Schabes,M. Doerner, M. Carey, B. Gurney, A. Moser, M. Best, G. Zeltzer, K. Rubin, and H. Rosen. [1]. Fullerton et al., Appl. Phys. Lett. 77, 3806 (2000). [2]. Abarra et al., Appl. Phys. Lett. 77, 2581 (2000).

  1. Antiferromagnetic ground state in NpCoGe

    NASA Astrophysics Data System (ADS)

    Colineau, E.; Griveau, J.-C.; Eloirdi, R.; Gaczyński, P.; Khmelevskyi, S.; Shick, A. B.; Caciuffo, R.

    2014-03-01

    NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, 237Np Mössbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a Néel temperature TN≈13 K and an average ordered magnetic moment <μNp>=0.80μB. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (≈3 T). The value of the isomer shift suggests a Np3+ charge state (configuration 5f4). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol-1 K-2) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the c axis. Mössbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to a magnetic quantum critical point.

  2. Antiferromagnetic order in MnO spherical nanoparticles

    SciTech Connect

    Wang, Cuihuan; Baker, Sheila N; Lumsden, Mark D; Nagler, Stephen E; Heller, William T; Baker, Gary A; Deen, P P; Cranswick, Lachlan M.D.; Su, Y.; Christianson, Andrew D

    2011-01-01

    We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8- and 13-nm antiferromagnetic MnO nanoparticles. For the 8-nm sample, the antiferromagnetic transition temperature T{sub N} (114 K) is suppressed compared to that in the bulk material (119 K), while for the 13-nm sample T{sub N} (120 K) is comparable to that in the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2 {+-} 0.3 {micro}{sub B}/Mn for the 8-nm sample and 3.9 {+-} 0.2 {micro}{sub B}/Mn for the 13-nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80% of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.

  3. Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice

    PubMed Central

    Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

    2014-01-01

    The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems. PMID:24777282

  4. Antiferromagnetic Kondo lattice compound CePt3P.

    PubMed

    Chen, Jian; Wang, Zhen; Zheng, Shiyi; Feng, Chunmu; Dai, Jianhui; Xu, Zhu'an

    2017-02-03

    A new ternary platinum phosphide CePt3P was synthesized and characterized by means of magnetic, thermodynamic and transport measurements. The compound crystallizes in an antiperovskite tetragonal structure similar to that in the canonical family of platinum-based superconductors APt3P (A = Sr, Ca, La) and closely related to the noncentrosymmetric heavy fermion superconductor CePt3Si. In contrast to all the superconducting counterparts, however, no superconductivity is observed in CePt3P down to 0.5 K. Instead, CePt3P displays a coexistence of antiferromagnetic ordering, Kondo effect and crystalline electric field effect. A field-induced spin-flop transition is observed below the magnetic ordering temperature TN1 of 3.0 K while the Kondo temperature is of similar magnitude as TN1. The obtained Sommerfeld coefficient of electronic specific heat is γCe = 86 mJ/mol·K(2) indicating that CePt3P is a moderately correlated antiferromagnetic Kondo lattice compound.

  5. Antiferromagnetic Kondo lattice compound CePt3P

    PubMed Central

    Chen, Jian; Wang, Zhen; Zheng, Shiyi; Feng, Chunmu; Dai, Jianhui; Xu, Zhu’an

    2017-01-01

    A new ternary platinum phosphide CePt3P was synthesized and characterized by means of magnetic, thermodynamic and transport measurements. The compound crystallizes in an antiperovskite tetragonal structure similar to that in the canonical family of platinum-based superconductors APt3P (A = Sr, Ca, La) and closely related to the noncentrosymmetric heavy fermion superconductor CePt3Si. In contrast to all the superconducting counterparts, however, no superconductivity is observed in CePt3P down to 0.5 K. Instead, CePt3P displays a coexistence of antiferromagnetic ordering, Kondo effect and crystalline electric field effect. A field-induced spin-flop transition is observed below the magnetic ordering temperature TN1 of 3.0 K while the Kondo temperature is of similar magnitude as TN1. The obtained Sommerfeld coefficient of electronic specific heat is γCe = 86 mJ/mol·K2 indicating that CePt3P is a moderately correlated antiferromagnetic Kondo lattice compound. PMID:28157184

  6. Electrical control of antiferromagnetic metal up to 15 nm

    NASA Astrophysics Data System (ADS)

    Zhang, PengXiang; Yin, GuFan; Wang, YuYan; Cui, Bin; Pan, Feng; Song, Cheng

    2016-08-01

    Manipulation of antiferromagnetic (AFM) spins by electrical means is on great demand to develop the AFM spintronics with low power consumption. Here we report a reversible electrical control of antiferromagnetic moments of FeMn up to 15 nm, using an ionic liquid to exert a substantial electric-field effect. The manipulation is demonstrated by the modulation of exchange spring in [Co/Pt]/FeMn system, where AFM moments in FeMn pin the magnetization rotation of Co/Pt. By carrier injection or extraction, the magnetic anisotropy of the top layer in FeMn is modulated to influence the whole exchange spring and then passes its influence to the [Co/Pt]/FeMn interface, through a distance up to the length of exchange spring that fully screens electric field. Comparing FeMn to IrMn, despite the opposite dependence of exchange bias on gate voltages, the same correlation between carrier density and exchange spring stiffness is demonstrated. Besides the fundamental significance of modulating the spin structures in metallic AFM via all-electrical fashion, the present finding would advance the development of low-power-consumption AFM spintronics.

  7. Pressure effects in the itinerant antiferromagnetic metal TiAu

    DOE PAGES

    Wolowiec, C. T.; Fang, Y.; McElroy, C. A.; ...

    2017-06-07

    Here, we report the pressure dependence of the Néel temperature TN up to P ≈ 27 GPa for the recently discovered itinerant antiferromagnet (IAFM) TiAu. The TN(P) phase boundary exhibits unconventional behavior in which the Néel temperature is enhanced from TN ≈ 33 K at ambient pressure to a maximum of TN ≈ 35 K occurring at P ≈ 5.5 GPa. Upon a further increase in pressure, TN is monotonically suppressed to ~22 K at P ≈ 27 GPa. We also find a crossover in the temperature dependence of the electrical resistivity ρ in the antiferromagnetic (AFM) phase that ismore » coincident with the peak in TN(P), such that the temperature dependence of ρ = ρ0 + AnTn changes from n≈3 during the enhancement of TN to n ≈ 2 during the suppression of TN. Based on an extrapolation of the TN(P) data to a possible pressure-induced quantum critical point, we estimate the critical pressure to be Pc ≈ 45 GPa.« less

  8. Antiferromagnetic coupling in soft amorphous ferromagnet/semiconductor multilayers

    NASA Astrophysics Data System (ADS)

    Velez, Maria

    2005-03-01

    Antiferromagnetic coupling between ferromagnetic layers separated by nonmagnetic metallic interlayers has been intensively studied due to the fundamental and technological interest in such behaviour. In this work, the presence of antiferromagnetic (AF) coupling has been investigated in multilayers where the nonmagnetic interlayers are not metallic but semiconducting. The analyzed samples are amorphous (CoxSi1-x)5 nm /(Si)d multilayers obtained by co-sputtering on Si substrates, and the Si layer thickness has been varied in the range 1 nm < d < 15 nm. X-ray diffraction analysis has shown that the multilayered structure is well defined. The individual (CoxSi1-x)5 nm ferromagnetic layer presents an uniaxial anisotropy and a soft magnetic behaviour (with coercivity smaller than 1 Oe for fields applied along its easy axis), being suitable to detect the possible AF coupling in the multilayer. Magneto-optical kerr effect and alternating gradient magnetometry measurements have revealed that these multilayers do present AF coupling at room temperature for d < 8 nm. Moreover, the magnetic field required to switch between antiparallel and parallel configurations is as low as 3 Oe and varies slightly with the Si layer thickness [1]. [1] C. Quiros et al., Phys. Rev. B (in press)

  9. Magnetic excitations of a doped two-dimensional antiferromagnet

    SciTech Connect

    Sherman, A. ); Schreiber, M. )

    1993-09-01

    Magnetic excitations of the two-dimensional (2D) [ital t]-[ital J] model are considered in the presence of a small concentration of holes [ital c]. The spin-wave approximation used implies long-range antiferromagnetic ordering from the beginning. Migdal's theorem is shown to be valid for the model considered. The energy spectrum of the magnons is determined with the help of the one-pole approximation for the hole Green's function. If the concentration of mobile holes is larger than a critical value an additional branch of overdamped magnons arises near the [Gamma] and [ital M] points of the Brillouin zone. This is connected with the generation of electron-hole pairs (the Stoner excitations) by magnons. The appearance of such excitations means the destruction of the long-range antiferromagnetic order. For parameters presumably realized in cuprate perovskites this happens for several percent of holes per site. The relation between the critical concentration and the hole concentration destroying the 3D long-range ordering in La[sub 2[minus][ital x

  10. Fractional excitations in the square-lattice quantum antiferromagnet

    SciTech Connect

    Dalla Piazza, Bastien; Mourigal, M.; Christensen, N. B.; Nilsen, G. J.; Tregenna-Piggott, P.; Perring, T. G.; Enderle, M.; McMorrow, D. F.; Ivanov, D. A.; Ronnow, H. M.

    2014-12-15

    Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). Here, we use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Lastly, our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.

  11. Fractional excitations in the square lattice quantum antiferromagnet

    PubMed Central

    Christensen, N. B.; Nilsen, G. J.; Tregenna-Piggott, P.; Perring, T. G.; Enderle, M.; McMorrow, D. F.; Ivanov, D. A.; Rønnow, H. M.

    2014-01-01

    Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration. PMID:25729400

  12. Three-sublattice skyrmion crystal in the antiferromagnetic triangular lattice

    NASA Astrophysics Data System (ADS)

    Rosales, H. D.; Cabra, D. C.; Pujol, Pierre

    2015-12-01

    The frustrated classical antiferromagnetic Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of an antiferromagnetic skyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating skyrmion crystals (one by sublattice), and most importantly, the AF-SkX state seems to survive in the limit of zero temperature. To characterize the phase diagram we compute the average of the topological order parameter which can be associated with the number of topological charges or skyrmions. As the magnetic field increases this parameter presents a clear jump, indicating a discontinuous transition from a spiral phase into the AF-SkX phase, where multiple Bragg peaks coexist in the spin structure factor. For higher fields, a second (probably continuous) transition occurs into a featureless paramagnetic phase.

  13. Spin Seebeck effect through antiferromagnetic NiO

    NASA Astrophysics Data System (ADS)

    Prakash, Arati; Brangham, Jack; Yang, Fengyuan; Heremans, Joseph P.

    2016-07-01

    We report temperature-dependent spin Seebeck measurements on Pt/YIG bilayers and Pt/NiO/YIG trilayers, where YIG (yttrium iron garnet, Y3F e5O12 ) is an insulating ferrimagnet and NiO is an antiferromagnet at low temperatures. The thickness of the NiO layer is varied from 0 to 10 nm. In the Pt/YIG bilayers, the temperature gradient applied to the YIG stimulates dynamic spin injection into the Pt, which generates an inverse spin Hall voltage in the Pt. The presence of a NiO layer dampens the spin injection exponentially with a decay length of 2 ± 0.6 nm at 180 K. The decay length increases with temperature and shows a maximum of 5.5 ± 0.8 nm at 360 K. The temperature dependence of the amplitude of the spin Seebeck signal without NiO shows a broad maximum of 6.5 ± 0.5 μV/K at 20 K. In the presence of NiO, the maximum shifts sharply to higher temperatures, likely correlated to the increase in decay length. This implies that NiO is most transparent to magnon propagation near the paramagnet-antiferromagnet transition. We do not see the enhancement in spin current driven into Pt reported in other papers when 1-2 nm NiO layers are sandwiched between Pt and YIG.

  14. Theory of the spin Seebeck effect in antiferromagnets

    NASA Astrophysics Data System (ADS)

    Rezende, S. M.; Rodríguez-Suárez, R. L.; Azevedo, A.

    2016-01-01

    The spin Seebeck effect (SSE) consists in the generation of a spin current by a temperature gradient applied in a magnetic film. The SSE is usually detected by an electric voltage generated in a metallic layer in contact with the magnetic film resulting from the conversion of the spin current into charge current by means of the inverse spin Hall effect. The SSE has been widely studied in bilayers made of the insulating ferrimagnet yttrium iron garnet (YIG) and metals with large spin-orbit coupling such as platinum. Recently the SSE has been observed in bilayers made of the antiferromagnet Mn F2 and Pt, revealing dependences of the SSE voltage on temperature and field very different from the ones observed in YIG/Pt. Here we present a theory for the SSE in structures with an antiferromagnetic insulator (AFI) in contact with a normal metal (NM) that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the AFI/NM bilayer. The theory explains quite well the measured dependences of the SSE voltage on the sample temperature and on the applied magnetic field in Mn F2/Pt .

  15. Evidence for Intertwining of Superconductivity and Antiferromagnetism in a Cuprate

    NASA Astrophysics Data System (ADS)

    Tranquada, John; Xu, Zhijun; Stock, C.; Chi, S. X.; Kolesnikov, A. I.; Xu, G. Y.; Gu, G. D.

    2014-03-01

    We have used inelastic neutron scattering to measure the low-energy, incommensurate antiferromagnetic spin excitations both above and below the superconducting transition temperature (Tc = 32 K) of La1.905Ba0.095CuO4. While the magnetic excitations in optimally-doped cuprates typically show the development of a spin gap and magnetic resonance below Tc, our sample shows no such effect. Instead strong, gapless spin excitations coexist with bulk superconductivity. To understand this, we note that previous transport measurements have shown that the superconducting layers are decoupled by a magnetic field applied along the c-axis, resulting in a state with frustrated interlayer Josephson coupling, similar to LBCO with x = 1 / 8 , where it has been proposed that pair-density-wave superconductivity occurs. This suggests that, in a similar fashion, the spatially modulated antiferromagnetic correlations (which we see directly in the x = 0 . 095 sample) are intertwined with a spatially modulated superconducting pair wave function. Work at BNL supported by Office of Basic Energy Sciences, US DOE, under Contract No. DE-AC02-98CH10886.

  16. Strain fields and electronic structure of antiferromagnetic CrN

    NASA Astrophysics Data System (ADS)

    Rojas, Tomas; Ulloa, Sergio E.

    2017-09-01

    We present a theoretical analysis of the role that strain plays on the electronic structure of chromium nitride (CrN) crystals. We use local spin-density approximation + U calculations to study the elastic constants, deformation potentials, and strain dependence of electron and hole masses near the fundamental gap. We consider the lowest energy antiferromagnetic models believed to describe CrN at low temperatures, and apply strain along different directions. We find relatively large deformation potentials for all models, and find increasing gaps for tensile strain along most directions. Most interestingly, we find that compressive strains should be able to close the relatively small indirect gap (≃100 meV) at moderate amplitudes ≃1.3 % . We also find large and anisotropic changes in the effective masses with strain, with principal axes closely related to the magnetic ordering of neighboring layers in the antiferromagnet. It would be interesting to consider the role that these effects may have on typical film growth on different substrates, and the possibility of monitoring optical and transport properties of thin films as strain is applied.

  17. Dynamical current-induced ferromagnetic and antiferromagnetic resonances

    NASA Astrophysics Data System (ADS)

    Guimarães, F. S. M.; Lounis, S.; Costa, A. T.; Muniz, R. B.

    2015-12-01

    We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain.

  18. Highly tunable perpendicularly magnetized synthetic antiferromagnets for biotechnology applications

    NASA Astrophysics Data System (ADS)

    Vemulkar, T.; Mansell, R.; Petit, D. C. M. C.; Cowburn, R. P.; Lesniak, M. S.

    2015-07-01

    Magnetic micro and nanoparticles are increasingly used in biotechnological applications due to the ability to control their behavior through an externally applied field. We demonstrate the fabrication of particles made from ultrathin perpendicularly magnetized CoFeB/Pt layers with antiferromagnetic interlayer coupling. The particles are characterized by zero moment at remanence, low susceptibility at low fields, and a large saturated moment created by the stacking of the basic coupled bilayer motif. We demonstrate the transfer of magnetic properties from thin films to lithographically defined 2 μm particles which have been lifted off into solution. We simulate the minimum energy state of a synthetic antiferromagnetic bilayer system that is free to rotate in an applied field and show that the low field susceptibility of the system is equal to the magnetic hard axis followed by a sharp switch to full magnetization as the field is increased. This agrees with the experimental results and explains the behaviour of the particles in solution.

  19. Striped spin liquid crystal ground state instability of kagome antiferromagnets.

    PubMed

    Clark, Bryan K; Kinder, Jesse M; Neuscamman, Eric; Chan, Garnet Kin-Lic; Lawler, Michael J

    2013-11-01

    The Dirac spin liquid ground state of the spin 1/2 Heisenberg kagome antiferromagnet has potential instabilities. This has been suggested as the reason why it does not emerge as the ground state in large-scale numerical calculations. However, previous attempts to observe these instabilities have failed. We report on the discovery of a projected BCS state with lower energy than the projected Dirac spin liquid state which provides new insight into the stability of the ground state of the kagome antiferromagnet. The new state has three remarkable features. First, it breaks spatial symmetry in an unusual way that may leave spinons deconfined along one direction. Second, it breaks the U(1) gauge symmetry down to Z(2). Third, it has the spatial symmetry of a previously proposed "monopole" suggesting that it is an instability of the Dirac spin liquid. The state described herein also shares a remarkable similarity to the distortion of the kagome lattice observed at low Zn concentrations in Zn-paratacamite and in recently grown single crystals of volborthite suggesting it may already be realized in these materials.

  20. Highly tunable perpendicularly magnetized synthetic antiferromagnets for biotechnology applications.

    PubMed

    Vemulkar, T; Mansell, R; Petit, D C M C; Cowburn, R P; Lesniak, M S

    2015-07-06

    Magnetic micro and nanoparticles are increasingly used in biotechnological applications due to the ability to control their behavior through an externally applied field. We demonstrate the fabrication of particles made from ultrathin perpendicularly magnetized CoFeB/Pt layers with antiferromagnetic interlayer coupling. The particles are characterized by zero moment at remanence, low susceptibility at low fields, and a large saturated moment created by the stacking of the basic coupled bilayer motif. We demonstrate the transfer of magnetic properties from thin films to lithographically defined 2 μm particles which have been lifted off into solution. We simulate the minimum energy state of a synthetic antiferromagnetic bilayer system that is free to rotate in an applied field and show that the low field susceptibility of the system is equal to the magnetic hard axis followed by a sharp switch to full magnetization as the field is increased. This agrees with the experimental results and explains the behaviour of the particles in solution.

  1. Fractional excitations in the square-lattice quantum antiferromagnet

    DOE PAGES

    Dalla Piazza, Bastien; Mourigal, M.; Christensen, N. B.; ...

    2014-12-15

    Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). Here, we use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experimentsmore » reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Lastly, our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.« less

  2. Fractional excitations in the square lattice quantum antiferromagnet.

    PubMed

    Piazza, B Dalla; Mourigal, M; Christensen, N B; Nilsen, G J; Tregenna-Piggott, P; Perring, T G; Enderle, M; McMorrow, D F; Ivanov, D A; Rønnow, H M

    2015-01-01

    Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.

  3. Antiferromagnetic exchange bias of a ferromagnetic semiconductor by a ferromagnetic metal

    SciTech Connect

    Olejnik, K.; Wadley, P.; Haigh, J.; Edmonds, K. W.; Campion, R. P.; Rushforth, A. W.; Gallagher, B. L.; Foxon, C. T.; Jungwirth, T.; Wunderlich, J.; Dhesi, S. S.; Cavill, S.; van der Laan, G.; Arenholz, E.

    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.

  4. Combined effects of transition metal (Ni and Rh) substitution and annealing/quenching on the physical properties of CaFe2As2

    SciTech Connect

    Ran, S; Bud'ko, S L; Straszheim, W E; Canfield, P C

    2014-08-01

    We performed systematic studies of the combined effects of annealing/quenching temperature (TA/Q) and T=Ni, Rh substitution (x) on the physical properties of Ca(Fe1-xTx)2As2. We constructed two-dimensional, TA/Q-x phase diagrams for the low-temperature states for both substitutions to map out the relations between ground states and compared them with that of Co substitution. Ni substitution, which brings one more extra electron per substituted atom and suppresses the c-lattice parameter at roughly the same rate as Co substitution, leads to a similar parameter range of antiferromagnetic/orthorhombic phase space in the TA/Q-x space as that found for Co substitution, but the parameter range for superconductivity has been shrunk (roughly by a factor of 2). This result is similar to what is found when Co- and Ni-substituted BaFe2As2 are compared. On the other hand, Rh substitution, which brings the same amount of extra electrons as does Co substitution, but suppresses the c-lattice parameter more rapidly, has a different phase diagram. The collapsed tetragonal phase exists much more pervasively, to the exclusion of the normal, paramagnetic, tetragonal phase. The range of antiferromagnetic/orthorhombic phase space is noticeably reduced, and the superconducting region is substantially suppressed, essentially truncated by the collapsed tetragonal phase. In addition, we found that whereas for Co substitution there was no difference between phase diagrams for samples annealed for 1 or 7 days, for Ni and Rh substitutions a second, reversible effect of annealing was revealed by 7-day anneals.

  5. Pressure-induced switching of magnetic anisotropy in the antiferromagnetic ordered phase in CeRu2Al10

    NASA Astrophysics Data System (ADS)

    Tanida, H.; Tanaka, D.; Nonaka, Y.; Kobayashi, S.; Sera, M.; Nishioka, T.; Matsumura, M.

    2013-07-01

    We have examined the La substitution and its pressure effect on the Kondo semiconductor CeRu2Al10, which shows an unusual antiferromagnetic (AFM) order at T0=27 K with the AFM ordered moment (mAF) parallel to the orthorhombic c axis, not expected from the large anisotropy of the magnetic susceptibility (χ) of χa≫χc≫χb in the paramagnetic state. mAF could not be aligned along the a axis, but could only be in the bc plane. By only 10% La substitution corresponding to a negative chemical pressure effect, the direction of mAF in the AFM ordered phase is changed from the c to the b axis. However, by applying a small pressure of P˜0.3 GPa, this mAF∥b is easily changed to mAF∥c. Thus, the magnetic anisotropy could be easily controlled by tuning the pressure slightly. This indicates that the c-f hybridization dominates the magnetic anisotropy in the AFM ordered state and plays an essential role in the unusual AFM order in CeT2Al10 (T=Ru,Os).

  6. Chiral Magnetic Effect and Anomalous Hall Effect in Antiferromagnetic Insulators with Spin-Orbit Coupling.

    PubMed

    Sekine, Akihiko; Nomura, Kentaro

    2016-03-04

    We search for dynamical magnetoelectric phenomena in three-dimensional correlated systems with spin-orbit coupling. We focus on the antiferromagnetic insulator phases where the dynamical axion field is realized by the fluctuation of the antiferromagnetic order parameter. It is shown that the dynamical chiral magnetic effect, an alternating current generation by magnetic fields, emerges due to such time dependences of the order parameter as antiferromagnetic resonance. It is also shown that the anomalous Hall effect arises due to such spatial variations of the order parameter as antiferromagnetic domain walls. Our study indicates that spin excitations in antiferromagnetic insulators with spin-orbit coupling can result in nontrivial charge responses. Moreover, observing the chiral magnetic effect and anomalous Hall effect in our system is equivalent to detecting the dynamical axion field in condensed matter.

  7. Combined effect of magnetic field and charge current on antiferromagnetic domain-wall dynamics

    NASA Astrophysics Data System (ADS)

    Yamane, Yuta; Gomonay, Olena; Velkov, Hristo; Sinova, Jairo

    2017-08-01

    We theoretically examine a cross effect of magnetic field and charge current on antiferromagnetic domain wall dynamics. Since antiferromagnetic materials are largely insensitive to external magnetic fields in general, charge current has been shown recently as an alternative and efficient way to manipulate antiferromagnets. We find a new role of the magnetic field in the antiferromagnetic dynamics that appears when it is combined with charge current, demonstrating a domain wall motion in the presence of both field and current. We show that a spatially varying magnetic field can shift the current-driven domain-wall velocity, depending on the domain-wall structure and the direction of the field gradient. Our result suggests a novel concept of field control of current-driven antiferromagnetic dynamics.

  8. Antiferromagnetic CuMnAs multi-level memory cell with microelectronic compatibility.

    PubMed

    Olejník, K; Schuler, V; Marti, X; Novák, V; Kašpar, Z; Wadley, P; Campion, R P; Edmonds, K W; Gallagher, B L; Garces, J; Baumgartner, M; Gambardella, P; Jungwirth, T

    2017-05-19

    Antiferromagnets offer a unique combination of properties including the radiation and magnetic field hardness, the absence of stray magnetic fields, and the spin-dynamics frequency scale in terahertz. Recent experiments have demonstrated that relativistic spin-orbit torques can provide the means for an efficient electric control of antiferromagnetic moments. Here we show that elementary-shape memory cells fabricated from a single-layer antiferromagnet CuMnAs deposited on a III-V or Si substrate have deterministic multi-level switching characteristics. They allow for counting and recording thousands of input pulses and responding to pulses of lengths downscaled to hundreds of picoseconds. To demonstrate the compatibility with common microelectronic circuitry, we implemented the antiferromagnetic bit cell in a standard printed circuit board managed and powered at ambient conditions by a computer via a USB interface. Our results open a path towards specialized embedded memory-logic applications and ultra-fast components based on antiferromagnets.

  9. Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging.

    PubMed

    Chauleau, J-Y; Haltz, E; Carrétéro, C; Fusil, S; Viret, M

    2017-08-01

    Among the variety of magnetic textures available in nature, antiferromagnetism is one of the most 'discrete' because of the exact cancellation of its staggered internal magnetization. It is therefore very challenging to probe. However, its insensitivity to external magnetic perturbations, together with the intrinsic sub-picosecond dynamics, make it very appealing for tomorrow's information technologies. Thus, it is essential to understand the microscopic mechanisms governing antiferromagnetic domains to achieve accurate manipulation and control. Using optical second-harmonic generation, a unique and laboratory-available tool, we succeeded in imaging with sub-micrometre resolution both electric and antiferromagnetic orders in the model multiferroic BiFeO3. We show here that antiferromagnetic domains can be manipulated with low power consumption, using sub-coercive electric fields and sub-picosecond light pulses. Interestingly, we also show that antiferromagnetic and ferroelectric domains can behave independently, thus revealing that magneto-electric coupling can lead to various arrangements of the two orders.

  10. Barkhausen-like antiferromagnetic to ferromagnetic phase transition driven by spin polarized current

    NASA Astrophysics Data System (ADS)

    Suzuki, Ippei; Naito, Tomoyuki; Itoh, Mitsuru; Taniyama, Tomoyasu

    2015-08-01

    We provide clear evidence for the effect of a spin polarized current on the antiferromagnetic to ferromagnetic phase transition of an FeRh wire at Co/FeRh wire junctions, where the antiferromagnetic ground state of FeRh is suppressed by injecting a spin polarized current. We find a discrete change in the current-voltage characteristics with increasing current density, which we attribute to the Barkhausen-like motion of antiferromagnetic/ferromagnetic interfaces within the FeRh wire. The effect can be understood via spin transfer, which exerts a torque to the antiferromagnetic moments of FeRh, together with non-equilibrium magnetic effective field at the interface. The conclusion is reinforced by the fact that spin unpolarized current injection from a nonmagnetic Cu electrode has no effects on the antiferromagnetic state of FeRh.

  11. Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging

    NASA Astrophysics Data System (ADS)

    Chauleau, J.-Y.; Haltz, E.; Carrétéro, C.; Fusil, S.; Viret, M.

    2017-08-01

    Among the variety of magnetic textures available in nature, antiferromagnetism is one of the most `discrete' because of the exact cancellation of its staggered internal magnetization. It is therefore very challenging to probe. However, its insensitivity to external magnetic perturbations, together with the intrinsic sub-picosecond dynamics, make it very appealing for tomorrow's information technologies. Thus, it is essential to understand the microscopic mechanisms governing antiferromagnetic domains to achieve accurate manipulation and control. Using optical second-harmonic generation, a unique and laboratory-available tool, we succeeded in imaging with sub-micrometre resolution both electric and antiferromagnetic orders in the model multiferroic BiFeO3. We show here that antiferromagnetic domains can be manipulated with low power consumption, using sub-coercive electric fields and sub-picosecond light pulses. Interestingly, we also show that antiferromagnetic and ferroelectric domains can behave independently, thus revealing that magneto-electric coupling can lead to various arrangements of the two orders.

  12. Random Ising antiferromagnet on Bethe-like lattices with triangular loops

    NASA Astrophysics Data System (ADS)

    Yokota, Terufumi

    2016-04-01

    Phase diagrams for a random Ising antiferromagnet on Bethe-like lattices with triangular loops are obtained. Triangular loops cause strong geometrical frustration for the Ising antiferromagnet. Spin glass states appear by introducing randomness in the interaction between Ising spins. The random Ising antiferromagnet is studied by the replica method using global order parameter. The phase diagrams are compared with those for the corresponding random Ising ferromagnet to see the effects of the geometrical frustration. Antiferromagnetic phase does not appear for M ≤ 4 where M is the number of the corner sharing triangles on the Bethe-like lattices. In these cases, spin glass phase appears with a reentrant behavior. Spin glass phase in the random antiferromagnet appears for much weaker randomness than that in the corresponding random ferromagnet.

  13. Antiferromagnetic CuMnAs multi-level memory cell with microelectronic compatibility

    NASA Astrophysics Data System (ADS)

    Olejník, K.; Schuler, V.; Marti, X.; Novák, V.; Kašpar, Z.; Wadley, P.; Campion, R. P.; Edmonds, K. W.; Gallagher, B. L.; Garces, J.; Baumgartner, M.; Gambardella, P.; Jungwirth, T.

    2017-05-01

    Antiferromagnets offer a unique combination of properties including the radiation and magnetic field hardness, the absence of stray magnetic fields, and the spin-dynamics frequency scale in terahertz. Recent experiments have demonstrated that relativistic spin-orbit torques can provide the means for an efficient electric control of antiferromagnetic moments. Here we show that elementary-shape memory cells fabricated from a single-layer antiferromagnet CuMnAs deposited on a III-V or Si substrate have deterministic multi-level switching characteristics. They allow for counting and recording thousands of input pulses and responding to pulses of lengths downscaled to hundreds of picoseconds. To demonstrate the compatibility with common microelectronic circuitry, we implemented the antiferromagnetic bit cell in a standard printed circuit board managed and powered at ambient conditions by a computer via a USB interface. Our results open a path towards specialized embedded memory-logic applications and ultra-fast components based on antiferromagnets.

  14. Antiferromagnetic CuMnAs multi-level memory cell with microelectronic compatibility

    PubMed Central

    Olejník, K.; Schuler, V.; Marti, X.; Novák, V.; Kašpar, Z.; Wadley, P.; Campion, R. P.; Edmonds, K. W.; Gallagher, B. L.; Garces, J.; Baumgartner, M.; Gambardella, P.; Jungwirth, T.

    2017-01-01

    Antiferromagnets offer a unique combination of properties including the radiation and magnetic field hardness, the absence of stray magnetic fields, and the spin-dynamics frequency scale in terahertz. Recent experiments have demonstrated that relativistic spin-orbit torques can provide the means for an efficient electric control of antiferromagnetic moments. Here we show that elementary-shape memory cells fabricated from a single-layer antiferromagnet CuMnAs deposited on a III–V or Si substrate have deterministic multi-level switching characteristics. They allow for counting and recording thousands of input pulses and responding to pulses of lengths downscaled to hundreds of picoseconds. To demonstrate the compatibility with common microelectronic circuitry, we implemented the antiferromagnetic bit cell in a standard printed circuit board managed and powered at ambient conditions by a computer via a USB interface. Our results open a path towards specialized embedded memory-logic applications and ultra-fast components based on antiferromagnets. PMID:28524862

  15. Impact of antiferromagnetism on the optical properties of rare-earth nickelates

    NASA Astrophysics Data System (ADS)

    Ruppen, J.; Teyssier, J.; Ardizzone, I.; Peil, O. E.; Catalano, S.; Gibert, M.; Triscone, J.-M.; Georges, A.; van der Marel, D.

    2017-07-01

    We study the temperature dependence of the optical conductivity of rare-earth nickelate films of varying composition and strain close to the antiferromagnetic ordering temperature, TN. Two prominent peaks at 0.6 and 1.3 eV, which are characteristic of the insulating phase, display a small but significant increase in intensity when the material passes from para- to antiferromagnetic. This observation indicates the presence of a positive feedback between antiferromagnetic (AF) and bond disproportionation (BD) order. By analyzing the temperature dependence near TN, and using a Landau-type free-energy expression for BD and AF order, we infer that BD order is a necessary condition for the AF phase to appear, and that the antiferromagnetism contributes to stabilization of the bond disproportionation. This model also explains why hysteresis is particularly strong when the transition into the insulating state occurs simultaneously with antiferromagnetic order.

  16. Barkhausen-like antiferromagnetic to ferromagnetic phase transition driven by spin polarized current

    SciTech Connect

    Suzuki, Ippei; Naito, Tomoyuki; Itoh, Mitsuru; Taniyama, Tomoyasu

    2015-08-24

    We provide clear evidence for the effect of a spin polarized current on the antiferromagnetic to ferromagnetic phase transition of an FeRh wire at Co/FeRh wire junctions, where the antiferromagnetic ground state of FeRh is suppressed by injecting a spin polarized current. We find a discrete change in the current-voltage characteristics with increasing current density, which we attribute to the Barkhausen-like motion of antiferromagnetic/ferromagnetic interfaces within the FeRh wire. The effect can be understood via spin transfer, which exerts a torque to the antiferromagnetic moments of FeRh, together with non-equilibrium magnetic effective field at the interface. The conclusion is reinforced by the fact that spin unpolarized current injection from a nonmagnetic Cu electrode has no effects on the antiferromagnetic state of FeRh.

  17. Chiral Magnetic Effect and Anomalous Hall Effect in Antiferromagnetic Insulators with Spin-Orbit Coupling

    NASA Astrophysics Data System (ADS)

    Sekine, Akihiko; Nomura, Kentaro

    2016-03-01

    We search for dynamical magnetoelectric phenomena in three-dimensional correlated systems with spin-orbit coupling. We focus on the antiferromagnetic insulator phases where the dynamical axion field is realized by the fluctuation of the antiferromagnetic order parameter. It is shown that the dynamical chiral magnetic effect, an alternating current generation by magnetic fields, emerges due to such time dependences of the order parameter as antiferromagnetic resonance. It is also shown that the anomalous Hall effect arises due to such spatial variations of the order parameter as antiferromagnetic domain walls. Our study indicates that spin excitations in antiferromagnetic insulators with spin-orbit coupling can result in nontrivial charge responses. Moreover, observing the chiral magnetic effect and anomalous Hall effect in our system is equivalent to detecting the dynamical axion field in condensed matter.

  18. Performing Substitute Teaching

    ERIC Educational Resources Information Center

    Bletzer, Keith V.

    2010-01-01

    Formal education is both a right and an obligation bestowed on young people in most all nations of the world. Teachers (adults) and students (youth) form a co-present dyadic contract that must be maintained within the classroom. Substitute teachers fill a role in sustaining the integrity of this teacher-student link, whenever teachers are absent.…

  19. The Age of Substitutability

    ERIC Educational Resources Information Center

    Goeller, H. E.; Weinberg, Alvin M.

    1976-01-01

    Dwindling mineral resources might cause a shift from nonrenewable resources to renewable resources and inexhaustible elements such as iron and aluminum. Alternative energy sources such as breeder, fusion, solar, and geothermal power must be developed for production and recycling of materials. Substitution and, hence, living standards ultimately…

  20. No cheap substitutes.

    PubMed

    Griffiths, Peter

    2016-06-15

    The Nuffield Trust report on reshaping the healthcare workforce was published last month. Its conclusions were widely reported as a recommendation to 'train up' nurses as a solution to junior doctor shortages, with support workers, in turn, substituting for registered nurses.

  1. The Age of Substitutability

    ERIC Educational Resources Information Center

    Goeller, H. E.; Weinberg, Alvin M.

    1976-01-01

    Dwindling mineral resources might cause a shift from nonrenewable resources to renewable resources and inexhaustible elements such as iron and aluminum. Alternative energy sources such as breeder, fusion, solar, and geothermal power must be developed for production and recycling of materials. Substitution and, hence, living standards ultimately…

  2. Performing Substitute Teaching

    ERIC Educational Resources Information Center

    Bletzer, Keith V.

    2010-01-01

    Formal education is both a right and an obligation bestowed on young people in most all nations of the world. Teachers (adults) and students (youth) form a co-present dyadic contract that must be maintained within the classroom. Substitute teachers fill a role in sustaining the integrity of this teacher-student link, whenever teachers are absent.…

  3. Screening Substitute Teachers.

    ERIC Educational Resources Information Center

    Kakkuri, Mark

    2000-01-01

    The screening process a school district uses in hiring substitute teachers is critical to striking a balance between required qualifications and immediate need. Typically, screening involves at least one of the following: pre-screening, paper and pencil screening, interviews, and background checks, each of which is used to different degrees…

  4. Organizing Substitute Teachers.

    ERIC Educational Resources Information Center

    Vail, Kathleen

    2000-01-01

    Many substitute teachers are underpaid, undertrained, and "invisible" employees lacking health benefits and respect from regular colleagues. Grass-roots organizing efforts have improved working conditions for subs in two Florida and California districts. Districts' improvement initiatives should be guided by concerns for student…

  5. Bismuth cuprate high-Tc superconductors using cationic substitution

    NASA Astrophysics Data System (ADS)

    Tarascon, J.-M.; Barboux, P.; Hull, G. W.; Ramesh, R.; Greene, L. H.; Giroud, M.; Hegde, M. S.; McKinnon, W. R.

    1989-03-01

    The Bi4Sr4Ca2-xRxCu4BOy materials (R is a rare-earth element) were studied to determine their structural and physical properties. For most of the rare-earth elements, a complete solid solution exists up to x=2. Below x=0.5,Tc is not affected and for each added rare-earth element we find that about 0.5 oxygen atom is added to the structure. However, the structural modulation observed along the b axis for the undoped material persists and remains of the same amplitude for the rare-earth-doped samples. When more than one R(x>=1) is substituted, Tc is depressed and the compound becomes semiconducting beyond x=1.5. The depression in the Tc from 85 K (x=0) to less than 4.2 K (x=1.5) correlates to a decrease in the formal valence of copper and is independent whether the rare-earth element is magnetic or nonmagnetic. No evidence for magnetic ordering over the range of temperature 1.7-400 K has been observed in all the substituted compounds. The substitution for Cu by 3d metals or for Sr by rare-earth elements fails for the 85-K Bi phase but succeeds for the 10-K Bi phase. Consequently, the following series Bi2Sr2Cu1-xMxOy (M=Fe, Co) and Bi2RCaCuOy (R=La, Pr, Nd, Sm) were made for study. These substitutions result in an uptake of oxygen (0.5 for each substituted element). But the materials become semiconducting even though the formal valence of Cu remains greater than 2. An antiferromagnetic transition at 140 K has been found for the Co sample for which Co is found to be in the +3 state.

  6. NMR studies of the incommensurate helical antiferromagnet EuCo2P2 : Determination of antiferromagnetic propagation vector

    NASA Astrophysics Data System (ADS)

    Higa, Nonoka; Ding, Qing-Ping; Yogi, Mamoru; Sangeetha, N. S.; Hedo, Masato; Nakama, Takao; Ōnuki, Yoshichika; Johnston, D. C.; Furukawa, Yuji

    2017-07-01

    Recently, Q.-P. Ding et al. [Phys. Rev. B 95, 184404 (2017), 10.1103/PhysRevB.95.184404] reported that their nuclear magnetic resonance (NMR) study on EuCo2As2 successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out 153Eu, 31P, and 59Co NMR measurements on the helical antiferromagnet EuCo2P2 with an AFM ordering temperature TN=66.5 K. An incommensurate helical AFM structure was clearly confirmed by 153Eu and 31P NMR spectra on single-crystalline EuCo2P2 in zero magnetic field at 1.6 K and its external magnetic field dependence. Furthermore, based on 59Co NMR data in both the paramagnetic and incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector k =(0 ,0 ,0.73 ±0.09 )2 π /c , where c is the c -axis lattice parameter. The temperature dependence of k is also discussed.

  7. NMR studies of the incommensurate helical antiferromagnet EuCo2P2: Determination of antiferromagnetic propagation vector

    DOE PAGES

    Higa, Nonoka; Ding, Qing -Ping; Yogi, Mamoru; ...

    2017-07-06

    Recently, Q.-P. Ding et al. reported that their nuclear magnetic resonance (NMR) study on EuCo2As2 successfully characterized the antiferromagnetic (AFM) propagation vector of the incommensurate helix AFM state, showing that NMR is a unique tool for determination of the spin structures in incommensurate helical AFMs. Motivated by this work, we have carried out 153Eu, 31P, and 59Co NMR measurements on the helical antiferromagnet EuCo2P2 with an AFM ordering temperature TN = 66.5 K. An incommensurate helical AFM structure was clearly confirmed by 153Eu and 31P NMR spectra on single-crystalline EuCo2P2 in zero magnetic field at 1.6 K and its externalmore » magnetic field dependence. Furthermore, based on 59Co NMR data in both the paramagnetic and incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector k = (0,0,0.73±0.09)2π/c, where c is the c-axis lattice parameter. As a result, the temperature dependence of k is also discussed.« less

  8. Temperature-induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic Bi0.9Sm0.1FeO3 driven by f -d induced magnetic anisotropy

    NASA Astrophysics Data System (ADS)

    Johnson, R. D.; McClarty, P. A.; Khalyavin, D. D.; Manuel, P.; Svedlindh, P.; Knee, C. S.

    2017-02-01

    In multiferroic BiFeO3 a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that substituting samarium for 10% of the bismuth ions increases the periodicity of the room-temperature cycloid, and upon cooling to below ˜15 K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from f -d exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order—a result of general significance regarding the stability of noncollinear magnetic structures in the presence of multiple magnetic sublattices.

  9. Charge dynamics of the antiferromagnetically ordered Mott insulator

    NASA Astrophysics Data System (ADS)

    Han, Xing-Jie; Liu, Yu; Liu, Zhi-Yuan; Li, Xin; Chen, Jing; Liao, Hai-Jun; Xie, Zhi-Yuan; Normand, B.; Xiang, Tao

    2016-10-01

    We introduce a slave-fermion formulation in which to study the charge dynamics of the half-filled Hubbard model on the square lattice. In this description, the charge degrees of freedom are represented by fermionic holons and doublons and the Mott-insulating characteristics of the ground state are the consequence of holon-doublon bound-state formation. The bosonic spin degrees of freedom are described by the antiferromagnetic Heisenberg model, yielding long-ranged (Néel) magnetic order at zero temperature. Within this framework and in the self-consistent Born approximation, we perform systematic calculations of the average double occupancy, the electronic density of states, the spectral function and the optical conductivity. Qualitatively, our method reproduces the lower and upper Hubbard bands, the spectral-weight transfer into a coherent quasiparticle band at their lower edges and the renormalisation of the Mott gap, which is associated with holon-doublon binding, due to the interactions of both quasiparticle species with the magnons. The zeros of the Green function at the chemical potential give the Luttinger volume, the poles of the self-energy reflect the underlying quasiparticle dispersion with a spin-renormalised hopping parameter and the optical gap is directly related to the Mott gap. Quantitatively, the square-lattice Hubbard model is one of the best-characterised problems in correlated condensed matter and many numerical calculations, all with different strengths and weaknesses, exist with which to benchmark our approach. From the semi-quantitative accuracy of our results for all but the weakest interaction strengths, we conclude that a self-consistent treatment of the spin-fluctuation effects on the charge degrees of freedom captures all the essential physics of the antiferromagnetic Mott-Hubbard insulator. We remark in addition that an analytical approximation with these properties serves a vital function in developing a full understanding of the

  10. Unusual magnetic excitations in the weakly ordered spin- 12 chain antiferromagnet Sr2CuO3: Possible evidence for Goldstone magnon coupled with the amplitude mode

    DOE PAGES

    Sergeicheva, E. G.; Sosin, S. S.; Prozorova, L. A.; ...

    2017-01-18

    We report on an electron spin resonance (ESR) study of a nearly one-dimensional (1D) spin-1/2 chain antiferromagnet, Sr2CuO3, with extremely weak magnetic ordering. The ESR spectra at T > TN, in the disordered Luttinger-spin-liquid phase, reveal nearly ideal Heisenberg-chain behavior with only a very small, field-independent linewidth, ~1/T. In the ordered state, below TN, we identify field-dependent antiferromagnetic resonance modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet. Additionally, we observe a major resonant mode with unusual and strongly anisotropic properties, which is not anticipated by the conventional theory of Goldstone spin waves.more » Lastly, we propose that this unexpected magnetic excitation can be attributed to a field-independent magnon mode renormalized due to its interaction with the high-energy amplitude (Higgs) mode in the regime of weak spontaneous symmetry breaking.« less

  11. Covalent magnetism, exchange interactions and anisotropy of the high temperature layered antiferromagnet MnB₂.

    PubMed

    Khmelevskyi, S; Mohn, P

    2012-01-11

    The investigation of the electronic structure and magnetism for the compound MnB(2) with crystal structure type AlB(2) has been revisited to resolve contradictions between various experimental and theoretical results present in the literature. We find that MnB(2) exhibits an interesting example of a Kübler's covalent magnetism (Williams et al 1981 J. Appl. Phys. 52 2069). The covalent magnetism also appears to be the source of some disagreement between the calculated values of the magnetic moments and those given by neutron diffraction experiments. We show that this shortcoming is due to the atomic sphere approximation applied in earlier calculations. The application of the disordered local moment approach and the calculation of the inter-atomic exchange interactions within the Liechtenstein formalism reveal strong local moment antiferromagnetism with a high Néel temperature predicted from Monte Carlo simulations. A fully relativistic band structure calculation and then the application of the torque method yields a strong in-plane anisotropy of the Mn magnetic moments. The agreement of these results with neutron diffraction studies rules out any possible weak itinerant electron magnetism scenarios as proposed earlier for MnB(2).

  12. Barlowite: a spin-1/2 antiferromagnet with a geometrically perfect Kagome motif.

    SciTech Connect

    Han, Tian-Heng; Singleton, John; Schlueter, John A.

    2014-11-25

    We present thermodynamic studies of a new spin-1/2 antiferromagnet containing undistorted kagome lattices-barlowite Cu-4(OH)(6)FBr. Magnetic susceptibility gives theta(CW) = -136 K, while long-range order does not happen until T-N = 15 K with a weak ferromagnetic moment mu < 0.1 mu(B)/Cu. A 60 T magnetic field induces a moment less than 0.5 mu(B)/Cu at T = 0.6 K. Specific-heat measurements have observed multiple phase transitions at T << vertical bar theta(CW)vertical bar. The magnetic entropy of these transitions is merely 18% of k(B) ln 2 per Cu spin. These observations suggest that nontrivial spin textures are realized in barlowite with magnetic frustration. Comparing with the leading spin-liquid candidate herbertsmithite, the superior interkagome environment of barlowite sheds light on new spin-liquid compounds with minimum disorder. The robust perfect geometry of the kagome lattice makes charge doping promising.

  13. Barlowite: A Spin-1/2 Antiferromagnet with a Geometrically Perfect Kagome Motif.

    PubMed

    Han, Tian-Heng; Singleton, John; Schlueter, John A

    2014-11-28

    We present thermodynamic studies of a new spin-1/2 antiferromagnet containing undistorted kagome lattices-barlowite Cu_{4}(OH)_{6}FBr. Magnetic susceptibility gives θ_{CW}=-136  K, while long-range order does not happen until T_{N}=15  K with a weak ferromagnetic moment μ<0.1μ_{B}/Cu. A 60 T magnetic field induces a moment less than 0.5μ_{B}/Cu at T=0.6  K. Specific-heat measurements have observed multiple phase transitions at T≪∣θ_{CW}∣. The magnetic entropy of these transitions is merely 18% of k_{B}ln2 per Cu spin. These observations suggest that nontrivial spin textures are realized in barlowite with magnetic frustration. Comparing with the leading spin-liquid candidate herbertsmithite, the superior interkagome environment of barlowite sheds light on new spin-liquid compounds with minimum disorder. The robust perfect geometry of the kagome lattice makes charge doping promising.

  14. Calculation of nonlinear magnetic susceptibility tensors for a uniaxial antiferromagnet

    NASA Astrophysics Data System (ADS)

    Lim, Siew-Choo; Osman, Junaidah; Tilley, D. R.

    2000-11-01

    In this paper, we present a derivation of the nonlinear susceptibility tensors for a two-sublattice uniaxial antiferromagnet up to the third-order effects within the standard definition by which the rf magnetization m is defined as a power series expansion in the rf fields h with the susceptibility tensors χ(q) as the coefficients. The starting point is the standard set of torque equations of motion for this problem. A complete set of tensor elements is derived for the case of a single-frequency input wave. Within a circular polarization frame (pnz) expressions are given for the first-order susceptibility, second-harmonic generation, optical rectification, third-harmonic generation and intensity-dependent susceptibility. Some of the coefficients with representative resonance features in the far infrared are illustrated graphically and we conclude with a brief discussion of the implications of the resonance features arising from the calculations and their potential applications.

  15. Ferrimagnetism in delta chain with anisotropic ferromagnetic and antiferromagnetic interactions

    NASA Astrophysics Data System (ADS)

    Dmitriev, D. V.; Krivnov, V. Ya

    2016-12-01

    We consider analytically and numerically an anisotropic spin-\\frac{1}{2} delta-chain (sawtooth chain) in which exchange interactions between apical and basal spins are ferromagnetic and those between basal spins are antiferromagnetic. In the limit of strong anisotropy of exchange interactions this model can be considered as the Ising delta chain with macroscopic degenerate ground state perturbed by transverse quantum fluctuations. These perturbations lift the ground state degeneracy and the model reduces to the basal XXZ spin chain in the magnetic field induced by static apical spins. We show that the ground state of such a model is ferrimagnetic. The excitations of the model are formed by ferrimagnetic domains separated by domain walls with a finite energy. At low temperatures the system is effectively divided into two independent subsystems, the apical subsystem described by the Ising spin-\\frac{1}{2} chain and the basal subsystem described by the XXZ chain with infinite zz interactions.

  16. Ab initio dynamical exchange interactions in frustrated antiferromagnets

    NASA Astrophysics Data System (ADS)

    Simoni, Jacopo; Stamenova, Maria; Sanvito, Stefano

    2017-08-01

    The ultrafast response to an optical pulse excitation of the spin-spin exchange interaction in transition metal antiferromagnets is studied within the framework of the time-dependent spin-density functional theory. We propose a formulation for the full dynamical exchange interaction, which is nonlocal in space, and it is derived starting from ab initio arguments. Then, we investigate the effect of the laser pulse on the onset of the dynamical process. It is found that we can distinguish two types of excitations, both activated immediately after the action of the laser pulse. While the first one can be associated to a Stoner-like excitation and involves the transfer of spin from one site to another, the second one is related to the ultrafast modification of a Heisenberg-like exchange interaction and can trigger the formation of spin waves in the first few hundred femtoseconds of the time evolution.

  17. Low-temperature broken-symmetry phases of spiral antiferromagnets.

    PubMed

    Capriotti, Luca; Sachdev, Subir

    2004-12-17

    We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T

  18. Antiferromagnetic Critical Fluctuations in BaFe$_2$As$_2$

    SciTech Connect

    Wilson, Stephen D; Yamani, Z.; Rotundu, C. R.; Freelon, B.; Valdivia, P. N.; Bourret-Courchesne, E. D.; Lynn, J W; Chi, Songxue; Hong, Tao; Birgeneau, R. J.

    2010-01-01

    Magnetic correlations near the magnetostructural phase transition in the bilayer iron-pnictide parent compound, BaFe{sub 2}As{sub 2}, are measured. In close proximity to the antiferromagnetic phase transition in BaFe{sub 2}As{sub 2}, a crossover to three-dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T{sub N}. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe{sub 2}As{sub 2} and their evolution near the anticipated crossover to three-dimensional critical behavior and long-range order are discussed.

  19. Two Dimensional Antiferromagnetic Chern Insulator: NiRuCl6.

    PubMed

    Zhou, P; Sun, C Q; Sun, L Z

    2016-10-12

    Density functional theory (DFT) and Berry curvature calculations show that quantum anomalous Hall effect (QAHE) can be realized in two-dimensional(2D) antiferromagnetic (AFM) NiRuCl6. The results indicate that NiRuCl6 behaves as an AFM Chern insulator and its spin-polarized electronic structure and strong spin-orbit coupling (SOC) are responsible for the QAHE. By tuning SOC, we found that the topological property of NiRuCl6 arises from its energy band inversion. Considering the compatibility between the AFM and insulators, AFM Chern insulator provides a new way to archive high temperature QAHE in experiments due to its different magnetic coupling mechanism from that of ferromagnetic (FM) Chern insulator.

  20. Kagome-like chains with anisotropic ferromagnetic and antiferromagnetic interactions.

    PubMed

    Dmitriev, D V; Krivnov, V Ya

    2017-06-01

    We consider a spin-[Formula: see text] kagome-like chain with competing ferro- and antiferromagnetic anisotropic exchange interactions. The ground state phase diagram of this model consists of the ferromagnetic and ferrimagnetic phases. We study the ground state and the low-temperature properties on the phase boundary between these phases. The ground state on this phase boundary is macroscopically degenerate and consists of localized magnon states. We calculate the ground state degeneracy and corresponding residual entropy. The spontaneous magnetization has a jump on the phase boundary confirming the first-order type of the phase transition. In the limit of a strong anisotropy, the spectrum of the low-energy excitations has multi-scale structure governing the peculiar features of the specific heat behavior.