Primordial magnetic seed field amplification by gravitational waves
NASA Astrophysics Data System (ADS)
Betschart, Gerold; Zunckel, Caroline; Dunsby, Peter K. S.; Marklund, Mattias
2005-12-01
Using second-order gauge-invariant perturbation theory, a self-consistent framework describing the nonlinear coupling between gravitational waves and a large-scale homogeneous magnetic field is presented. It is shown how this coupling may be used to amplify seed magnetic fields to strengths needed to support the galactic dynamo. In situations where the gravitational wave background is described by an “almost“ Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology we find that the magnitude of the original magnetic field is amplified by an amount proportional to the magnitude of the gravitational wave induced shear anisotropy and the square of the field’s initial comoving scale. We apply this mechanism to the case where the seed field and gravitational wave background are produced during inflation and find that the magnitude of the gravitational boost depends significantly on the manner in which the estimate of the shear anisotropy at the end of inflation is calculated. Assuming a seed field of 10-34G spanning a comoving scale of about 10 kpc today, the shear anisotropy at the end of inflation must be at least as large as 10-40 in order to obtain a generated magnetic field of the same order of magnitude as the original seed. Moreover, contrasting the weak-field approximation to our gauge-invariant approach, we find that while both methods agree in the limit of high conductivity, their corresponding solutions are otherwise only compatible in the limit of infinitely long-wavelength gravitational waves.
Interplanetary magnetic holes: Theory
NASA Technical Reports Server (NTRS)
Burlaga, L. F.; Lemaire, J. F.
1978-01-01
Magnetic holes in the interplanetary medium are explained as stationary, non-propagating, equilibrium structures in which there are field-aligned enhancements of the plasma density and/or temperature. Magnetic anti-holes are considered to be associated with depressions in the plasma pressure. In this model, the observed changes in the magnetic field intensity and direction are due to diamagnetic currents that are carried by ions which drift in a sheath as the result of gradients in the magnetic field and in the plasma pressure within the sheath. The thickness of the sheaths considered is approximately a few ion Larmor radii. An electric field is normal to the magnetic field in the sheath. Solutions of Vlasov's equation and Maxwell's equations are presented which account for several types of magnetic holes, including null-sheets, that were observed.
Theory of nuclear magnetic relaxation
NASA Technical Reports Server (NTRS)
Mcconnell, J.
1983-01-01
A theory of nuclear magnetic interaction is based on the study of the stochastic rotation operator. The theory is applied explicitly to relaxation by anisotropic chemical shift and to spin-rotational interactions. It is applicable also to dipole-dipole and quadrupole interactions.
Planetary magnetism. [emphasizing dynamo theories
NASA Technical Reports Server (NTRS)
Stevenson, D.
1974-01-01
The origin and maintenance of planetary magnetic fields are discussed. The discussion is not limited to dynamo theories, although these are almost universally favored. Thermoelectric currents are found to be a possible alternative for Jupiter. Two energy sources for dynamos are considered: convection and precessionally induced fluid flow. The earth is the most favorable planet for precessionally driven dynamo, although Neptune is a possibility. Jupiter is likely to have a convectionally driven dynamo, as may Saturn, but the relevant properties of Saturn are not yet well known. Conclusions for each planet are given.
Theories for the origin of lunar magnetism
NASA Technical Reports Server (NTRS)
Daily, W. D.; Dyal, P.
1979-01-01
This paper reviews the major theories which have been proposed to explain the remanent magnetism found in the lunar crust. A total of nine different mechanisms for lunar magnetism are discussed and evaluated in light of the theoretical and experimental constraints pertinent to lunar magnetism. It is concluded that none of these theories in their present state of development satisfy all the known constraints. However, the theories which agree best with the present understanding of the moon are meteorite impact magnetization, thermoelectric dynamo field generation, and an early solar wind field.
Predictability of magnetic hysteresis and thermoremanent magnetization using Preisach theory
NASA Astrophysics Data System (ADS)
Newell, A. J.; Niemerg, M.; Bates, D.
2014-12-01
Preisach theory is a phenomenological model of hysteresis that is the basis for FORC analysis in rock magnetism. In FORC analysis, a system is characterized using first-order reversal curves (FORCs), each of which is a magnetization curve after a reversal in the direction of change of the magnetic field. Preisach theory uses the same curves to predict the magnetic response to changes in the magnetic field. In rock magnetism, the Preisach model has been adapted to predict general properties of thermoremanent magnetization (TRM), and even to inferpaleointensity from room-temperature FORCs. Preisach theory represents hysteresis by a collection of hysteresis units called hysterons; the distribution of hysterons is inferred from FORC measurements. Each hysteron represents a two-state system. This is similar to a single-domain (SD) magnet, but the first-order theory cannot represent the magnetism of a simple system of randomly oriented SD magnets. Such a system can be represented by a second-order Preisach theory, which requires the measurement of magnetization curves after two reversals of the direction of change. One can generalize this process to higher order reversal curves, although each increase in the number of reversals greatly increases the number of measurements that are needed. The magnetic hysteresis of systems of interacting SD magnets is calculated using numerical homotopy, a method that can find all the solutions of the equilibrium equations for such a system. The hysteresis frequently has features that cannot be represented by any order of Preisach theory. Furthermore, there are stable magnetic states that are not reachable during isothermal hysteresis unless thermal fluctuations are large enough. Such states would not be visible at room temperature but would contribute to TRM.
Theory of antiskyrmions in magnets
NASA Astrophysics Data System (ADS)
Koshibae, Wataru; Nagaosa, Naoto
2016-01-01
Skyrmions and antiskyrmions are swirling topological magnetic textures realized as emergent particles in magnets. A skyrmion is stabilized by the Dzyaloshinskii-Moriya interaction in chiral magnets and/or a dipolar interaction in thin film magnets, which prefer the twist of the magnetic moments. Here we show by a numerical simulation of the Landau-Lifshitz-Gilbert equation that pairs of skyrmions and antiskyrmions are created from the helix state as the magnetic field is increased. Antiskyrmions are unstable and disappear immediately in chiral magnets, whereas they are metastable and survive in dipolar magnets. The collision between a skyrmion and an antiskyrmion in a dipolar magnet is also studied. It is found that the collision depends on their relative direction, and the pair annihilation occurs in some cases and only the antiskyrmion is destroyed in the other cases. These results indicate that the antiskyrmion offers a unique opportunity to study particles and antiparticles in condensed-matter systems.
Theory of antiskyrmions in magnets.
Koshibae, Wataru; Nagaosa, Naoto
2016-01-01
Skyrmions and antiskyrmions are swirling topological magnetic textures realized as emergent particles in magnets. A skyrmion is stabilized by the Dzyaloshinskii-Moriya interaction in chiral magnets and/or a dipolar interaction in thin film magnets, which prefer the twist of the magnetic moments. Here we show by a numerical simulation of the Landau-Lifshitz-Gilbert equation that pairs of skyrmions and antiskyrmions are created from the helix state as the magnetic field is increased. Antiskyrmions are unstable and disappear immediately in chiral magnets, whereas they are metastable and survive in dipolar magnets. The collision between a skyrmion and an antiskyrmion in a dipolar magnet is also studied. It is found that the collision depends on their relative direction, and the pair annihilation occurs in some cases and only the antiskyrmion is destroyed in the other cases. These results indicate that the antiskyrmion offers a unique opportunity to study particles and antiparticles in condensed-matter systems. PMID:26821932
Theory of antiskyrmions in magnets
Koshibae, Wataru; Nagaosa, Naoto
2016-01-01
Skyrmions and antiskyrmions are swirling topological magnetic textures realized as emergent particles in magnets. A skyrmion is stabilized by the Dzyaloshinskii–Moriya interaction in chiral magnets and/or a dipolar interaction in thin film magnets, which prefer the twist of the magnetic moments. Here we show by a numerical simulation of the Landau–Lifshitz–Gilbert equation that pairs of skyrmions and antiskyrmions are created from the helix state as the magnetic field is increased. Antiskyrmions are unstable and disappear immediately in chiral magnets, whereas they are metastable and survive in dipolar magnets. The collision between a skyrmion and an antiskyrmion in a dipolar magnet is also studied. It is found that the collision depends on their relative direction, and the pair annihilation occurs in some cases and only the antiskyrmion is destroyed in the other cases. These results indicate that the antiskyrmion offers a unique opportunity to study particles and antiparticles in condensed-matter systems. PMID:26821932
Magnetization dissipation in ferromagnets from scattering theory
NASA Astrophysics Data System (ADS)
Brataas, Arne; Tserkovnyak, Yaroslav; Bauer, Gerrit E. W.
2011-08-01
The magnetization dynamics of ferromagnets is often formulated in terms of the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation describes the response of the magnetization in terms of effective fields, whereas the dissipative part is parametrized by the Gilbert damping tensor. We formulate a scattering theory for the magnetization dynamics and map this description on the linearized LLG equation by attaching electric contacts to the ferromagnet. The reactive part can then be expressed in terms of the static scattering matrix. The dissipative contribution to the low-frequency magnetization dynamics can be described as an adiabatic energy pumping process to the electronic subsystem by the time-dependent magnetization. The Gilbert damping tensor depends on the time derivative of the scattering matrix as a function of the magnetization direction. By the fluctuation-dissipation theorem, the fluctuations of the effective fields can also be formulated in terms of the quasistatic scattering matrix. The theory is formulated for general magnetization textures and worked out for monodomain precessions and domain-wall motions. We prove that the Gilbert damping from scattering theory is identical to the result obtained by the Kubo formalism.
Magnetic fields and density functional theory
Salsbury Jr., Freddie
1999-02-01
A major focus of this dissertation is the development of functionals for the magnetic susceptibility and the chemical shielding within the context of magnetic field density functional theory (BDFT). These functionals depend on the electron density in the absence of the field, which is unlike any other treatment of these responses. There have been several advances made within this theory. The first of which is the development of local density functionals for chemical shieldings and magnetic susceptibilities. There are the first such functionals ever proposed. These parameters have been studied by constructing functionals for the current density and then using the Biot-Savart equations to obtain the responses. In order to examine the advantages and disadvantages of the local functionals, they were tested numerically on some small molecules.
Effects of non-linearities on magnetic field generation
Nalson, Ellie; Malik, Karim A.; Christopherson, Adam J. E-mail: achristopherson@gmail.com
2014-09-01
Magnetic fields are present on all scales in the Universe. While we understand the processes which amplify the fields fairly well, we do not have a ''natural'' mechanism to generate the small initial seed fields. By using fully relativistic cosmological perturbation theory and going beyond the usual confines of linear theory we show analytically how magnetic fields are generated. This is the first analytical calculation of the magnetic field at second order, using gauge-invariant cosmological perturbation theory, and including all the source terms. To this end, we have rederived the full set of governing equations independently. Our results suggest that magnetic fields of the order of 10{sup -30}- 10{sup -27} G can be generated (although this depends on the small scale cut-off of the integral), which is largely in agreement with previous results that relied upon numerical calculations. These fields are likely too small to act as the primordial seed fields for dynamo mechanisms.
Magnetic monopoles in Kaluza Klein theory
Sundaresan, M.K.; Tanaka, K.
1985-01-01
We start with an introduction to magnetic monopoles and then discuss the magnetic monopoles in 5-dimensions, the stability of solution with respect to small changes in the metric, and finally end with remarks.
Suppression of superheavy magnetic monopoles in grand unified theories
Pi, S.Y.
1980-08-01
The superheavy magnetic monopoles predicted by grand unified theories would not be produced in significant numbers if electromagnetic gauge invariance is spontaneously broken when the temperature T is greater than T/sub c/ >approx. 1 TeV.
The theory of an active magnetic regenerative refrigerator
NASA Technical Reports Server (NTRS)
Barclay, J. A.
1983-01-01
The adiabatic temperature change with field which is limited to about 2 K/Tesla for ferromagnets near their Curie temperatures by the change of magnetization with temperature and the lattice heat capacity is discussed. Practical magnetic refrigerators operate on a regenerative cycle such as the Brayton cycle. This cycle can be executed through the use of an active magnetic regenerator, i.e., a regenerator composed of magnetic material that is cycled in an out of a magnetic field with appropriate fluid flows. The theory of these devices is predicted by solving the partial differential equations that describe fluid and the magnetic solid. The active magnetic regenerator is described along with the method of calculation. Temperature profiles for a normal regenerator and a magnetic regenerative refrigerator are shown.
Topological magnetic crystalline insulators and co-representation theory
NASA Astrophysics Data System (ADS)
Zhang, Ruixing; Liu, Chaoxing
2014-03-01
We introduce a new type of topological insulator protected by magnetic group symmetry, which is a combined symmetry of point group symmetry and time reversal symmetry. Based on the Herring rule of the co-representation theory of magnetic group, we systematically show that systems with certain magnetic group symmetries can have Kramers'-like degeneracies and admit a Z2 classification. We establish a tight-binding model describing a layered magnetic structure with combined C4 rotation and time reversal symmetry. We show that this model can support non-trivial topological phases by calculating its gapless surface states and defining its Z2 topological invariant.
Asymptotic theory of relativistic, magnetized jets
Lyubarsky, Yuri
2011-01-15
The structure of a relativistically hot, strongly magnetized jet is investigated at large distances from the source. Asymptotic equations are derived describing collimation and acceleration of the externally confined jet. Conditions are found for the transformation of the thermal energy into the fluid kinetic energy or into the Poynting flux. Simple scalings are presented for the jet collimation angle and Lorentz factors.
Neutral Vlasov kinetic theory of magnetized plasmas
Tronci, Cesare; Camporeale, Enrico
2015-02-15
The low-frequency limit of Maxwell equations is considered in the Maxwell-Vlasov system. This limit produces a neutral Vlasov system that captures essential features of plasma dynamics, while neglecting radiation effects. Euler-Poincaré reduction theory is used to show that the neutral Vlasov kinetic theory possesses a variational formulation in both Lagrangian and Eulerian coordinates. By construction, the new model recovers all collisionless neutral models employed in plasma simulations. Then, comparisons between the neutral Vlasov system and hybrid kinetic-fluid models are presented in the linear regime.
Lattice Study of Magnetic Catalysis in Graphene Effective Field Theory
NASA Astrophysics Data System (ADS)
Winterowd, Christopher; Detar, Carleton; Zafeiropoulos, Savvas
2016-03-01
The discovery of graphene ranks as one of the most important developments in condensed matter physics in recent years. As a strongly interacting system whose low-energy excitations are described by the Dirac equation, graphene has many similarities with other strongly interacting field theories, particularly quantum chromodynamics (QCD). Graphene, along with other relativistic field theories, have been predicted to exhibit spontaneous symmetry breaking (SSB) when an external magnetic field is present. Using nonperturbative methods developed to study QCD, we study the low-energy effective field theory (EFT) of graphene subject to an external magnetic field. We find strong evidence supporting the existence of SSB at zero-temperature and characterize the dependence of the chiral condensate on the external magnetic field. We also present results for the mass of the Nambu-Goldstone boson and the dynamically generated quasiparticle mass that result from the SSB.
Electromagnetic scattering by magnetic spheres: Theory and algorithms
NASA Astrophysics Data System (ADS)
Milham, Merill E.
1994-10-01
The theory for the scattering of magnetic spheres is developed by means of scaling functions. This theory leads in a natural way to the development of scattering algorithms which use exponential scaling to overcome computational overflow problems. The design and testing of the algorithm is described. Fortran codes which implement the algorithmic design are presented and examples of code use are given. Listings of the code are included.
The Theory of Magnetic Reconnection: Past, Present, and Future
NASA Astrophysics Data System (ADS)
Cassak, P. A.
2008-05-01
Magnetic reconnection underlies the energy release observed in eruptive events in the solar corona (such as solar flares and coronal mass ejections) and in the Earth's magnetosphere. The theory of magnetic reconnection was originally developed to understand observations by Ron Giovanelli, who discovered that solar flares occur where the coronal magnetic field changes directions. Pioneers in space plasma theory such as James Dungey, Peter Sweet, Eugene Parker, and Harry Petschek first elucidated the underlying physical effects that lead to this massive energy release. Since then, much effort has been made to understand what process or processes cause magnetic reconnection to be fast enough to be consistent with observations, such as anomalous resistivity, secondary instabilities, and the Hall effect. However, a thorough understanding of this important process remains a topic of intense study. In celebration of the 50th anniversary of Parker's paper predicting the high-speed solar wind, this talk will review the history of the theory of magnetic reconnection. The present status of the field will be discussed, and remaining unanswered questions will be summarized.
Decoherence in Quantum Magnets: Theory and Experiment on T2
NASA Astrophysics Data System (ADS)
Tupitsyn, Igor; Stamp, Philip; Takahashi, Susumu; Sherwin, Mark; van Tol, Johan; Beedle, Christopher; Hendrickson, David
2010-03-01
The individual properties of molecular magnets are controlled by chemistry rather than nanoengineering, and are highly tunable. This makes them ideal candidates for solid-state qubits. However decoherence in many solid-state systems is anomalously high, and their advantages cannot be exploited until decoherence is understood and suppressed. In molecular magnets decoherence is caused by coupling to the nuclear spin bath, to phonons, and to each other via dipole-dipole and exchange interactions. Here we study decoherence in 2 different crystals of Fe8 nanomolecules, in several field orientations, both theoretically and experimentally. The experimental results for the decoherence time T2 agree with the existing theory (Morello et al., Phys Rev Lett 97, 207206 (2006)). To our knowledge this is the first time that experimental decoherence rates agree with theory in magnetic systems.
Magnetic mirrors at the nanoscale: theory
NASA Astrophysics Data System (ADS)
Chang, Mark P. J. L.; Jia, Dongdong; Nazari, Haedeh
2004-08-01
The control of charged particles at sub micrometer and nanometer length scales presents an intrinsically interesting challenge, as well as being a rich field for the study of trapped ions and plasmas. Motivated by this, we obtain the exact solution for the vector potential for a wire of finite length and of arbitrary form. Closed form solutions can then be deduced describing the electromagnetic waves propagating from the wire. This allows us to investigate design parameters, so that we may produce spiral wire shapes which, when injected with oscillatory currents, produce effects similar to conventional magnetic mirrors, except at the submicron and nanometre scale. Nanoscale devices present an added complication: very closely placed surfaces can exchange heat through the tunneling of evanescent radiation modes. This can augment the local heating effect when compared to blackbody emission, so any fabrication defects on the surface of the wire spirals could be problematic. We show that the evanescent contributions scale as a function of separation and dominate the heat exchange process when the spacing is much less than the characteristic wavelength of a given temperature. We expect that excess material might be deposited erroneously during fabrication of the spiral wires, so the transfer of heat from one wire coil to the defect will be higher than the rate due to uniform blackbody radiation. In the case of tungsten, for our typical spiral geometry, the heating rate is enhanced by a factor of 15. In the case of a carbon or other high conductivity composite material this rate can be raised by as much as 106, which is evidently not appropriate.
Rock magnetism: Studies in theory, data manipulation, and application
NASA Astrophysics Data System (ADS)
Carter-Stiglitz, Brian Scott
This collection of studies in rock magnetism encompasses theoretical modeling, data analysis, and an environmental magnetism application. Rock magnetism is essentially the study of the magnetic properties of naturally occurring material for application in the geosciences or for its own sake as a basic science. Over the last two decades low-temperature magnetic measurements have become common in rock magnetism studies. Some applications include identification and quantification of specific target minerals and nanophase material (ferrimagnetic or antiferromagnetic); low-temperature demagnetization (in, e.g., paleointensity studies); applications in biomagnetism (e.g., the magnetic detection of magnetotactic bacteria). Despite the wide range of applications a strong theoretical understanding of the low-temperature magnetism of even the most common terrestrial magnetic materials has been slow in coming. Theoretical modeling of the low-temperature magnetic behavior of single-domain magnetite (chapters 1 and 2) yield results that are in general agreement with empirical data. In particular magnetosome chains (as produced by magnetotactic bacteria) are studied. The observed magnetic behavior can be explained, at least in part, by maghemitization. A new study of the low-temperature magnetic properties of multidomain magnetite (chapter 4) shows behavior that seems to be a unique to "true" multidomain magnetite; this is explained with a combination of "twinning pinning" and easy axis bias effects. A new technique to "unmix" mixtures of magnetic materials (chapter 5) is based on fitting data with empirical basis functions, assuming linearity of magnetic moments as a function of mixing ratio. This assumption and the technique are successfully tested on a set of artificial mixtures. The technique is then applied to a small set of natural samples. Finally an environmental magnetism study of Argentine loess (chapter 6) combines rock magnetic measurements, geochemistry, and
On the theory of phase transitions in magnetic fluids
Zubarev, A. Yu. Iskakova, L. Yu.
2007-11-15
Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.
Magnetic MAX phases from theory and experiments; a review.
Ingason, A S; Dahlqvist, M; Rosen, J
2016-11-01
This review presents MAX phases (M is a transition metal, A an A-group element, X is C or N), known for their unique combination of ceramic/metallic properties, as a recently uncovered family of novel magnetic nanolaminates. The first created magnetic MAX phases were predicted through evaluation of phase stability using density functional theory, and subsequently synthesized as heteroepitaxial thin films. All magnetic MAX phases reported to date, in bulk or thin film form, are based on Cr and/or Mn, and they include (Cr,Mn)2AlC, (Cr,Mn)2GeC, (Cr,Mn)2GaC, (Mo,Mn)2GaC, (V,Mn)3GaC2, Cr2AlC, Cr2GeC and Mn2GaC. A variety of magnetic properties have been found, such as ferromagnetic response well above room temperature and structural changes linked to magnetic anisotropy. In this paper, theoretical as well as experimental work performed on these materials to date is critically reviewed, in terms of methods used, results acquired, and conclusions drawn. Open questions concerning magnetic characteristics are discussed, and an outlook focused on new materials, superstructures, property tailoring and further synthesis and characterization is presented. PMID:27602484
Electrical polarization and orbital magnetization: the modern theories.
Resta, Raffaele
2010-03-31
Macroscopic polarization P and magnetization M are the most fundamental concepts in any phenomenological description of condensed media. They are intensive vector quantities that intuitively carry the meaning of dipole per unit volume. But for many years both P and the orbital term in M evaded even a precise microscopic definition, and severely challenged quantum-mechanical calculations. If one reasons in terms of a finite sample, the electric (magnetic) dipole is affected in an extensive way by charges (currents) at the sample boundary, due to the presence of the unbounded position operator in the dipole definitions. Therefore P and the orbital term in M--phenomenologically known as bulk properties--apparently behave as surface properties; only spin magnetization is problemless. The field has undergone a genuine revolution since the early 1990s. Contrary to a widespread incorrect belief, P has nothing to do with the periodic charge distribution of the polarized crystal: the former is essentially a property of the phase of the electronic wavefunction, while the latter is a property of its modulus. Analogously, the orbital term in M has nothing to do with the periodic current distribution in the magnetized crystal. The modern theory of polarization, based on a Berry phase, started in the early 1990s and is now implemented in most first-principle electronic structure codes. The analogous theory for orbital magnetization started in 2005 and is partly work in progress. In the electrical case, calculations have concerned various phenomena (ferroelectricity, piezoelectricity, and lattice dynamics) in several materials, and are in spectacular agreement with experiments; they have provided thorough understanding of the behaviour of ferroelectric and piezoelectric materials. In the magnetic case the very first calculations are appearing at the time of writing (2010). Here I review both theories on a uniform ground in a density functional theory (DFT) framework, pointing out
Theory and computation of nuclear magnetic resonance parameters.
Vaara, Juha
2007-10-28
The art of quantum chemical electronic structure calculation has over the last 15 years reached a point where systematic computational studies of magnetic response properties have become a routine procedure for molecular systems. One of their most prominent areas of application are the spectral parameters of nuclear magnetic resonance (NMR) spectroscopy, due to the immense importance of this experimental method in many scientific disciplines. This article attempts to give an overview on the theory and state-of-the-art of the practical computations in the field, in terms of the size of systems that can be treated, the accuracy that can be expected, and the various factors that would influence the agreement of even the most accurate imaginable electronic structure calculation with experiment. These factors include relativistic effects, thermal effects, as well as solvation/environmental influences, where my group has been active. The dependence of the NMR spectra on external magnetic and optical fields is also briefly touched on. PMID:17925967
Toward the Theory of Turbulence in Magnetized Plasmas
Boldyrev, Stanislav
2013-07-26
The goal of the project was to develop a theory of turbulence in magnetized plasmas at large scales, that is, scales larger than the characteristic plasma microscales (ion gyroscale, ion inertial scale, etc.). Collisions of counter-propagating Alfven packets govern the turbulent cascade of energy toward small scales. It has been established that such an energy cascade is intrinsically anisotropic, in that it predominantly supplies energy to the modes with mostly field-perpendicular wave numbers. The resulting energy spectrum of MHD turbulence, and the structure of the fluctuations were studied both analytically and numerically. A new parallel numerical code was developed for simulating reduced MHD equations driven by an external force. The numerical setting was proposed, where the spectral properties of the force could be varied in order to simulate either strong or weak turbulent regimes. It has been found both analytically and numerically that weak MHD turbulence spontaneously generates a “condensate”, that is, concentration of magnetic and kinetic energy at small k{sub {parallel}}. A related topic that was addressed in the project is turbulent dynamo action, that is, generation of magnetic field in a turbulent flow. We were specifically concentrated on the generation of large-scale magnetic field compared to the scales of the turbulent velocity field. We investigate magnetic field amplification in a turbulent velocity field with nonzero helicity, in the framework of the kinematic Kazantsev-Kraichnan model.
Magnetic Separations with Magnetite: Theory, Operation, and Limitations
G. B. Cotten
2000-08-01
This dissertation documents the theory development and experimental plan followed to describe how a magnetite-based column under the influence of an external magnetic field functions as a magnetic separator. Theoretical simulations predict that weekly paramagnetic particles in the sub-micron range can be magnetically separated while diamagnetic particles as large as 2 microns in diameter may pass. Magnetite-based columns were evaluated as magnetically-controllable enhanced filtration devices. There was no evidence of enhanced filtration for diamagnetic particles by the magnetite-based bed. Magnetite-based magnetic separators have proven to be effective in specific laboratory experiments, indicating a potential feasibility for scale-up operations. Column media-filter type filtration effects indicate a magnetite-based column would not be suitable for treatment of a waste stream with a high diamagnetic solids content or high volume throughput requirements. Specific applications requiring removal of sub-micron para- or ferromagnetic particles under batch or Stokes flow conditions would be most applicable.
Toward functional magnetic stimulation (FMS) theory and experiment.
Davey, K; Luo, L; Ross, D A
1994-11-01
This paper examines the use of magnetic fields to functionally stimulate peripheral nerves. All electric fields are induced via a changing magnetic field whose flux is entirely confined within a closed magnetic circuit. Induced electric fields are simulated using a nonlinear boundary element solver. The induced fields are solved using duality theory. The accuracy of these predictions is verified by saline bath experiments. Next, the theory is applied to the stimulation of nerves using small, partially occluded ferrite and laminated vanadium permendur cores. Experiments demonstrate the successful stimulation of peripheral nerves in the African bullfrog with 11 mA, 153 mV excitations. These results offer a new vista of possibilities in the area of functional nerve stimulation. Unlike functional electric stimulation (FES), FMS does not involve any half cell reactions, and thus would not have the commensurate FES restrictions regarding balanced biphasic stimulation, strength duration balances, and oxidation issues, always exercising care that the electrodes remain in the reversible operating regime. PMID:8001991
Nuclear chiral and magnetic rotation in covariant density functional theory
NASA Astrophysics Data System (ADS)
Meng, Jie; Zhao, Pengwei
2016-05-01
Excitations of chiral rotation observed in triaxial nuclei and magnetic and/or antimagnetic rotations (AMR) seen in near-spherical nuclei have attracted a lot of attention. Unlike conventional rotation in well-deformed or superdeformed nuclei, here the rotational axis is not necessary coinciding with any principal axis of the nuclear density distribution. Thus, tilted axis cranking (TAC) is mandatory to describe these excitations self-consistently in the framework of covariant density functional theory (CDFT). We will briefly introduce the formalism of TAC–CDFT and its application for magnetic and AMR phenomena. Configuration-fixed CDFT and its predictions for nuclear chiral configurations and for favorable triaxial deformation parameters are also presented, and the discoveries of the multiple chiral doublets in 133Ce and 103Rh are discussed.
Kinetic theory of weak turbulence in magnetized plasmas: Perpendicular propagation
Yoon, Peter H.
2015-08-15
The present paper formulates a weak turbulence theory in which electromagnetic perturbations are assumed to propagate in directions perpendicular to the ambient magnetic field. By assuming that all wave vectors lie in one direction transverse to the ambient magnetic field, the linear solution and second-order nonlinear solutions to the equation for the perturbed distribution function are obtained. Nonlinear perturbed current from the second-order nonlinearity is derived in general form, but the limiting situation of cold plasma temperature is taken in order to derive an explicit nonlinear wave kinetic equation that describes three-wave decay/coalescence interactions among X and Z modes. A potential application of the present formalism is also discussed.
Nonlinear resonance theory of Pc 3 magnetic pulsation
Yumoto, K.; Saito, T.
1982-07-01
Compressional Pc 3 magnetic pulsations with large wave numbers normal to the static magnetic field in the magnetosphere and the beating type Pc 3 pulsations on the ground are hardly interpreted with respect to the linear resonance theory based on the idea of a resonance coupling between a monochromatic surface wave at the magnetopause and a shear Alfven wave at a local field line in the inner magnetosphere. A parametric excitation of an Alfven wave (k/sup A/, ..omega../sub A/) by a magnetosonic pump wave (k/sup tsf/s/sub 1/, ..omega../sub 1f/s), which propagates obliquely to the static magnetic field, has been analyzed. The resonance conditions are chosen such that k/sup tsf/s/sub 1/ = k/sup tsf/s/sub 2/+k/sup A/ and ..omega../sub 1f/s-..omega../sub A/ = deltaapprox...omega../sub 2f/s. For both standing and propagating pumps the growth rates of the excited HM waves depend not only on the pump power but also on ..beta... It is found that large growth rates of parametric excitation of Alfven waves by the fast magnetosonic pump waves arise if theta/sub 1f/ = angle(k/sup tsf//sub 1/, B/sub 0/) approx.70/sup 0/--80/sup 0/ and the regions of parametric excitation are localized at the resonance point in the magnetosphere where ..beta..approx.m/sub e//m/sub i/. It is concluded that parametric excitation of Pc 3 range HM waves is a more attractive theory of the beating type geomagnetic pulsations than the linear resonance theory.
High-. beta. theory of low-frequency magnetic pulsations
Migliuolo, S.
1983-03-01
The theory of low-frequency (compared to ion cyclotron) arbitrary-..beta.. modes is developed for the following system: a two-component (hot and cold) inhomogeneous plasma, and a straight inhomogeneous magnetic field. This system is taken to model the magnetosphere, near the geomagnetic equator. The stability properties of three modes are presented in detail: the drift-compressional mode (driven by pressure gradients) the firehose mode (driven by T/sub parallel/>T/sub perpendicular/), and the drift mirror mode (driven by T/sub perpendicular/>T/sub parallel/). Comparisons to earlier models and to one observed event are also presented.
Diamagnetic boundary layers - A kinetic theory. [for collisionless magnetized plasmas
NASA Technical Reports Server (NTRS)
Lemaire, J.; Burlaga, L. F.
1976-01-01
A kinetic theory is presented for boundary layers associated with MHD tangential 'discontinuities' in a collisionless magnetized plasma, such as those observed in the solar wind. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found to have a thickness of the order of a few electron gyroradii, but the drift speed of the current-carrying electrons is found to exceed the Alfven speed, and accordingly such layers are not stable. Several types of layers in which the current is carried by protons are discussed; in particular, cases are considered in which the magnetic-field intensity, direction, or both, changed across the layer. In every case, the thickness was of the order of a few proton gyroradii, and the field changed smoothly, although the characteristics depended somewhat on the boundary conditions. The drift speed was always less than the Alfven speed, consistent with stability of such structures. These results are consistent with observations of boundary layers in the solar wind near 1 AU.
Disorder-induced magnetic memory: experiments and theories
Pierce, M.S.; Buechler, C.R.; Sorensen, L.B.; Kevan, S.D.; Jagla,E.A.; Deutsch, J.M.; Mai, T.; Narayan, O.; Davies, J.E.; Liu, K.; Zimanyi, G.T.; Katzgraber, H.G.; Hellwig, O.; Fullerton, E.E.; Fischer,P.; Kortright, J.B.
2007-01-04
Beautiful theories of magnetic hysteresis based on randommicroscopic disorder have been developed over the past ten years. Ourgoal was to directly compare these theories with precise experiments. Todo so, we first developed and then applied coherent x-ray specklemetrology to a series of thin multilayer perpendicular magneticmaterials. To directly observe the effects of disorder, we deliberatelyintroduced increasing degrees of disorder into our films. We usedcoherent x rays, produced at the Advanced Light Source at LawrenceBerkeley National Laboratory, to generate highly speckled magneticscattering patterns. The apparently "random" arrangement of the specklesis due to the exact configuration of the magnetic domains in the sample.In effect, each speckle pattern acts as a unique fingerprint for themagnetic domain configuration. Small changes in the domain structurechange the speckles, and comparison of the different speckle patternsprovides a quantitative determination of how much the domain structurehas changed. Our experiments quickly answered one longstanding question:How is the magnetic domain configuration at one point on the majorhysteresis loop related to the configurations at the same point on theloop during subsequent cycles? This is called microscopic return-pointmemory "RPM". We found that the RPM is partial and imperfect in thedisordered samples, and completely absent when the disorder is below athreshold level. We also introduced and answered a second importantquestion: How are the magnetic domains at one point on the major looprelated to the domains at the complementary point, the inversionsymmetric point on the loop, during the same and during subsequentcycles? This is called microscopic complementary-point memory "CPM". Wefound that the CPM is also partial and imperfect in the disorderedsamples and completely absent when the disorder is not present. Inaddition, we found that the RPM is always a little larger than the CPM.We also studied the
Theory for magnetic excitations in quantum spin ice
NASA Astrophysics Data System (ADS)
Onoda, Shigeki; Datta, Trinanjan
Magnetic excitations in magnetic rare-earth pyrochlore oxides called quantum spin ice (QSI) systems such as Yb2Ti2O7, Pr2Zr2O7, and Tb2Ti2O7 have attracted great interest for possible observations of the quantum dynamics of spin ice monopoles and emergent photon excitations. However, their spectral properties remain open especially for cases relevant to experimental systems. Here, we develop a theoretical framework that incorporates gauge fluctuations into a modified gauge mean-field approach, so that it reproduces key features of recent quantum Monte-Carlo results on the double broad specific heat in the simplest QSI model and can describe a continuous growth of a coherence in gauge-field correlations on cooling down to Coulomb-phase ground states. Using this new approach, we provide a theory for magnetic neutron-scattering spectra. It is found that spin-flip exchange interactions produce dispersive QSI monopole excitations which create a particle-hole continuum neutron-scattering spectrum. Gauge fluctuations give multi-particle contributions to the spectrum, which will be possibly detected in Higgs phases.
Status of magnetically-insulated power transmission theory
Mendel, C.W. Jr.
1995-12-31
The theory of magnetically-insulated power flow has improved dramatically over the last two decades since the early works of Creedon, Lovelace and Ott, Ron, Mondelli, and Rostoker, and of Bergeron. During the intervening years theoretical improvements included a complete general kinetic theory that involved distributions of electrons based on quasi-conserved canonical variables and was used to study flow stability and to analyze simulations and pulsers with voltage adders. The status of theory at this time allowed them to understand many features of these flows, but did not allow detailed analysis for design and data interpretation. Recent theoretical advances have drastically changed this situation. Two recent static models based on layered flows have allowed them to understand and to improve power coupling in voltage adders, current adders, plasma opening switches and in systems where the vacuum impedance varies along the flow. A dynamic model based upon electrons flowing in one or more thin layers has permitted detailed self-consistent time-dependent calculations which include electron flow. This model accurately predicts experimental and simulational data.
Mean-field theory for Bose-Hubbard model under a magnetic field
Oktel, M. Oe.; Tanatar, B.; Nita, M.
2007-01-15
We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength (t/U){sub c} increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.
Basic Models in the Quantum Theory of Magnetism
NASA Astrophysics Data System (ADS)
Izyumov, Yu. A.
2003-08-01
We regard the Heisenberg model, the Hubbard model, the tJ-model and the sd-model as the basic models of the quantum theory of magnetism in solids. They can describe localized and itinerant magnets and strongly correlated electron systems. This review is devoted to analytical approaches for these models: diagrammatic techniques and the method of generating functional. The diagrammatic techniques are based on a generalization of the Wick theorem for spin and X operators. Peculiarities of such techniques for the basic models appear because the spin and X operators do not commute on a C-value, but their commutator (anticommutator) is an operator itself. The method of generating functional is a generalization of the Kadanoff-Baym approach, developed earlier for usual Fermi systems. The generating functional describes the interaction of a system with fluctuating fields, and different Green's functions can be treated as variational derivatives with respect to these fields. Such approach allows to derive the equation of motion for the Green's functions in each model in terms of functional derivatives. These equations help to find common features in the behavior of the basic models, particularly in finding the multiplicative structure of one-particle Green's functions. Iteration of the equations generates perturbation theory, which is compared with the diagrammatic techniques. Both approaches are applied to the calculation of the quasiparticle spectrum of the models and of collective excitations. A generalized random phase approximation (GRPA) is suggested for calculation of different dynamical susceptibilities. This approximation is developed in both approaches: the diagrammatic technique and the generating functional method.
Magnetic fields from heterotic cosmic strings
Gwyn, Rhiannon; Alexander, Stephon H.; Brandenberger, Robert H.; Dasgupta, Keshav
2009-04-15
Large-scale magnetic fields are observed today to be coherent on galactic scales. While there exists an explanation for their amplification and their specific configuration in spiral galaxies--the dynamo mechanism--a satisfying explanation for the original seed fields required is still lacking. Cosmic strings are compelling candidates because of their scaling properties, which would guarantee the coherence on cosmological scales of any resultant magnetic fields at the time of galaxy formation. We present a mechanism for the production of primordial seed magnetic fields from heterotic cosmic strings arising from M theory. More specifically, we make use of heterotic cosmic strings stemming from M5-branes wrapped around four of the compact internal dimensions. These objects are stable on cosmological time scales and carry charged zero modes. Therefore a scaling solution of such defects will generate seed magnetic fields which are coherent on galactic scales today.
Magnetostatic potential theory and the lunar magnetic dipole field
NASA Technical Reports Server (NTRS)
Goldstein, M. L.
1975-01-01
The lunar magnetic dipole moment is discussed. It is proposed that if a primordial core magnetic field existed, it would give rise to a present day nonzero external dipole magnetic field. This conclusion is based on the assumption that the lunar mantle is at least slightly ferromagnetic, and thus would maintain a permanent magnetization after the disappearance of the core magnetic field. Using a simple mathematical model of the moon, calculations are performed which support this hypothesis.
The theory of the failure of magnetic fusion
NASA Astrophysics Data System (ADS)
Zakharov, Leonid E.
2007-11-01
In the physics of the 20th century, fusion represents an extraordinary failure which eroded expectations of society on an ``unexhaustible'' energy source. The question is if these 50 years of research did really prove that fusion will be forever a ``carrot'' on a stick and always 35 years from its implementation. When a person is asking fusion people why this program is full of broken promises, he (besides conventional complaints on the lack of funding) is typically getting the answer that the problem itself is the most difficult one that physics ever faced. In the FSU, such characterizations were done as early as in the 60s by Lev Artsimovich, the leader in the field. This view is only partially applicable in the 21st century. Since the times of Artsimovich, fusion, as a ``difficult'' problem, has been converted into the ``complicated'' one. The presented theory makes a clear distinction between these two kinds of problems, which require significantly different management approaches, and explains the current stagnation in magnetic fusion by the lack of understanding this crucial difference.
On the unsteady-motion theory of magnetic forces for maglev
Chen, S.S.; Zhu, S.; Cai, Y.
1993-11-01
Motion-dependent magnetic forces are the key elements in the study of magnetically levitated vehicle (maglev) system dynamics. In the past, most maglev-system designs were based on a quasisteady-motion theory of magnetic forces. This report presents an experimental and analytical study that will enhance our understanding of the role of unsteady-motion-dependent magnetic forces and demonstrate an experimental technique that can be used to measure those unsteady magnetic forces directly. The experimental technique provides a useful tool to measure motion-dependent magnetic forces for the prediction and control of maglev systems.
Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields.
Elfimova, Ekaterina A; Ivanov, Alexey O; Camp, Philip J
2012-05-21
Anisotropic pair correlations in ferrofluids exposed to magnetic fields are studied using a combination of statistical-mechanical theory and computer simulations. A simple dipolar hard-sphere model of the magnetic colloidal particles is studied in detail. A virial-expansion theory is constructed for the pair distribution function (PDF) which depends not only on the length of the pair separation vector, but also on its orientation with respect to the field. A detailed comparison is made between the theoretical predictions and accurate simulation data, and it is found that the theory works well for realistic values of the dipolar coupling constant (λ = 1), volume fraction (φ ≤ 0.1), and magnetic field strength. The structure factor is computed for wavevectors either parallel or perpendicular to the field. The comparison between theory and simulation is generally very good with realistic ferrofluid parameters. For both the PDF and the structure factor, there are some deviations between theory and simulation at uncommonly high dipolar coupling constants, and with very strong magnetic fields. In particular, the theory is less successful at predicting the behavior of the structure factors at very low wavevectors, and perpendicular Gaussian density fluctuations arising from strongly correlated pairs of magnetic particles. Overall, though, the theory provides reliable predictions for the nature and degree of pair correlations in ferrofluids in magnetic fields, and hence should be of use in the design of functional magnetic materials. PMID:22612098
Mathematical developments regarding the general theory of the Earth magnetism
NASA Technical Reports Server (NTRS)
Schmidt, A.
1983-01-01
A literature survey on the Earth's magnetic field, citing the works of Gauss, Erman-Petersen, Quintus Icilius and Neumayer is presented. The general formulas for the representation of the potential and components of the Earth's magnetic force are presented. An analytical representation of magnetic condition of the Earth based on observations is also made.
Quantum theory of the dielectric constant of a magnetized plasma and astrophysical applications. I.
NASA Technical Reports Server (NTRS)
Canuto, V.; Ventura, J.
1972-01-01
A quantum mechanical treatment of an electron plasma in a constant and homogeneous magnetic field is considered, with the aim of (1) defining the range of validity of the magnetoionic theory (2) studying the deviations from this theory, in applications involving high densities, and intense magnetic field. While treating the magnetic field exactly, a perturbation approach in the photon field is used to derive general expressions for the dielectric tensor. Numerical estimates on the range of applicability of the magnetoionic theory are given for the case of the 'one-dimensional' electron gas, where only the lowest Landau level is occupied.
Theory and Application of Magnetic Flux Leakage Pipeline Detection.
Shi, Yan; Zhang, Chao; Li, Rui; Cai, Maolin; Jia, Guanwei
2015-01-01
Magnetic flux leakage (MFL) detection is one of the most popular methods of pipeline inspection. It is a nondestructive testing technique which uses magnetic sensitive sensors to detect the magnetic leakage field of defects on both the internal and external surfaces of pipelines. This paper introduces the main principles, measurement and processing of MFL data. As the key point of a quantitative analysis of MFL detection, the identification of the leakage magnetic signal is also discussed. In addition, the advantages and disadvantages of different identification methods are analyzed. Then the paper briefly introduces the expert systems used. At the end of this paper, future developments in pipeline MFL detection are predicted. PMID:26690435
Theory and Application of Magnetic Flux Leakage Pipeline Detection
Shi, Yan; Zhang, Chao; Li, Rui; Cai, Maolin; Jia, Guanwei
2015-01-01
Magnetic flux leakage (MFL) detection is one of the most popular methods of pipeline inspection. It is a nondestructive testing technique which uses magnetic sensitive sensors to detect the magnetic leakage field of defects on both the internal and external surfaces of pipelines. This paper introduces the main principles, measurement and processing of MFL data. As the key point of a quantitative analysis of MFL detection, the identification of the leakage magnetic signal is also discussed. In addition, the advantages and disadvantages of different identification methods are analyzed. Then the paper briefly introduces the expert systems used. At the end of this paper, future developments in pipeline MFL detection are predicted. PMID:26690435
Theory of using magnetic deflections to combine charged particle beams.
Doyle, Barney Lee; Steckbeck, Mackenzie K.
2014-09-01
Several radiation effects projects in the Ion Beam Lab (IBL) have recently required two disparate charged particle beams to simultaneously strike a single sample through a single port of the target chamber. Because these beams have vastly different mass- energy products (MEP), the low MEP beam requires a large angle of deflection toward the sample by a bending electromagnet. A second electromagnet located further upstream provides a means to compensate for the small angle deflection experienced by the high MEP beam during its path through the bending magnet. This paper derives the equations used to select the magnetic fields required by these two magnets to achieve uniting both beams at the target sample. A simple result was obtained when the separation of the two magnets was equivalent to the distance from the bending magnet to the sample, and the equations is given by: , 1 2 c s c s r B B r where and are the magnetic fields in the steering and bending magnet and is s B c B c s r r the ratio of the radii of the bending magnet to that of the steering magnet. This result is not dependent upon the parameters of the high MEP beam, i.e. energy, mass, charge state. Therefore, once the field of the bending magnet is set for the low MEP beam, and the field in the steering magnet is set as indicted in the equation, the trajectory path of any high MEP beam will be directed into the sample. (page intentionally left blank)
Magnetic particle motions within living cells. Physical theory and techniques.
Valberg, P A; Butler, J P
1987-01-01
Body tissues are not ferromagnetic, but ferromagnetic particles can be present as contaminants or as probes in the lungs and in other organs. The magnetic domains of these particles can be aligned by momentary application of an external magnetic field; the magnitude and time course of the resultant remanent field depend on the quantity of magnetic material and the degree of particle motion. The interpretation of magnetometric data requires an understanding of particle magnetization, agglomeration, random motion, and both rotation and translation in response to magnetic fields. We present physical principles relevant to magnetometry and suggest models for intracellular particle motion driven by thermal, elastic, or cellular forces. The design principles of instrumentation for magnetizing intracellular particles and for detecting weak remanent magnetic fields are described. Such magnetic measurements can be used for noninvasive studies of particle clearance from the body or of particle motion within body tissues and cells. Assumptions inherent to this experimental approach and possible sources of artifact are considered and evaluated. PMID:3676435
Theory of multichannel magnetic stimulation: toward functional neuromuscular rehabilitation.
Ruohonen, J; Ravazzani, P; Grandori, F; Ilmoniemi, R J
1999-06-01
Human excitable cells can be stimulated noninvasively with externally applied time-varying electromagnetic fields. The stimulation can be achieved either by directly driving current into the tissue (electrical stimulation) or by means of electro-magnetic induction (magnetic stimulation). While the electrical stimulation of the peripheral neuromuscular system has many beneficial applications, peripheral magnetic stimulation has so far only a few. This paper analyzes theoretically the use of multiple magnetic stimulation coils to better control the excitation and also to eventually mimic electrical stimulation. Multiple coils allow electronic spatial adjustment of the shape and location of the stimulus without moving the coils. The new properties may enable unforeseen uses for peripheral magnetic stimulation, e.g., in rehabilitation of patients with neuromuscular impairment. PMID:10356871
Quantum theory of spin alignment in a circular magnetic nanotube
NASA Astrophysics Data System (ADS)
Bergmann, Gerd; Thompson, Richard S.; Lu, Jia G.
2015-12-01
When electron spin and momentum couple in a solid, one generally obtains intriguing and unexpected phenomena. Metallic ferromagnetic nanotubes of cobalt with circular magnetization, which have been prepared by us and others, are a particularly interesting system. Here the spins of the conduction electrons are frustrated. They would like to align parallel to the magnetic field of the magnetization, but as the electrons move quickly around the tube the spins cannot follow the magnetization direction. In a previous short theoretical paper we solved the spin dynamics using a classical model. Here we generalize our work to a quantum mechanical model. The surprising result is that the spin of most conduction electrons is not parallel or anti-parallel to the circumferential magnetization but mostly parallel or anti-parallel to the axis of the nanotube. This result means that such a cobalt nanotube is a different ferromagnet from a cobalt film or bulk cobalt.
Advanced quantitative magnetic nondestructive evaluation methods - Theory and experiment
NASA Technical Reports Server (NTRS)
Barton, J. R.; Kusenberger, F. N.; Beissner, R. E.; Matzkanin, G. A.
1979-01-01
The paper reviews the scale of fatigue crack phenomena in relation to the size detection capabilities of nondestructive evaluation methods. An assessment of several features of fatigue in relation to the inspection of ball and roller bearings suggested the use of magnetic methods; magnetic domain phenomena including the interaction of domains and inclusions, and the influence of stress and magnetic field on domains are discussed. Experimental results indicate that simplified calculations can be used to predict many features of these results; the data predicted by analytic models which use finite element computer analysis predictions do not agree with respect to certain features. Experimental analyses obtained on rod-type fatigue specimens which show experimental magnetic measurements in relation to the crack opening displacement and volume and crack depth should provide methods for improved crack characterization in relation to fracture mechanics and life prediction.
Theory of carrier mediated ferromagnetism in dilute magnetic oxides
Calderon, M.J. Das Sarma, S.
2007-11-15
We analyze the origin of ferromagnetism as a result of carrier mediation in diluted magnetic oxide semiconductors in the light of the experimental evidence reported in the literature. We propose that a combination of percolation of magnetic polarons at lower temperature and Ruderman-Kittel-Kasuya-Yosida ferromagnetism at higher temperature may be the reason for the very high critical temperatures measured (up to {approx}700 K)
Verification of a magnetic island in gyro-kinetics by comparison with analytic theory
Zarzoso, D. Casson, F. J.; Poli, E.; Hornsby, W. A.; Peeters, A. G.
2015-02-15
A rotating magnetic island is imposed in the gyrokinetic code GKW, when finite differences are used for the radial direction, in order to develop the predictions of analytic tearing mode theory and understand its limitations. The implementation is verified against analytics in sheared slab geometry with three numerical tests that are suggested as benchmark cases for every code that imposes a magnetic island. The convergence requirements to properly resolve physics around the island separatrix are investigated. In the slab geometry, at low magnetic shear, binormal flows inside the island can drive Kelvin-Helmholtz instabilities which prevent the formation of the steady state for which the analytic theory is formulated.
Electric-magnetic dualities in non-abelian and non-commutative gauge theories
NASA Astrophysics Data System (ADS)
Ho, Jun-Kai; Ma, Chen-Te
2016-08-01
Electric-magnetic dualities are equivalence between strong and weak coupling constants. A standard example is the exchange of electric and magnetic fields in an abelian gauge theory. We show three methods to perform electric-magnetic dualities in the case of the non-commutative U (1) gauge theory. The first method is to use covariant field strengths to be the electric and magnetic fields. We find an invariant form of an equation of motion after performing the electric-magnetic duality. The second method is to use the Seiberg-Witten map to rewrite the non-commutative U (1) gauge theory in terms of abelian field strength. The third method is to use the large Neveu Schwarz-Neveu Schwarz (NS-NS) background limit (non-commutativity parameter only has one degree of freedom) to consider the non-commutative U (1) gauge theory or D3-brane. In this limit, we introduce or dualize a new one-form gauge potential to get a D3-brane in a large Ramond-Ramond (R-R) background via field redefinition. We also use perturbation to study the equivalence between two D3-brane theories. Comparison of these methods in the non-commutative U (1) gauge theory gives different physical implications. The comparison reflects the differences between the non-abelian and non-commutative gauge theories in the electric-magnetic dualities. For a complete study, we also extend our studies to the simplest abelian and non-abelian p-form gauge theories, and a non-commutative theory with the non-abelian structure.
Magnetic Fields Around the Heliosphere: Theory vs Observations
NASA Astrophysics Data System (ADS)
Pogorelov, Nikolai
2016-07-01
Voyager in situ measurements of the magnetic field around the heliosphere are the source of invaluable information about the interface between the solar wind (SW) and local interstellar medium (LISM). On the other hand, they are quite challenging for theoretical analysis unless accompanied by remote observations of neutral atoms the Interstellar Boundary Explorer (IBEX) and Ulysses missions. Of particular interest is the fine structure of the heliopause due to its instability and possible magnetic reconnection. Both phenomena may have contributed to the remarkable changes in the galactic and anomalous cosmic ray fluxes observed by Voyager 1 within a one-month period of 2012 after which the spacecraft penetrated into the LISM. Draping of the heliopause by the interstellar magnetic field affects the position of the bright ribbon of enhanced ENA flux observed by IBEX on the celestial sphere and 2-3 kHz radio emission caused by shock propagation through the outer heliosheath observed by Voyager 1. Interstellar magnetic field determines the structure of the bow wave in front of the heliopause. Moreover, magnetic fields define the orientation and shape of the heliotail, the features of which have been observed by IBEX. Recent numerical simulations show that the details of the large-scale interstellar magnetic field modification caused by the presence of the heliotail may be the source of the observed 1-10 TeV cosmic ray anisotropy studied in detail in numerous air shower measurements around the world. In this paper, an overview will be given of the recent theoretical and simulations results describing the magnetic field distribution around the heliosphere. The objective of the talk is to connect observational and theoretical results, and outline challenges that are going to inspire the heliospheric community in the coming years.
Airborne Bacteria in the Atmospheric Surface Layer: Temporal Distribution above a Grass Seed Field
Lighthart, B.; Shaffer, B. T.
1995-01-01
Temporal airborne bacterial concentrations and meteorological conditions were measured above a grass seed field in the Willamette River Valley, near Corvallis, Oreg., in the summer of 1993. The concentration of airborne bacteria had a maximum of 1,368.5 CFU/m(sup3), with a coefficient of variation of 90.5% and a mean of 121.3 CFU/m(sup3). The lowest concentration of bacteria occurred during the predawn hours, with an average of 32.2 CFU/m(sup3), while sunrise and early evening hours had the highest averages (164.7 and 158.1 CFU/m(sup3), respectively). The concentrations of bacteria in the atmosphere varied greatly, with a maximum difference between two 2-min samples of 1,995 CFU/m(sup3). The concentrations of bacteria in the atmosphere could be divided into five time periods during the day that were thought to be related to the local diurnal sea breeze and Pacific Coast monsoon weather conditions as follows: (i) the nighttime minimum concentration, i.e., 2300 to 0600 h; (ii) the sunrise peak concentration, i.e., 0600 to 0800 h; (iii) the midday accumulating concentration, i.e., 0800 to 1515 h; (iv) the late-afternoon sea breeze trough concentration, i.e., 1515 to 1700 h; and (v) the evening decrease to the nighttime minimum concentration, i.e., 1700 to 2300 h. The sunrise peak concentration (period ii) is thought to be a relatively general phenomenon dependent on ground heating by the sun, while the afternoon trough concentration is thought to be a relatively local phenomenon dependent on the afternoon sea breeze. Meteorological conditions are thought to be an important regulating influence on airborne bacterial concentrations in the outdoor atmosphere in the Willamette River Valley. PMID:16534998
Coupled magnetoelastic theory of magnetic and magnetostrictive hysteresis
Sablik, M.J. ); Jiles, D.C. . Ames Lab.)
1993-07-01
A physical model is developed for the coupling between magnetic and magnetostrictive hysteresis and for the effect of mechanical stress on both types of hysteresis. The Jiles-Atherton-Sablik model for magnetomechanical hysteresis is reviewed and interpreted. In that model, under applied stress, the magnetization is coupled to magnetostriction through the derivative of the magnetostriction with respect to magnetization. The magnetostriction is also a function of the magnetization even in the absence of stress. An expression for the magnetostriction is derived from minimization of the internal energy with respect to strains, which is necessary for mechanical equilibrium. In the case where stress [sigma]/Y, where Y is Young's modulus, and a magnetostrain which goes to zero at saturation ([Delta]E effect). From the magnetostrain, the magnetostriction is obtained, using the convention that magnetostriction is zero in the unmagnetized state. By taking into account fluctuations in the magnetic energy due to hysteresis, one finds that the magnetostriction initially moves to higher values as the magnitude of the flux density B decreases from its extremum value in [lambda] versus B plots. Various numerical cases are evaluated, and the modeling is compared to previous measurements in polycrystalline iron and steel and in terfenol and Ni-Zn ferrites.
Localized magnetism on the surface of niobium: experiments and theory
NASA Astrophysics Data System (ADS)
Proslier, Thomas; Zasadzinski, John; Ciovati, Gianluigi; Pellin, Mike
2011-03-01
The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field > 4 T , consistentwiththeAppelbaummodelforspinfliptunneling . ViewedtogethertheexperimentsstronglysuggestthatthenativeoxidesofNbareintrinsicallydefective , andconsistentlyexhibitlocalizedparamagneticmomentscausedbyoxygenvacanciesinNb 2 O 5 . Thecomputationofthesurfaceimpedance (R S) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance. Work supported by DOE-HEP office, under contract No. DE-AC02-06CH11357.
Effects of the substrate on graphone magnetism: A density functional theory study
NASA Astrophysics Data System (ADS)
Buonocore, Francesco; Mosca Conte, Adriano; Lisi, Nicola
2016-04-01
The magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. In the present density functional theory study, the effects the adhesion to either Cu(111) or α-quartz (0001) surface on the magnetic properties of graphone have been investigated. The total magnetization of the graphone adsorbed to copper and quartz surface is reduced by four and two times, respectively, with respect to the isolated graphone. We have shown there is electronic charge transfer from surface towards three-fold coordinated C atoms of graphone, but the main role in the partial magnetism quenching is played by bond formation and the consequent electron pairing of p-electrons. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature.
Magnetic-Field-Induced Insulator-Conductor Transition in SU(2) Quenched Lattice Gauge Theory
Buividovich, P.V.; Kharzeev, D.; Chernodub, M.N., Kalaydzhyan, T., Luschevskaya, E.V., and M.I. Polikarpov
2010-09-24
We study the correlator of two vector currents in quenched SU(2) lattice gauge theory with a chirally invariant lattice Dirac operator with a constant external magnetic field. It is found that in the confinement phase the correlator of the components of the current parallel to the magnetic field decays much slower than in the absence of a magnetic field, while for other components the correlation length slightly decreases. We apply the maximal entropy method to extract the corresponding spectral function. In the limit of zero frequency this spectral function yields the electric conductivity of quenched theory. We find that in the confinement phase the external magnetic field induces nonzero electric conductivity along the direction of the field, transforming the system from an insulator into an anisotropic conductor. In the deconfinement phase the conductivity does not exhibit any sizable dependence on the magnetic field.
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; Klepeis, J. E.
2015-01-01
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments. PMID:26514238
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; Klepeis, J. E.
2015-10-30
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment (DLM) method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, However, the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.
Phonon and magnetic structure in δ-plutonium from density-functional theory.
Söderlind, Per; Zhou, F; Landa, A; Klepeis, J E
2015-01-01
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments. PMID:26514238
Astrophysical dynamos and the growth of magnetic fields in high-redshift galaxies
NASA Astrophysics Data System (ADS)
Rieder, Michael; Teyssier, Romain
2015-08-01
The origin and evolution of magnetic fields in the Universe is still an open question. Observations of galaxies at high-redshift give evidence for strong galactic magnetic fields even in the early Universe which are consistently measured at later times up to the present age. However, primordial magnetic fields and seed field generation by battery processes cannot explain such high field strengths, suggesting the presence of a rapid growth mechanism in those high-redshift galaxies and subsequent maintenance against decay. Astrophysical dynamo theory provides efficient means of field amplification where even weak initial fields can grow exponentially on sufficiently fast timescales, driving the conversion of kinetic energy into magnetic energy. We investigate the role which feedback mechanisms play in the creation of the turbulence necessary for dynamos to operate. Performing magnetohydrodynamic simulations of cooling halos of dwarf and Milky Way-like high-redshift progenitors, we compare the magnetic field evolution of weak seed fields with various topologies and stellar feedback mechanisms. We find that strong feedback can drive galactic gas turbulence which gives rise to velocity fields with fast exponential magnetic field growth. The simulations display a high gas fraction and a clumpy morphology with kinematics resembling Kolmogorov turbulence and magnetic energy spectra as predicted by Kazantsev dynamo theory. Magnetic fields reach equipartition with $\\mu$G field strength. In a final quiescent phase where feedback is turned off, gas turbulence is reduced and a quadrupole symmetry is observed in the magnetic field. These findings support the theory of rapid magnetic field amplification inside high-redshift galaxies, when the Universe was still young.
Theory of Magnetization Plateaux in the Shastry-Sutherland Model
Dorier, J.; Mila, F.; Schmidt, K. P.
2008-12-19
Motivated by the remarkable properties of SrCu{sub 2}(BO{sub 3}){sub 2} in a magnetic field, we use perturbative continuous unitary transformations to determine the magnetization plateaux of the Shastry-Sutherland model, unveiling an unexpected sequence of plateaux progressively appearing at 2/9, 1/6, 1/9, and 2/15 upon increasing the interdimer coupling. We predict that a 1/6 plateau should be present in SrCu{sub 2}(BO{sub 3}){sub 2}, even if residual interactions beyond the Shastry-Sutherland are strong enough to modify the other plateaux below 1/3. The method is extended to calculate the magnetization profile within the plateaux, leading to a local structure around triplons that agrees with NMR results on SrCu{sub 2}(BO{sub 3}){sub 2}.
Szalai, I; Nagy, S; Dietrich, S
2013-11-20
The influence of polydispersity on the magnetization of ferrofluids is studied based on a previously published magnetization equation of state (Szalai and Dietrich, 2011 J. Phys.: Condens. Matter 23 326004) and computer simulations. The polydispersity of the particle diameter is described by the gamma distribution function. Canonical ensemble Monte Carlo simulations have been performed in order to test these theoretical results for the initial susceptibility and the magnetization. The results for the magnetic properties of the polydisperse systems turn out to be in quantitative agreement with our present simulation data. In addition, we find good agreement between our theory and experimental data for magnetite-based ferrofluids. PMID:24153397
NASA Astrophysics Data System (ADS)
Braguta, V. V.; Buividovich, P. V.; Chernodub, M. N.; Kotov, A. Yu.; Polikarpov, M. I.
2012-12-01
Using numerical simulations of quenched SU (2) gauge theory we demonstrate that an external magnetic field leads to spontaneous generation of quark condensates with quantum numbers of electrically charged ρ mesons if the strength of the magnetic field exceeds the critical value eBc = 0.927 (77) GeV2 or Bc = (1.56 ± 0.13) ṡ1016 Tesla. The condensation of the charged ρ mesons in strong magnetic field is a key feature of the magnetic-field-induced electromagnetic superconductivity of the vacuum.
Theory of the inverse Faraday effect in view of ultrafast magnetization experiments
NASA Astrophysics Data System (ADS)
Popova, Daria; Bringer, Andreas; Blügel, Stefan
2011-12-01
We supplement the theory of the inverse Faraday effect, which was developed in the 1960s, to the conditions used today in ultrafast magnetization experiments. We show that assumptions used to derive the effective Hamiltonian and magnetization are not valid under these conditions. We extended the approach to be applicable to describe magnetization dynamics at femtosecond time scales. We show that after the action of an ultrafast laser pulse the system is brought with a certain probability to a state, the magnetic signature of which is different from before the excitation.
Theory of electromagnetic fluctuations for magnetized multi-species plasmas
Navarro, Roberto E. Muñoz, Víctor; Araneda, Jaime; Moya, Pablo S.; Viñas, Adolfo F.; Valdivia, Juan A.
2014-09-15
Analysis of electromagnetic fluctuations in plasma provides relevant information about the plasma state and its macroscopic properties. In particular, the solar wind persistently sustains a small but detectable level of magnetic fluctuation power even near thermal equilibrium. These fluctuations may be related to spontaneous electromagnetic fluctuations arising from the discreteness of charged particles. Here, we derive general expressions for the plasma fluctuations in a multi-species plasma following arbitrary distribution functions. This formalism, which generalizes and includes previous works on the subject, is then applied to the generation of electromagnetic fluctuations propagating along a background magnetic field in a plasma of two proton populations described by drifting bi-Maxwellians.
Theory of Magnetic Edge States in Chiral Graphene Nanoribbons
NASA Astrophysics Data System (ADS)
Capaz, Rodrigo; Yazyev, Oleg; Louie, Steven
2011-03-01
Using a model Hamiltonian approach including electron Coulomb interactions, we systematically investigate the electronic structure and magnetic properties of chiral graphene nanoribbons. We show that the presence of magnetic edge states is an intrinsic feature of any smooth graphene nanoribbons with chiral edges, and discover a number of structure-property relations. Specifically, we describe how the edge-state energy gap, zone-boundary edge-state energy splitting, and magnetic moment per edge length depend on the nanoribbon width and chiral angle. The role of environmental screening effects is also studied. Our results address a recent experimental observation of signatures of magnetic ordering at smooth edges of chiral graphene nanoribbons and provide an avenue towards tuning their properties via the structural and environmental degrees of freedom. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and the ONR MURI program. RBC acknowledges financial support from Brazilian agencies CNPq, FAPERJ and INCT-Nanomateriais de Carbono.
Magnetic transitions in ultra-small nanoscopic magnetic rings: Theory and experiments
NASA Astrophysics Data System (ADS)
Singh, Deepak K.; Krotkov, Robert; Tuominen, Mark T.
2009-05-01
In this paper, we report on experimental and theoretical investigations of magnetic transitions in cobalt rings of size (diameter, width and thickness) comparable to the exchange length of cobalt. Magnetization measurements and calculations were performed for two sets of magnetic ring arrays: ultra-small magnetic rings (outer diameter 13 nm, inner diameter 5 nm and thickness 5 nm) and small magnetic rings (outer diameter 150 nm, width 5 nm, and thickness 5 nm). Our calculations suggest that if the linear dimensions of a magnetic ring are comparable to, or smaller than, the exchange length of the magnetic material, then only one magnetic state is important—the pure single-domain state. Vortex and onion-shape magnetic states do not arise. For a ring of larger diameter, magnetization reversal at zero field occurs via a vortex state. Theoretical calculations are based on an energetic analysis of pure and slightly distorted single-domain and vortex magnetic states. The calculations have been verified by micromagnetic simulations for ultra-small and small ring geometries. The hysteresis curves measured for small rings are consistent with the calculations, but there is a discrepancy for ultra-small rings. Micromagnetic simulations suggest that the discrepancies may be due to the variations in the shape and size of the ultra-small rings in the measured sample.
NASA Astrophysics Data System (ADS)
Frandsen, Benjamin; Page, Katharine; Brunelli, Michela; Staunton, Julie; Billinge, Simon
Short-range magnetic correlations are known to exist in a variety of strongly correlated electron systems, but our understanding of the role they play is challenged by the difficulty of experimentally probing such correlations. Magnetic pair distribution function (mPDF) analysis is a newly developed neutron total scattering method that can reveal short-range magnetic correlations directly in real space, and may therefore help ameliorate this difficulty. We present temperature-dependent mPDF measurements of the short-range magnetic correlations in the paramagnetic phase of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. We observe significant correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range-ordered spin arrangement. With no free parameters, ab initio calculations using the self-interaction-corrected local spin density approximation of density functional theory quantitatively reproduce the magnetic correlations to a high degree of accuracy. These results yield valuable insight into the magnetic exchange in MnO and showcase the utility of the mPDF technique for studying magnetic properties of strongly correlated electron systems.
Solar dynamo theory : Solar dynamo theory: a new look at the origin of small-scale magnetic fields
NASA Astrophysics Data System (ADS)
Cattaneo, Fausto; Hughes, David W.
2001-06-01
Fausto Cattaneo and David W Hughes delve beneath the surface of the Sun with numerical models of turbulent convection. Although magnetic dynamo action is traditionally associated with rotation, fast dynamo theory shows that chaotic flows, even without rotation, can act as efficient small-scale dynamos. Indeed, numerical simulations suggest that granular and supergranular convection may generate locally a substantial part of the field in the quiet photosphere.
Multiscale theory of thin film magnetic shape memory alloy microactuators
NASA Astrophysics Data System (ADS)
Stoilov, Vesselin
2007-04-01
This paper investigates the nano-macro transition in magnetic shape memory alloy(MSMA) thin films using a recently developed sharp phase front-based three-dimensional (3D) constitutive model outlined by Stoilov (JSMS 2007), and originally proposed in the 1D context by Stoilov and Bhattacharyya (Acta Mat 2002). The key ingredient in the model is the recognition of martensitic variants as separate phases in a MSMA domain. Evolution of the interface between these phases is taken as an indicator of the process of reorientation in progress. A formulation of the Helmholtz free energy potential based on Ising model has been derived. The implications of the external magnetic field on the initiation of phase transformation are studied for various mechanical loading modes.
Modeling of two-phase magnetic materials based on Jiles-Atherton theory of hysteresis
NASA Astrophysics Data System (ADS)
Raghunathan, A.; Melikhov, Y.; Snyder, J. E.; Jiles, D. C.
2012-01-01
The Jiles-Atherton (JA) theory of hysteresis has been extended in the present paper to model hysteresis in two-phase magnetic materials. Two-phase materials are those that exhibit two magnetic phases in one hysteresis cycle: one at lower fields and the other at higher fields. In magnetic hysteresis, the transition from one phase to the other i.e. low field phase to high field phase depends mainly on the exchange field. Hence, the material-dependent microstructural parameters of JA theory: spontaneous magnetization, MS, pinning factor, k, domain density, a, domain coupling, α, and reversibility factor, c, are represented as functions of the exchange field. Several cases based on this model have been discussed and compared with the measured data from existing literature. The shapes of the calculated and measured hysteresis loops are in excellent agreement.
Spin-Wave Theory for the Dynamics Induced by Direct Currents in Magnetic Multilayers
NASA Astrophysics Data System (ADS)
Rezende, S. M.; de Aguiar, F. M.; Azevedo, A.
2005-01-01
A spin-wave theory is presented for the magnetization dynamics in a ferromagnetic film that is traversed by spin-polarized carriers at high direct-current densities. It is shown that nonlinear effects due to four-magnon interactions arising from dipolar and surface anisotropy energies limit the growth of the driven spin wave and produce shifts in the microwave frequency oscillations. The theory explains quantitatively recent experimental results in nanometric point contacts onto magnetic multilayers showing downward frequency shifts (redshifts) with increasing current, if the external field is on the film plane, and upward shifts (blueshifts), if the field is perpendicular to the film.
Theory and application of maximum magnetic energy in toroidal plasmas
Chu, T.K.
1992-02-01
The magnetic energy in an inductively driven steady-state toroidal plasma is a maximum for a given rate of dissipation of energy (Poynting flux). A purely resistive steady state of the piecewise force-free configuration, however, cannot exist, as the periodic removal of the excess poloidal flux and pressure, due to heating, ruptures the static equilibrium of the partitioning rational surfaces intermittently. The rupture necessitates a plasma with a negative q{prime}/q (as in reverse field pinches and spheromaks) to have the same {alpha} in all its force-free regions and with a positive q{prime}/q (as in tokamaks) to have centrally peaked {alpha}'s.
Theory and application of maximum magnetic energy in toroidal plasmas
Chu, T.K.
1992-02-01
The magnetic energy in an inductively driven steady-state toroidal plasma is a maximum for a given rate of dissipation of energy (Poynting flux). A purely resistive steady state of the piecewise force-free configuration, however, cannot exist, as the periodic removal of the excess poloidal flux and pressure, due to heating, ruptures the static equilibrium of the partitioning rational surfaces intermittently. The rupture necessitates a plasma with a negative q{prime}/q (as in reverse field pinches and spheromaks) to have the same {alpha} in all its force-free regions and with a positive q{prime}/q (as in tokamaks) to have centrally peaked {alpha}`s.
Magnetically insulated theory with both electron and ion flows
Wang Huihui; Meng Lin; Liu Dagang; Liu Laqun; Yang Chao
2012-10-15
Both the ion emission from anode surface and the electron emission from cathode surface may occur in the magnetically insulated transmission line (MITL) with a very high pulsed power and a very large current density. A model for the MITL with both electron and ion flow is developed. In this model, physical quantities (such as space-charge sheath thicknesses and flow currents) in the MITL are theoretically analyzed, and the specific expression for the voltage on the line by the terms of currents is derived. Furthermore, particle-in-cell simulations are carried out to verify the theoretical results.
Theory of pulsed Reaction Yield Detected Magnetic Resonance.
Nasibulov, Egor A; Kulik, Leonid V; Kaptein, Robert; Ivanov, Konstantin L
2012-10-14
We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors. PMID:22930135
Theory of the negative magnetoresistance in magnetic metallic multilayers
Hood, R.Q.; Falicov, L.M. |
1993-04-01
The Boltzman equation is solved for a system consisting of alternating ferromagnetic normal metallic layers. The in-plane conductance of the film is calculated for two configurations: successive ferromagnetic layers aligned parallel and antiparallel to each other. Results explain the giant negative magnetoresistance encountered in these systems when an initial antiparallel arrangement is changed into a parallel configuration by application of an extemal magnetic field. The calculation depends on geometric parameters (the thicknesses of the layers); intrinsic metal parameters (number of conduction electrons, magnetization and effective masses in the layers); bulk sample properties (conductivity relaxation times); and interface scattering properties (diffuse scattering versus potential scattering at the interfaces). It is found that a large negative magnetoresistance requires, in general, considerable asymmetry in the interface scattering for the two spin orienmtions. All qualitative features of the experiments are reproduced. Quantitative agreement can be achieved with sensible values of the parameters. The effect can be conceptually explained based on considerations of phase-space availability for an electron of a given spin orientation as it travels through the multilayer sample in the various configurations and traverses the interfaces.
NASA Astrophysics Data System (ADS)
Milham, Merrill E.
1994-10-01
In this report, relevant parts of the scattering theory for magnetic spheres are presented. Mass extinction coefficients, and the lognormal size distribution are defined. The theory and algorithms for integrating scattering parameters over size distributions are developed. The integrations are carried out in terms of dimensionless scattering, and size distribution parameters, which are simply related to the usual mass scattering coefficients. Fortran codes, which implement the algorithmic design, are presented, and examples of code use are given. Code listings are included.
Linearized kinetic theory of spin-1/2 particles in magnetized plasmas
Lundin, J.; Brodin, G.
2010-11-15
We have considered linear kinetic theory, including the electron-spin properties in a magnetized plasma. The starting point is a mean-field Vlasov-like equation, derived from a fully quantum-mechanical treatment, where effects from the electron-spin precession and the magnetic dipole force are taken into account. The general conductivity tensor is derived, including both the free current contribution and the magnetization current associated with the spin contribution. We conclude the paper with an extensive discussion of the quantum-mechanical boundary where we list parameter conditions that must be satisfied for various quantum effects to be influential.
Yang, Jiehui; Ma, Shengcan; Xu, You
2009-03-01
The pronounced anisotropy of the magnetization caused by the Ho(3+) ions in the ferrimagnetic holmium iron garnet has been investigated based on quantum theory. The strong anisotropy of the magnetization of the Ho(3+) ions originates mainly from the effect of the crystal field upon the Ho(3+) ions and the anisotropic Ho(3+)-Fe(3+) superexchange interaction. Following the expression of the Yb(3+)-Fe(3+) exchange interaction used by Alben, the anisotropy of the Ho(3+)-Fe(3+) exchange interaction is defined by three principal values of the exchange tensor G. Because the six Ho(3+) sublattices are magnetically non-equivalent, we calculate the magnetic quantities of the Ho(3+) at the six sublattices and compare the average value of the so-obtained six quantities with the measured values. The calculated results are in good agreement with experiments. An interpretation on the anisotropy of the magnetic properties of HoIG is given. PMID:21817410
Electromagnetic fluctuations in magnetized plasmas. I. The rigorous relativistic kinetic theory
Schlickeiser, R. E-mail: yoonp@umd.edu; Yoon, P. H. E-mail: yoonp@umd.edu
2015-07-15
Using the system of the Klimontovich and Maxwell equations, the general linear fluctuation theory for magnetized plasmas is developed. General expressions for the electromagnetic fluctuation spectra (electric and magnetic fields) from uncorrelated plasma particles in plasmas with a uniform magnetic field are derived, which are covariantly correct within the theory of special relativity. The general fluctuation spectra hold for plasmas of arbitrary composition, arbitrary momentum dependences of the plasma particle distribution functions, and arbitrary orientations of the wave vector with respect to the uniform magnetic field. Moreover, no restrictions on the values of the real and the imaginary parts of the frequency are made. The derived fluctuation spectra apply to both non-collective fluctuations and collective plasma eigenmodes in magnetized plasmas. In the latter case, kinetic equations for the components of fluctuating electric and magnetic fields in magnetized plasmas are derived that include the effect of spontaneous emission and absorption. In the limiting case of an unmagnetized plasmas, the general fluctuation spectra correctly reduce to the unmagnetized fluctuation spectra derived before.
Roles of Magnetic Reconnection and Developments of Modern Theory^*
NASA Astrophysics Data System (ADS)
Coppi, B.
2007-11-01
The role of reconnection was recognized in Solar and Space Physics and auroral substorms were suggested to originate in the night-side of the Earth's magnetosphere as a result collisionless reconnectionootnotetextB. Coppi, Nature 205, 998 (1965). well before the kind of modern theory employed for this became applied to laboratory plasmas. Experiments have reached low collisionality regimes where, like in space plasmas, the features of the electron distribution and in particular of the electron temperature gradient become important and the factors contributing to the electron thermal energy balance equation (transverse thermal and longitudinal diffusivities, or electron Landau dampingootnotetextB. Coppi, J.W.-K. Mark, L. Sugiyama, G. Bertin, Phys. Rev. Letters 42, 1058 (1978) and J. Drake, et al., Phys. Fluids 26, 2509 (1983). play a key role. For this an asymptotic theory of modes producing macroscopic islands has been developed involving 3 regions, the innermost one related to finite resistivity and the intermediate one to the finite ratio of the to thermal conductivitiesootnotetextB. Coppi, C. Crabtree, and V. Roytershteyn contribution to Paper TH/R2-19, I.A.E.A. Conference 2006.,^4. A background of excited micro-reconnecting modes, driven by the electron temperature gradient, is considered to make this ratio significantootnotetextB. Coppi, in``Collective Phenomena in Macroscopic Systems'' Eds. G. Bertin et al. (World Scientific, 2007) MIT-LNS Report 06/11(2006). ^*Supported in part by the US D.O.E.
Kinetic theory of a two-dimensional magnetized plasma.
NASA Technical Reports Server (NTRS)
Vahala, G.; Montgomery, D.
1971-01-01
Several features of the equilibrium and nonequilibrium statistical mechanics of a two-dimensional plasma in a uniform dc magnetic field are investigated. The charges are assumed to interact only through electrostatic potentials. The problem is considered both with and without the guiding-center approximation. With the guiding-center approximation, an appropriate Liouville equation and BBGKY hierarchy predict no approach to thermal equilibrium for the spatially uniform case. For the spatially nonuniform situation, a guiding-center Vlasov equation is discussed and solved in special cases. For the nonequilibrium, nonguiding-center case, a Boltzmann equation, and a Fokker-Planck equation are derived in the appropriate limits. The latter is more tractable than the former, and can be shown to obey conservation laws and an H-theorem, but contains a divergent integral which must be cut off on physical grounds. Several unsolved problems are posed.
Theory of the jitter radiation in a magnetized plasma accompanying a temperature gradient
NASA Astrophysics Data System (ADS)
Hattori, Makoto; Fujiki, Kazushiro
2016-04-01
The linear stability of a magnetized plasma accompanying a temperature gradient is reexamined by using plasma kinetic theory. We propose that the anisotropic velocity distribution function should be decomposed into two components. One is proportional to the temperature gradient parallel to the background magnetic field. The other is proportional to the temperature gradient perpendicular to the background magnetic field. Since the amplitude of the anisotropic velocity distribution function is proportional to the heat conductivity, and the heat conductivity perpendicular to the magnetic field is strongly reduced, the first component of the anisotropic velocity distribution function is predominant. The anisotropic velocity distribution function induced by the temperature gradient along the background magnetic field drives plasma kinetic instability and circular polarized magnetic plasma waves are excited. We show that the instability is almost identical to the Weibel instability in the weakly magnetized plasma. However, in the case of the instability caused by the temperature gradient, whether wave vectors of modes are parallel to or antiparallel to the background magnetic field, the growth rate of one mode is suppressed and the growth rate of the other mode is enhanced due to the background magnetic field. In the strongly magnetized plasma, one mode is stabilized and only one of the modes remains unstable. The formulae for the jitter radiation spectrum emitted by relativistic electrons when they travel through the magnetized plasma with the plasma waves driven by the instability are deduced at the first time. We show that the synchrotron emission and the jitter radiation are simultaneously emitted from the same relativistic electron. The jitter radiation is expected to be circularly polarized but with a very small polarization degree since almost the same amounts of left-handed and right-handed circular polarized magnetic waves are excited by the instability.
Bianchi type-I magnetized radiating cosmological model in self creation theory of gravitation
NASA Astrophysics Data System (ADS)
Jain, Vimal Chand; Jain, Nikhil
2015-06-01
We have investigated Bianchi type-I cosmological model in the presence of magnetized field with disordered radiation in Barber's second self-creation theory of gravitation. To obtain exact solution we assume that the component of shear tensor is proportional to expansion ( θ). Some geometrical and physical properties of the model have also been discussed.
Galvao, R. A.; Ziebell, L. F.
2012-09-15
In this work, we detail the derivation of a plasma kinetic theory leading to the components of the dielectric tensor for a magnetized dusty plasma with variable charge on the dust particles, considering that the dust component of the plasma contains spherical dust particles with different sizes, which are charged both by inelastic collisions of electrons and ions and by photoionization.
Ginzburg-Landau theory for skyrmions in inversion-symmetric magnets with competing interactions
Lin, Shi-Zeng; Hayami, Satoru
2016-02-01
Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. This description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axismore » spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the “spin” of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of freedoms of an individual skyrmion can couple to each other, and give rise to unusual behavior that is absent for the skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. Finally, the derived Ginzburg-Landau theory provides a convenient and general framework to discuss skyrmion physics and will facilitate the search for skyrmions in inversion-symmetric magnets.« less
Ginzburg-Landau theory for skyrmions in inversion-symmetric magnets with competing interactions
NASA Astrophysics Data System (ADS)
Lin, Shi-Zeng; Hayami, Satoru
2016-02-01
Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. Such a description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axis spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the "spin" of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of freedoms of an individual skyrmion can couple to each other, and give rise to unusual behavior that is absent for the skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. The derived Ginzburg-Landau theory provides a convenient and general framework to discuss skyrmion physics and will facilitate the search for skyrmions in inversion-symmetric magnets.
Theory and simulations of electron vortices generated by magnetic pushing
NASA Astrophysics Data System (ADS)
Richardson, A. S.; Angus, J. R.; Swanekamp, S. B.; Ottinger, P. F.; Schumer, J. W.
2013-08-01
Vortex formation and propagation are observed in kinetic particle-in-cell (PIC) simulations of magnetic pushing in the plasma opening switch. These vortices are studied here within the electron-magnetohydrodynamic (EMHD) approximation using detailed analytical modeling. PIC simulations of these vortices have also been performed. Strong v ×B forces in the vortices give rise to significant charge separation, which necessitates the use of the EMHD approximation in which ions are fixed and the electrons are treated as a fluid. A semi-analytic model of the vortex structure is derived, and then used as an initial condition for PIC simulations. Density-gradient-dependent vortex propagation is then examined using a series of PIC simulations. It is found that the vortex propagation speed is proportional to the Hall speed vHall≡cB0/4πneeLn. When ions are allowed to move, PIC simulations show that the electric field in the vortex can accelerate plasma ions, which leads to dissipation of the vortex. This electric field contributes to the separation of ion species that has been observed to occur in pulsed-power experiments with a plasma-opening switch.
Stoner-like theory of Magnetism in Silicon MOSFETs
NASA Astrophysics Data System (ADS)
Golosov, Denis
We consider quasi-two-dimensional gas of electrons in a typical Si-MOSFET, assuming contact repulsive interaction between electrons. Magnetisation and susceptibility are evaluated within the mean-field approach. The finite thickness of inversion layer results in an interaction-induced electron wave function change, not found in both purely two-dimensional and three-dimensional (bulk) cases. Taking this self-consistent change into account leads to an increased susceptibility and ultimately to a ferromagnetic transition deep in the high-density metallic regime. We further find that in the paramagnetic state, magnetisation increases sublinearly with increasing in-plane magnetic field. In the opposite limit of low carrier densities, the effects of long-range interaction become important and can be included phenomenologically via bandwidth renormalisation. Our treatment then suggests that with decreasing density, the metal-insulator transition is preceded by a ferromagnetic instability. We discuss the validity of our mean-field scheme, and relate the results to the available experimental data. Supported by Israeli Absorption Ministry.
Stochastic field-line wandering in magnetic turbulence with shear. I. Quasi-linear theory
NASA Astrophysics Data System (ADS)
Shalchi, A.; Negrea, M.; Petrisor, I.
2016-07-01
We investigate the random walk of magnetic field lines in magnetic turbulence with shear. In the first part of the series, we develop a quasi-linear theory in order to compute the diffusion coefficient of magnetic field lines. We derive general formulas for the diffusion coefficients in the different directions of space. We like to emphasize that we expect that quasi-linear theory is only valid if the so-called Kubo number is small. We consider two turbulence models as examples, namely, a noisy slab model as well as a Gaussian decorrelation model. For both models we compute the field line diffusion coefficients and we show how they depend on the aforementioned Kubo number as well as a shear parameter. It is demonstrated that the shear effect reduces all field line diffusion coefficients.
Implant assisted-magnetic drug targeting: Comparison of in vitro experiments with theory
NASA Astrophysics Data System (ADS)
Avilés, Misael O.; Ebner, Armin D.; Ritter, James A.
Implant assisted-magnetic drug targeting (IA-MDT) was studied both in vitro and theoretically, with extensive comparisons made between model and experiment. Magnetic drug carrier particles (MDCPs) comprised of magnetite encased in a polymer were collected magnetically using a ferromagnetic, coiled, wire stent as the implant and a NdFeB permanent magnet for the applied magnetic field. A 2-D mathematical model with no adjustable parameters was developed and compared to the 3-D experimental results. The effects of the fluid velocity, stent and MDCP properties, and magnetic field strength on the performance of the system were evaluated in terms of the capture efficiency (CE) of the MDCPs. In nearly all cases, the parametric trends predicted by the model were in good agreement with the experimental results: the CE always increased with decreasing velocity, increasing magnetic field strength, increasing MDCP size or magnetite content, or increasing wire size. The only exception was when experiments showed an increase in the CE with an increase in the number of loops in the wire, while the model showed no dependence. The discrepancies between experiment and theory were attributed to phenomena not accounted for by the model, such as 3-D to 2-D geometric and magnetic field orientation differences, and interparticle interactions between the MDCPs that lead to magnetic agglomeration and shearing force effects. Overall, this work showed the effectiveness of a stent-based IA-MDT system through both in vitro experimentation and corroborated theory, with the designs of the ferromagnetic wire and the MDCPs both being paramount to the CE.
On the theory of domain structure in ferromagnetic phase of diluted magnetic semiconductors
NASA Astrophysics Data System (ADS)
Stephanovich, V. A.
2006-09-01
We present a comprehensive analysis of domain structure formation in ferromagnetic phase of diluted magnetic semiconductors (DMS) of p-type. Our analysis is carried out on the base of effective magnetic free energy of DMS calculated by us earlier [Yu.G. Semenov, V.A. Stephanovich, Phys. Rev. B 67 (2003) 195203]. This free energy, substituting DMS (a disordered magnet) by effective ordered substance, permits to apply the standard phenomenological approach to domain structure calculation. Using coupled system of Maxwell equations with those obtained by minimization of above free energy functional, we show the existence of critical ratio ν of concentration of charge carriers and magnetic ions such that sample critical thickness L (such that at L
A compact theory of magnetic nerve stimulation: predicting how to aim
2014-01-01
Background A compact theory that predicts quantitatively when and where magnetic neurostimulation will occur is needed as a guide to therapy, ideally providing a single equation that defines the target volume of tissue excited by single or dual coils. Methods A first-principles analysis of magnetic stimulation incorporating a simplified description of electromagnetic fields and a simplified cable theory of the axon yields a mathematical synthesis predicting how to aim. Results Nerve stimulation produced by a single circular coil having one or more closely packed turns occurs in donut shaped volume of tissue beneath the coil. Axons spanning several millimeters are the sites of magnetic stimulation. The sites of maximal transmembrane depolarization in nerve fibers correspond to points where the axons enter or exit this volume of magnetically induced voltage and current. The axonal membrane at one end is depolarized locally during the rising phase of current in the coil. The axonal membrane at the opposite end is depolarized locally during the falling phase of current in the coil. Penetration depths of several centimeters from the skin surface or approximately one to two coil radii are practical. With two coils placed in a figure-of-eight configuration the separate clockwise and counterclockwise currents generate magnetic fields that add, producing maximal stimulation of a spindle shaped volume, centered at a depth of one-third to one-half coil radius from the body surface. Conclusions This condensed synthesis of electromagnetic theory and cable theories of axon physiology provides a partial solution to the targeting problem in peripheral and in transcranial magnetic stimulation. PMID:24885299
Itinerant Electron Magnets: Curie Temperature and Susceptibility in Density-Functional Theory
NASA Astrophysics Data System (ADS)
Mohn, Peter; Khmelevskyi, Sergei
Groundstate properties of solids are astonishingly well described by the local density functional approximation (LDA) [1]. This is also true of metallic magnets for which the situation was recently described by this author [2]. Excited-state properties of magnets (and other systems), however, are still a great challenge and it was believed until recently that the band-picture, based in the LDA, fails entirely in describing magnetism at elevated temperatures. We emphasize here that this is not so, attempting first to expose the reason why it was thought that the band-picture fails. Since historically the underlying physical picture was developed by Stoner and Wohlfarth, we begin with a discussion of their theory using, however, an approach that reveals the essential assumptions. This is Mermin's [3] finite-temperature density functional theory. Two points emerge: one is the essential noncollinearity of the magnetic moments at finite temperatures, the other is the form of the exchange-corr elation contribution to the thermodynamic potential. We know how to deal with noncollinear order in the LDA and we explain how we might use this knowledge to advance the issue. Exchange and correlation at finite temperatures are, however, at the present state not well understood. This statement not only applies to density functional theory but also to many-body treatments addressed at this workshop. We will show in particular that the theory of magnons in the band-LDA-picture at low temperatures is in good shape. At high temperatures we opt for a theory involving spin fluctuations and argue that, although broad features of the magnetic phase transition are described satisfactorily, many details await further improvements.
Basic theory for polarized, astrophysical maser radiation in a magnetic field
NASA Technical Reports Server (NTRS)
Watson, William D.
1994-01-01
Fundamental alterations in the theory and resulting behavior of polarized, astrophysical maser radiation in the presence of a magnetic field have been asserted based on a calculation of instabilities in the radiative transfer. I reconsider the radiative transfer and find that the relevant instabilities do not occur. Calculational errors in the previous investigation are identified. In addition, such instabilities would have appeared -- but did not -- in the numerous numerical solutions to the same radiative transfer equations that have been presented in the literature. As a result, all modifications that have been presented in a recent series of papers (Elitzur 1991, 1993) to the theory for polarized maser radiation in the presence of a magnetic field are invalid. The basic theory is thus clarified.
NASA Astrophysics Data System (ADS)
Solontsov, A.
2015-06-01
The paper critically overviews the recent developments of the theory of spatially dispersive spin fluctuations (SF) in itinerant electron magnetism with particular emphasis on spin-fluctuation coupling or spin anharmonicity. It is argued that the conventional self-consistent renormalized (SCR) theory of spin fluctuations is usually used aside of the range of its applicability actually defined by the constraint of weak spin anharmonicity based on the random phase approximation (RPA) arguments. An essential step in understanding SF in itinerant magnets beyond RPA-like arguments was made recently within the soft-mode theory of SF accounting for strong spin anharmonicity caused by zero-point SF. In the present paper we generalize it to apply for a wider range of temperatures and regimes of SF and show it to lead to qualitatively new results caused by zero-point effects.
NASA Astrophysics Data System (ADS)
Ruffolo, D. J.; Snodin, A. P.; Oughton, S.; Servidio, S.; Matthaeus, W. H.
2013-12-01
The random walk of magnetic field lines is examined analytically and numerically in the context of reduced magnetohydrodynamic (RMHD) turbulence, which provides a useful description of plasmas dominated by a strong mean field, such as in the solar corona. A nonperturbative theory of magnetic field line diffusion [1] is compared with the diffusion coefficients obtained by accurate numerical tracing of magnetic field lines for both synthetic models and direct numerical simulations of RMHD. Statistical analysis of an ensemble of trajectories confirms the applicability of the theory, which very closely matches the numerical field line diffusion coefficient as a function of distance z along the mean magnetic field for a wide range of the Kubo number R. The theory employs Corrsin's independence hypothesis, sometimes thought to be valid only at low R. However, the results demonstrate that it works well up to R=10, both for a synthetic RMHD model and an RMHD simulation. The numerical results from RMHD simulation are compared with and without phase randomization, demonstrating an effect of coherent structures on the field line random walk for low Kubo number. Partially supported by a postdoctoral fellowship from Mahidol University, the Thailand Research Fund, POR Calabria FSE-2007/2013, the US NSF (AGS-1063439 and SHINE AGS-1156094), NASA (Heliophysics Theory NNX08AI47G & NNX11AJ44G), by the Solar Probe Plus Project through the ISIS Theory team, by the MMS Theory and Modeling team, and by EU Marie Curie Project FP7 PIRSES-2010-269297 'Turboplasmas' at Università della Calabria. [1] D. Ruffolo and W. H. Matthaeus, Phys. Plasmas, 20, 012308 (2013).
Casanova, S.; Schlickeiser, R.
2012-02-01
Recently, a new transport theory of cosmic rays in magnetized space plasmas extending the quasilinear approximation to the particle orbit has been developed for the case of an axisymmetric incompressible magnetic turbulence. Here, we generalize the approach to the important physical case of a compressible plasma. As previously obtained in the case of an incompressible plasma, we allow arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field. For the case of quasi-stationary and spatially homogeneous magnetic turbulence we derive, in the small Larmor radius approximation, gyrophase-averaged cosmic-ray Fokker-Planck coefficients. Upper limits for the perpendicular and pitch-angle Fokker-Planck coefficients and for the perpendicular and parallel spatial diffusion coefficients are presented.
"Treasure maps" for magnetic high-entropy-alloys from theory and experiment
NASA Astrophysics Data System (ADS)
Körmann, F.; Ma, D.; Belyea, D. D.; Lucas, M. S.; Miller, C. W.; Grabowski, B.; Sluiter, M. H. F.
2015-10-01
The critical temperature and saturation magnetization for four- and five-component FCC transition metal alloys are predicted using a formalism that combines density functional theory and a magnetic mean-field model. Our theoretical results are in excellent agreement with experimental data presented in both this work and in the literature. The generality and power of this approach allow us to computationally design alloys with well-defined magnetic properties. Among other alloys, the method is applied to CoCrFeNiPd alloys, which have attracted attention recently for potential magnetic applications. The computational framework is able to predict the experimentally measured TC and to explore the dominant mechanisms for alloying trends with Pd. A wide range of ferromagnetic properties and Curie temperatures near room temperature in hitherto unexplored alloys is predicted in which Pd is replaced in varying degrees by, e.g., Ag, Au, and Cu.
Effects of simulated cosmological magnetic fields on the galaxy population
NASA Astrophysics Data System (ADS)
Marinacci, Federico; Vogelsberger, Mark
2016-02-01
We investigate the effects of varying the intensity of the primordial magnetic seed field on the global properties of the galaxy population in ideal magnetohydrodynamic cosmological simulations performed with the moving-mesh code AREPO. We vary the seed field in our calculations in a range of values still compatible with the current cosmological upper limits. We show that above a critical intensity of ≃10-9 G, the additional pressure arising from the field strongly affects the evolution of gaseous structures, leading to a suppression of the cosmic star formation history, which is stronger for larger seed fields. This directly reflects into a lower total galaxy count above a fixed stellar mass threshold at all redshifts, and a lower galaxy number density at fixed stellar mass and a less massive stellar component at fixed virial mass at all mass scales. These signatures may be used, in addition to the existing methods, to derive tighter constraints on primordial magnetic seed field intensities.
Theory and Validation of Magnetic Resonance Fluid Motion Estimation Using Intensity Flow Data
Wong, Kelvin Kian Loong; Kelso, Richard Malcolm; Worthley, Stephen Grant; Sanders, Prashanthan; Mazumdar, Jagannath; Abbott, Derek
2009-01-01
Background Motion tracking based on spatial-temporal radio-frequency signals from the pixel representation of magnetic resonance (MR) imaging of a non-stationary fluid is able to provide two dimensional vector field maps. This supports the underlying fundamentals of magnetic resonance fluid motion estimation and generates a new methodology for flow measurement that is based on registration of nuclear signals from moving hydrogen nuclei in fluid. However, there is a need to validate the computational aspect of the approach by using velocity flow field data that we will assume as the true reference information or ground truth. Methodology/Principal Findings In this study, we create flow vectors based on an ideal analytical vortex, and generate artificial signal-motion image data to verify our computational approach. The analytical and computed flow fields are compared to provide an error estimate of our methodology. The comparison shows that the fluid motion estimation approach using simulated MR data is accurate and robust enough for flow field mapping. To verify our methodology, we have tested the computational configuration on magnetic resonance images of cardiac blood and proved that the theory of magnetic resonance fluid motion estimation can be applicable practically. Conclusions/Significance The results of this work will allow us to progress further in the investigation of fluid motion prediction based on imaging modalities that do not require velocity encoding. This article describes a novel theory of motion estimation based on magnetic resonating blood, which may be directly applied to cardiac flow imaging. PMID:19270756
NASA Astrophysics Data System (ADS)
Jiles, David; Raghunathan, Arun; Melikhov, Yevgen; Snyder, John
2010-03-01
The Jiles-Atherton (JA) theory explains the ferromagnetic hysteresis through contributions of irreversible and reversible magnetization components [1]. Anhysteretic magnetization function, a function of energy of the moments in a domain, forms a basic building block of this model. This function has known forms for specific cases of anisotropy: axially anisotropic (one-dimensional), planar anisotropic (two-dimensional), and isotropic (three-dimensional) [1, 2]. Hence there is a need to generalize anhysteretic magnetization function to extend JA theory to other forms of anisotropy. In this work, a functional form of anhysteretic magnetization function has been derived. It was shown that this functional form of anhysteretic magnetization with necessary boundary conditions can be reduced to the familiar specific model equations in the particular cases. This work extends the applicability of the JA model to systems with various anisotropy dependences. This research was supported by the UK EPSRC (EP/D057094) and the US NSF (DMR-0402716). [1] D. C. Jiles et. al., JMMM. 61, 48 (1986). [2] Y. M. Shi et. al., JMMM. 187, 75 (1998).
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; Klepeis, J. E.
2015-10-30
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure andmore » (ii) the disordered-local-moment (DLM) method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, However, the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.« less
From liquid crystal models to the guiding-center theory of magnetized plasmas
NASA Astrophysics Data System (ADS)
Tronci, Cesare
2016-08-01
Upon combining Northrop's picture of charged particle motion with modern liquid crystal theories, this paper provides a new description of guiding center dynamics (to lowest order). This new perspective is based on a rotation gauge field (gyrogauge) that encodes rotations around the magnetic field. In liquid crystal theory, an analogue rotation field is used to encode the rotational state of rod-like molecules. Instead of resorting to sophisticated tools (e.g. Hamiltonian perturbation theory and Lie series expansions) that still remain essential in higher-order gyrokinetics, the present approach combines the WKB method with a simple kinematical ansatz, which is then replaced into the charged particle Lagrangian. The latter is eventually averaged over the gyrophase to produce the guiding-center equations. A crucial role is played by the vector potential for the gyrogauge field. A similar vector potential is related to liquid crystal defects and is known as wryness tensor in Eringen's micropolar theory.
Cosmological perturbations: Vorticity, isocurvature and magnetic fields
NASA Astrophysics Data System (ADS)
Christopherson, Adam J.
2014-10-01
In this paper, I review some recent, interlinked, work undertaken using cosmological perturbation theory — a powerful technique for modeling inhomogeneities in the universe. The common theme which underpins these pieces of work is the presence of nonadiabatic pressure, or entropy, perturbations. After a brief introduction covering the standard techniques of describing inhomogeneities in both Newtonian and relativistic cosmology, I discuss the generation of vorticity. As in classical fluid mechanics, vorticity is not present in linearized perturbation theory (unless included as an initial condition). Allowing for entropy perturbations, and working to second order in perturbation theory, I show that vorticity is generated, even in the absence of vector perturbations, by purely scalar perturbations, the source term being quadratic in the gradients of first order energy density and isocurvature, or nonadiabatic pressure perturbations. This generalizes Crocco's theorem to a cosmological setting. I then introduce isocurvature perturbations in different models, focusing on the entropy perturbation in standard, concordance cosmology, and in inflationary models involving two scalar fields. As the final topic, I investigate magnetic fields, which are a potential observational consequence of vorticity in the early universe. I briefly review some recent work on including magnetic fields in perturbation theory in a consistent way. I show, using solely analytical techniques, that magnetic fields can be generated by higher order perturbations, albeit too small to provide the entire primordial seed field, in agreement with some numerical studies. I close this paper with a summary and some potential extensions of this work.
Sketch of a unifying auroral theory. [based on magnetic mirroring of magnetic disturbances
NASA Technical Reports Server (NTRS)
Lennartsson, W.
1975-01-01
On the basis of field and particle observations, it is suggested that a bright auroral display is a part of a magnetosphere-ionosphere current system which is fed by a charge-separation process in the outer magnetosphere (or the solar wind). The upward magnetic-field-aligned current is flowing out of the display, carried mainly by down-flowing electrons from the hot-particle populations in the outer magnetosphere (the ambient cold electrons being depleted at high altitudes). As a result of the magnetic mirroring of these downflowing current carriers, a large potential drop is set up along the magnetic field, increasing both the number flux and the kinetic energy of the precipitating electrons. It is found that this simple basic model, when combined with wave-particle interactions, may be able to explain a highly diversified selection of auroral particle observations. It may thus be possible to explain both inverted-V events and auroral rays in terms of a static parallel electric field, and the electric field may be compatible with a strongly variable pitch-angle distribution of the precipitating electrons, including distributions peaked at 90 deg as well as 0 deg. This model may also provide a simple explanation of the simultaneous precipitation of electrons and collimated positive ions.
Theory of magnetically insulated electron flows in coaxial pulsed power transmission lines
NASA Astrophysics Data System (ADS)
Lawconnell, Robert I.; Neri, Jesse
1990-03-01
The Cartesian magnetically insulated transmission line (MITL) theory of Mendel et al. [Appl. Phys. 50, 3830 (1979); Phys. Fluids 26, 3628 (1983)] is extended to cylindrical coordinates. A set of equations that describe arbitrary electron flows in cylindrical coordinates is presented. These equations are used to derive a general theory for laminar magnetically insulated electron flows. The laminar theory allows one to specify the potentials, fields, and densities across a coaxial line undergoing explosive electron emission at the cathode. The theory is different from others available in cylindrical coordinates in that the canonical momentum and total energy for each electron may be nonzero across the electron sheath. A nonzero canonical momentum and total energy for the electrons in the sheath allows the model to produce one-dimensional flows that resemble flows from lines with impedance mismatches and perturbing structures. The laminar theory is used to derive two new self-consistent cylindrical flow solutions: (1) for a constant density profile and (2) for a quadratic density profile of the form ρ=ρc[(r2m-r2)/(r2m-r2c)]. This profile is of interest in that it is similar to profiles observed in a long MITL simulation [Appl. Phys. 50, 4996 (1979)]. The theoretical flows are compared to numerical results obtained with two-dimensional (2-D) electromagnetic particle-in-cell (PIC) codes.
Electronic and magnetic structure of transition-metal carbodiimides by means of GGA+U theory.
Xiang, Hongping; Dronskowski, Richard; Eck, Bernhard; Tchougréeff, Andrei L
2010-11-25
The electronic structures and magnetic properties of MNCN (M = Fe, Co, and Ni) have been investigated by density-functional theory including explicit electronic correlation through an ad hoc Coulomb potential (GGA+U). The results evidence CoNCN and NiNCN as type-II anti-ferromagnetic semiconductors (that is, intralayer ferromagnetic and interlayer anti-ferromagnetic), in accordance with experimental observations. Just like the prototype MnNCN, the MNCN phases, with M = Ni and Co, thus resemble the corresponding MO monoxides with respect to their magnetic and transport properties. By contrast, FeNCN remains (semi)metallic even upon applying a strong Coulomb correlation potential. This, most probably, is in contradiction with its observed optical transparency and expected insulating behavior and points toward a serious density-functional theory problem. PMID:21038908
NASA Astrophysics Data System (ADS)
Nekrasov, A. K.; Shadmehri, Mohsen
2011-06-01
We investigate electromagnetic buoyancy instabilities of the electron-ion plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations, but using the multicomponent plasma approach when the momentum equations are solved for each species. We consider a geometry in which the background magnetic field, gravity, and stratification are directed along one axis. The nonzero background electron thermal flux is taken into account. Collisions between electrons and ions are included in the momentum equations. No simplifications usual for the one-fluid MHD-approach in studying these instabilities are used. We derive a simple dispersion relation, which shows that the thermal flux perturbation generally stabilizes an instability for the geometry under consideration. This result contradicts to conclusion obtained in the MHD-approach. We show that the reason of this contradiction is the simplified assumptions used in the MHD analysis of buoyancy instabilities and the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. Our dispersion relation also shows that the medium with the electron thermal flux can be unstable, if the temperature gradients of ions and electrons have the opposite signs. The results obtained can be applied to the weakly collisional magnetized plasma objects in laboratory and astrophysics.
Coupled-cluster theory for atoms and molecules in strong magnetic fields
Stopkowicz, Stella Lange, Kai K.; Tellgren, Erik I.; Helgaker, Trygve; Gauss, Jürgen
2015-08-21
An implementation of coupled-cluster (CC) theory to treat atoms and molecules in finite magnetic fields is presented. The main challenges for the implementation stem from the magnetic-field dependence in the Hamiltonian, or, more precisely, the appearance of the angular momentum operator, due to which the wave function becomes complex and which introduces a gauge-origin dependence. For this reason, an implementation of a complex CC code is required together with the use of gauge-including atomic orbitals to ensure gauge-origin independence. Results of coupled-cluster singles–doubles–perturbative-triples (CCSD(T)) calculations are presented for atoms and molecules with a focus on the dependence of correlation and binding energies on the magnetic field.
Electron theory of perpendicular magnetic anisotropy of Co-ferrite thin films
Inoue, Jun-ichiro; Yanagihara, Hideto; Kita, Eiji; Niizeki, Tomohiko; AIMR, Tohoku University, Sendai 980-8577 ; Itoh, Hiroyoshi
2014-02-15
We develop an electron theory for the t{sub 2g} electrons of Co{sup 2+} ions to clarify the perpendicular magnetic anisotropy (PMA) mechanism of Co-ferrite thin films by considering the spin-orbit interaction (SOI) and crystal-field (CF) potentials induced by the local symmetry around the Co ions and the global tetragonal symmetry of the film. Uniaxial and in-plane MA constants K{sub u} and K{sub 1} at 0 K, respectively, are calculated for various values of SOI and CF. We show that reasonable parameter values explain the observed PMA and that the orbital moment for the in-plane magnetization reduces to nearly half of that of the out-of-plane magnetization.
Coupled-cluster theory for atoms and molecules in strong magnetic fields.
Stopkowicz, Stella; Gauss, Jürgen; Lange, Kai K; Tellgren, Erik I; Helgaker, Trygve
2015-08-21
An implementation of coupled-cluster (CC) theory to treat atoms and molecules in finite magnetic fields is presented. The main challenges for the implementation stem from the magnetic-field dependence in the Hamiltonian, or, more precisely, the appearance of the angular momentum operator, due to which the wave function becomes complex and which introduces a gauge-origin dependence. For this reason, an implementation of a complex CC code is required together with the use of gauge-including atomic orbitals to ensure gauge-origin independence. Results of coupled-cluster singles-doubles-perturbative-triples (CCSD(T)) calculations are presented for atoms and molecules with a focus on the dependence of correlation and binding energies on the magnetic field. PMID:26298118
Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory
Wohlfeld, K.; Chen, Cheng-Chien; van Veenendaal, M. ; Devereaux, T. P.
2015-02-01
Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation [Phys. Rev. B 91, 165102 (2015)], we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstances be regarded as more complex than the physics of a spin-orbital chain.
Generalized form of anhysteretic magnetization function for Jiles-Atherton theory of hysteresis
NASA Astrophysics Data System (ADS)
Raghunathan, A.; Melikhov, Y.; Snyder, J. E.; Jiles, D. C.
2009-10-01
A generalized form of anhysteretic magnetization function to extend Jiles-Atherton theory to different forms of anisotropy has been derived. The general equation for the function has been compared with those of calculations made on the basis of known equations for specific cases: axially anisotropic (one-dimensional), planar anisotropic (two-dimensional), and isotropic (three-dimensional). The Jiles-Atherton model using the proposed functional form of generalized anhysteretic magnetization function for anisotropy dependence has been validated and the necessary equations derived. It has been shown in this work that this functional form of anhysteretic magnetization with necessary boundary conditions can be reduced to the familiar specific model equations in the particular cases.
Linear spin wave theory for single-Q incommensurate magnetic structures.
Toth, S; Lake, B
2015-04-29
Linear spin wave theory provides the leading term in the calculation of the excitation spectra of long-range ordered magnetic systems as a function of 1/√S. This term is acquired using the Holstein-Primakoff approximation of the spin operator and valid for small δS fluctuations of the ordered moment. We propose an algorithm that allows magnetic ground states with general moment directions and single-Q incommensurate ordering wave vector using a local coordinate transformation for every spin and a rotating coordinate transformation for the incommensurability. Finally we show, how our model can determine the spin wave spectrum of the magnetic C-site langasites with incommensurate order. PMID:25817594
Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory
Wohlfeld, K.; Chen, Cheng-Chien; van Veenendaal, M.; Devereaux, T. P.
2015-02-01
Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation [Phys. Rev. B 91, 165102 (2015)], we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstancesmore » be regarded as more complex than the physics of a spin-orbital chain.« less
Zevenhoven, Koos C. J.; Busch, Sarah; Hatridge, Michael; Öisjöen, Fredrik; Ilmoniemi, Risto J.; Clarke, John
2014-01-01
Eddy currents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field—applied before each signal acquisition sequence to increase the signal—induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures. PMID:24753629
NASA Astrophysics Data System (ADS)
Lawler, Michael
I generalize the theory of phonon topological band structures of isostatic lattices to highly frustrated antiferromagnets. I achieve this with a discovery of a many-body supersymmetry (SUSY) in the phonon problem of balls and springs which also applies to geometrically frustrated magnets. The Witten index of the SUSY model, when restricted to the single body problem (meaningful for linearized phonons), is then shown to be the Calladine-Kane-Lubensky index of mechanical structures that forms the cornerstone of the phonon topological band structure theory. ``Spontaneous supersymmetry breaking'' is then identified as the need to gap all modes in the bulk to create the topological state. The many-body SUSY formulation shows that the topology is not restricted to a band structure problem but extends to systems of coupled bosons and fermions that are in principle also realizable in solid state systems. The analogus supersymmetry of the magnon problem turns out to be particularly useful for highly frustrated magnets with the kagome family of antiferromagnets an analog of topological isostatic lattices. Thus, a solid state realization of the theory of phonon topological band structure may be found in highly frustrated magnets. However, our results show that this topology is protected not
Tartakovskaya, E. V.; Tucker, J. W.; Ivanov, B. A.
2001-06-15
A self-consistent theory of the ground-state nonuniform magnetization distribution in small magnetic nanoelements is proposed, valid for thicknesses much less than the exchange length, and with natural fulfillment of boundary conditions allowing application to a variety of element shapes. The theory is applied to rectangular 2p{sub 1}l{times}2p{sub 2}l{times}2l permalloy elements. In contrast to that of square elements, there exists a range of particle sizes having an {open_quotes}intermediate{close_quotes} ground state (mixed flower and leaf symmetries) with average magnetization inclined at {var_phi} to the longer edge. With increasing p{sub 1}/p{sub 2} (p{sub 2} fixed), {var_phi} gradually decreases to zero (flower state). This intermediate{r_arrow}flower transition is of the second type, unlike the leaf{r_arrow}flower transition (first type) observed in square elements with reduction in p{sub 1}(=p{sub 2}). Simulation results support the analytic theory. {copyright} 2001 American Institute of Physics.
Band Theory for the Electronic and Magnetic Properties of VO2 Phases
NASA Astrophysics Data System (ADS)
Shen, Xiao; Xu, Sheng; Hallman, Kent; Haglund, Richard; Pantelides, Sokrates
VO2 is widely studied for the insulator-metal transition between the monoclinic M1 (insulator) and rutile R (metal) phases. Recent experiments show that in addition to the M1 and R phases, VO2 has a rich phase diagram including a recently identified metallic monoclinic phase, making the material particularly intriguing. The origin of the band gap in the insulating phase of VO2 has been a subject of debate. It was suggested that the insulating phase cannot be described by band theory and thus strong correlations must be invoked. However, recent band calculations using density functional theory (DFT) with a hybrid functional and standard pseudopotentials correctly obtains a band gap for the M1 insulating phase. Subsequent calculations, however, found that the magnetic properties of VO2 phases are not correctly described by such calculations. Here we present DFT calculations using a tuned hybrid functional and hard pseudopotentials that reproduce both the band gaps and the magnetic properties of the known VO2 phases. Thus, it is appropriate to use band theory to describe VO2 phases without invoking strong correlations. Furthermore, using the band theory treatment, we identify a candidate for the metallic monoclinic phase. Doe DE-FG02-09ER46554, NSF EECS-1509740.
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-28
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires. PMID:26219748
Search for grand-unified-theory magnetic monopoles at a flux level below the Parker limit
NASA Astrophysics Data System (ADS)
Price, P. B.; Guo, S.-L.; Ahlen, S. P.; Fleischer, R. L.
1984-04-01
Results are presented from the first directs search for grand-unified-theory magnetic monopoles with adequate sensitivity to detect a flux as small as the Parker flux (Turner et al., 1983). It is pointed out that if stable monopole-nucleus bound states exist, then the observed absence of monopole tracks in the 4.6 x 10 to the 8th-yr-old mica detector places an upper limit of 10 to the -17th to 10 to the-16th/sq cm-sr-sec on the flux of grand-unified-theory monopoles having a velocity of 3 x 10 to the -4th c to 1.5 x 10 to the -3rd c. The scenario treated here has four aspects. The first is that monopoles enter the earth's atmosphere with a net electric charge less than or equal to zero. The second is that as the monopoles pass through the earth they eventually capture nuclei in bound states through magnetic dipole-magnetic monopole interaction. The third is that the nucleus-monopole composite passes through a naturally occurring underground sample of muscovite mica, undergoing elastic nuclear collisions that result in the formation of a trail of lattice defects in the mica. The fourth is that the track survives as long as the mica remains unheated and may be enlarged to macroscopic dimensions by retrieving the mica and etching it in hydrofluoric acid.
Bogoliubov theory of interacting bosons on a lattice in a synthetic magnetic field
Powell, Stephen; Barnett, Ryan; Sensarma, Rajdeep; Das Sarma, Sankar
2011-01-15
We consider theoretically the problem of an artificial gauge potential applied to a cold atomic system of interacting neutral bosons in a tight-binding optical lattice. Using the Bose-Hubbard model, we show that an effective magnetic field leads to superfluid phases with simultaneous spatial order, which we analyze using Bogliubov theory. This gives a consistent expansion in terms of quantum and thermal fluctuations, in which the lowest order gives a Gross-Pitaevskii equation determining the condensate configuration. We apply an analysis based on the magnetic symmetry group to show how the spatial structure of this configuration depends on commensuration between the magnetic field and the lattice. Higher orders describe the quasiparticle excitations, whose spectrum combines the intricacy of the Hofstadter butterfly with the characteristic features of the superfluid phase. We use the depletion of the condensate to determine the range of validity of our approximations and also to find an estimate for the onset of the Mott insulator phase. Our theory provides concrete experimental predictions for both time-of-flight imagery and Bragg spectroscopy.
2+1 dimensional magnetically charged solutions in Einstein-power-Maxwell theory
NASA Astrophysics Data System (ADS)
Mazharimousavi, S. Habib; Gurtug, O.; Halilsoy, M.; Unver, O.
2011-12-01
We obtain a class of magnetically charged solutions in 2+1 dimensional Einstein-Power-Maxwell theory. In the linear Maxwell limit, such horizonless solutions are known to exist. We show that in 3D geometry, black hole solutions with magnetic charge do not exist even if it is sourced by the power-Maxwell field. Physical properties of the solution with particular power k of the Maxwell field is investigated. The true timelike naked curvature singularity develops when k>1 which constitutes one of the striking effects of the power-Maxwell field. For specific power parameter k, the occurrence of a timelike naked singularity is analyzed in the quantum mechanical point of view. Quantum test fields obeying the Klein-Gordon and the Dirac equations are used to probe the singularity. It is shown that the class of static pure magnetic spacetime in the power-Maxwell theory is quantum-mechanically singular when it is probed with fields obeying Klein-Gordon and Dirac equations in the generic case.
Technology Transfer Automated Retrieval System (TEKTRAN)
Slugs are common pests of grass seed fields in western Oregon and are currently controlled using bait pellets that often fail to give adequate protection. Here we demonstrate the loss of bait pellet products to earthworms and its adverse effects on controlling slugs. Three years of field and greenho...
NASA Technical Reports Server (NTRS)
Kulsrud, Russell M.; Anderson, Stephen W.
1992-01-01
The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.
Thermodynamic theory for thermally driven domain wall motion in magnetic nanostructures
NASA Astrophysics Data System (ADS)
Wang, Xiang Rong
2015-03-01
DWs. This theory should be applicable to other spin textures like skyrmions as well since bound spin waves generally exist in spin textures. The theory also naturally explains why the magnetic domain widths decrease with the increase of the temperature, a well-known experimental phenomenon. In collaboration with X.S. Wang, Hong Kong University of Science & Technology. This work was supported by Hong Kong GRF Grant (605413) and the grant from NNSF of China (11374249).
Magnetism in undoped ZnS studied from density functional theory
Xiao, Wen-Zhi E-mail: llwang@hun.edu.cn; Rong, Qing-Yan; Xiao, Gang; Wang, Ling-ling E-mail: llwang@hun.edu.cn; Meng, Bo
2014-06-07
The magnetic property induced by the native defects in ZnS bulk, thin film, and quantum dots are investigated comprehensively based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach. We find the origin of magnetism is closely related to the introduction of hole into ZnS systems. The relative localization of S-3p orbitals is another key to resulting in unpaired p-electron, due to Hund's rule. For almost all the ZnS systems under study, the magnetic moment arises from the S-dangling bonds generated by Zn vacancies. The charge-neutral Zn vacancy, Zn vacancy in 1− charge sate, and S vacancy in the 1+ charge sate produce a local magnetic moment of 2.0, 1.0, and 1.0 μ{sub B}, respectively. The Zn vacancy in the neutral and 1− charge sates are the important cause for the ferromagnetism in ZnS bulk, with a Curie temperature (T{sub C}) above room temperature. For ZnS thin film with clean (111) surfaces, the spins on each surface are ferromagnetically coupled but antiferromagnetically coupled between two surfaces, which is attributable to the internal electric field between the two polar (111) surfaces of the thin film. Only surface Zn vacancies can yield local magnetic moment for ZnS thin film and quantum dot, which is ascribed to the surface effect. Interactions between magnetic moments on S-3p states induced by hole-doping are responsible for the ferromagnetism observed experimentally in various ZnS samples.
Density Functional Theory applied to magnetic materials: Mn3O4 at different hybrid functionals
NASA Astrophysics Data System (ADS)
Ribeiro, R. A. P.; de Lazaro, S. R.; Pianaro, S. A.
2015-10-01
Antiferromagnetic Mn3O4 in spinel structure was investigated employing the Density Functional Theory at different hybrid functionals with default HF exchange percentage. Structural, electronic and magnetic properties were examined. Structural results were in agreement with experimental and Hartree-Fock results showing that the octahedral site was distorted by the Jahn-Teller effect, which changed the electron density distribution. Band-gap results for B3LYP and B3PW hybrid functionals were closer to the experimental when compared to PBE0. Mulliken Population Analysis revealed magnetic moments very close to ideal d4 and d5 electron configurations of Mn3+ and Mn2+, respectively. Electron density maps are useful to determine that oxygen atoms mediate the electron transfer between octahedral and tetrahedral clusters. Magnetic properties were investigated from theoretical results for exchange coupling constants. Intratetrahedral and tetra-octahedral interactions were observed to be antiferromagnetic, whereas, octahedral sites presented antiferromagnetic interactions in the same layer and ferromagnetic in adjacent layers. Results showed that only default B3LYP was successful to describe magnetic properties of antiferromagnetic materials in agreement with experimental results.
NASA Astrophysics Data System (ADS)
Erkan, K.; Jekeli, C.
2009-12-01
Today gravity and magnetic field measurements are acquired in grids with high resolution and accuracy. Magnetic field measurements have already been proven for superior accuracy and practicality. Modern gravity gradiometry instruments have boosted the practicality of gravity field measurements for many subsurface problems. As a result of this, advanced algorithms are needed for quantitative integration of the two fields for a specific subsurface problem. These fields are correlated by Poisson relation as a first order approximation. However, subsurface sources generally show large deviations from the ideal conditions; in this case a generalized Poisson relation may be proposed as a perturbation of the ideal conditions. In this study, we take advantage of the abstraction of the deformation theory between two metric fields, and implement it between the two geophysical fields. In this generalized approach, the different geophysical fields are loosely correlated by Poisson relation; so the calculated deformation reflects the deviations from ideal density/susceptibility relationships for the subsurface structure. The resulting deformation field can then be used for detection of a known target with an expected deformation field. The present method introduces a novel algorithm for integration of the gravity gradiometry and magnetic field data. In this method, the results can be directly interpreted without making individual density and magnetic susceptibility assumptions. The method also intrinsically overcomes the scale problem between the two potential fields.
Theory of isolated, small-scale magnetic islands in a high temperature tokamak plasma
Connor, J.W.; Wilson, H.R.
1995-12-01
A theory for the existence of noninteracting small-scale, ``drift`` magnetic islands in a high temperature tokamak plasma is presented. This situation contrasts with that discussed by Rebut and Hugon [Plasma Phys. Controlled Fusion {bold 33}, 1085 (1991)] which involves a background ``sea`` of magnetic turbulence caused by island overlap. The islands are driven by the effect of finite ion Larmor radius on the particle drifts and they propagate with a velocity comparable to the diamagnetic velocity. In contrast with the work of Smolyakov [Plasma Phys. Controlled Fusion {bold 35}, 657 (1993)] collisions are assumed to be rare. Although the saturated island size is independent of the collision frequency in the model discussed here, collisions play a crucial role in determining the frequency of the magnetic islands. An estimate is made of the anomalous heat transport which results from the fluctuations in the electrostatic potential associated with these magnetic islands. The predicted thermal diffusivity has several, but not all, of the characteristics of the Rebut--Lallia--Watkins transport model. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.
NASA Astrophysics Data System (ADS)
Khaira, Jobanpreet S.; Jain, Richa N.; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-01
The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 Å from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 µB due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.
Theory of triplet-triplet annihilation in optically detected magnetic resonance
NASA Astrophysics Data System (ADS)
Keevers, T. L.; McCamey, D. R.
2016-01-01
Triplet-triplet annihilation allows two low-energy photons to be upconverted into a single high-energy photon. By essentially engineering the solar spectrum, this allows solar cells to be made more efficient and even exceed the Shockley-Quiesser limit. Unfortunately, optimizing the reaction pathway is difficult, especially with limited access to the microscopic time scales and states involved in the process. Optical measurements can provide detailed information: triplet-triplet annihilation is intrinsically spin dependent and exhibits substantial magnetoluminescence in the presence of a static magnetic field. Pulsed optically detected magnetic resonance is especially suitable, since it combines high spin sensitivity with coherent manipulation. In this paper, we develop a time-domain theory of triplet-triplet annihilation for complexes with arbitrary spin-spin coupling. We identify unique "Rabi fingerprints" for each coupling regime and show that this can be used to characterize the microscopic Hamiltonian.
Khaira, Jobanpreet S.; Jain, Richa N.; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-24
The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 Å from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 µ{sub B} due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.
Green's function theory of orbital magnetic moment of interacting electrons in solids
NASA Astrophysics Data System (ADS)
Aryasetiawan, F.; Karlsson, K.; Miyake, T.
2016-04-01
A general formula for the orbital magnetic moment of interacting electrons in solids is derived using the many-electron Green's function method. The formula factorizes into two parts, a part that contains the information about the one-particle band structure of the system and a part that contains the effects of exchange and correlations carried by the Green's function. The derived formula provides a convenient yet rigorous means of including the effects of exchange and correlations beyond the commonly used local density approximation of density functional theory.
NASA Astrophysics Data System (ADS)
Glassmeier, K.-H.; Tsurutani, B. T.
2014-02-01
This is a translation of the Allgemeine Theorie des Erdmagnetismus published by Carl Friedrich Gauss in 1839 in the Resultate aus den Beobachtungen des Magnetischen Vereins im Jahre 1838. The current translation is based on an earlier translation by Elizabeth Juliana Sabine published in 1841. This earlier translation has been revised, corrected, and extended. Numerous biographical comments on the scientists named in the original text have been added as well as further information on the observational material used by Carl Friedrich Gauss. An attempt is made to provide a readable text to a wider scientific community, a text laying the foundation of today's understanding of planetary magnetic fields.
Exact image theory for the problem of dielectric/magnetic slab
NASA Technical Reports Server (NTRS)
Lindell, I. V.
1987-01-01
Exact image method, recently introduced for the exact solution of electromagnetic field problems involving homogeneous half spaces and microstrip-like geometries, is developed for the problem of homogeneous slab of dielectric and/or magnetic material in free space. Expressions for image sources, creating the exact reflected and transmitted fields, are given and their numerical evaluation is demonstrated. Nonradiating modes, guided by the slab and responsible for the loss of convergence of the image functions, are considered and extracted. The theory allows, for example, an analysis of finite ground planes in microstrip antenna structures.
Rezende, Carlos A; San Gil, Rosane A S; Borré, Leandro B; Pires, José Ricardo; Vaiss, Viviane S; Resende, Jackson A L C; Leitão, Alexandre A; De Alencastro, Ricardo B; Leal, Katia Z
2016-09-01
The experiments of carvedilol form II, form III, and hydrate by (13)C and (15)N cross-polarization magic-angle spinning (CP MAS) are reported. The GIPAW (gauge-including projector-augmented wave) method from DFT (density functional theory) calculations was used to simulate (13)C and (15)N chemical shifts. A very good agreement was found for the comparison between the global results of experimental and calculated nuclear magnetic resonance (NMR) chemical shifts for carvedilol polymorphs. This work aims a comprehensive understanding of carvedilol crystalline forms employing solution and solid-state NMR as well as DFT calculations. PMID:26372719
Theory of triplon dynamics in the quantum magnet BiCu2PO6
NASA Astrophysics Data System (ADS)
Kim, Yong Baek; Hwang, Kyusung
We provide a theory of triplon dynamics in the valence bond solid ground state of the coupled spin-ladders modeled for BiCu2PO6. Utilizing the recent neutron scattering experimental data as guides and a theory of interacting triplons via the bond operator formulation, we determine a minimal spin Hamiltonian for this system. It is shown that the splitting of the low energy triplon modes and the peculiar magnetic field dependence of the triplon dispersions can be explained by including substantial Dzyaloshinskii-Moriya and symmetric anisotropic spin interactions. Taking into account the interactions between triplons and the decay of the triplons to the two-triplon continuum via anisotropic spin interactions, we provide a theoretical picture that can be used to understand the main features of the recent neutron scattering experimental data.
Theory of triplon dynamics in the quantum magnet BiCu2PO6
NASA Astrophysics Data System (ADS)
Hwang, Kyusung; Kim, Yong Baek
2016-06-01
We provide a theory of triplon dynamics in the valence bond solid ground state of the coupled spin ladders modeled for BiCu2PO6 . Utilizing the recent high-quality neutron scattering data [K. W. Plumb et al., Nat. Phys. 12, 224 (2016), 10.1038/nphys3566] as guides and a theory of interacting triplons via the bond operator formulation, we determine a minimal spin Hamiltonian for this system. It is shown that the splitting of the low-energy triplon modes and the peculiar magnetic field dependence of the triplon dispersions can be explained by including substantial Dzyaloshinskii-Moriya and symmetric anisotropic spin interactions. Taking into account the interactions between triplons and the decay of the triplons to the two-triplon continuum via anisotropic spin interactions, we provide a theoretical picture that can be used to understand the main features of the recent neutron scattering experimental data.
Computation of magnetic suspension of maglev systems using dynamic circuit theory
He, J.L.; Rote, D.M.; Coffey, H.T.
1991-01-01
Dynamic circuit theory is applied to several magnetic suspensions associated with maglev systems. These suspension systems are the loop-shaped coil guideway, the figure-eight-shaped null-flux coil guideway, and the continuous sheet guideway. Mathematical models, which can be used for the development of computer codes, are provided for each of these suspension systems. The differences and similarities of the models in using dynamic circuit theory are discussed in the paper. The paper emphasizes the transient and dynamic analysis and computer simulation of maglev systems. In general, the method discussed here can be applied to many electrodynamic suspension system design concepts. It is also suited for the computation of the performance of maglev propulsion systems. Numerical examples are presented in the paper. 15 refs., 7 figs., 1 tab.
Computation of magnetic suspension of maglev systems using dynamic circuit theory
NASA Technical Reports Server (NTRS)
He, J. L.; Rote, D. M.; Coffey, H. T.
1992-01-01
Dynamic circuit theory is applied to several magnetic suspensions associated with maglev systems. These suspension systems are the loop-shaped coil guideway, the figure-eight-shaped null-flux coil guideway, and the continuous sheet guideway. Mathematical models, which can be used for the development of computer codes, are provided for each of these suspension systems. The differences and similarities of the models in using dynamic circuit theory are discussed in the paper. The paper emphasizes the transient and dynamic analysis and computer simulation of maglev systems. In general, the method discussed here can be applied to many electrodynamic suspension system design concepts. It is also suited for the computation of the performance of maglev propulsion systems. Numerical examples are presented in the paper.
A unified theory of electrodynamic coupling in coronal magnetic loops - The coronal heating problem
NASA Technical Reports Server (NTRS)
Ionson, J. A.
1984-01-01
The coronal heating problem is studied, and it is demonstrated that Ionson's (1982) LRC approach results in a unified theory of coronal heating which unveils a variety of new heating mechanisms and which links together previously proposed mechanisms. Ionson's LRC equation is rederived, focusing on various aspects that were not clarified in the original article and incorporating new processes that were neglected. A parameterized heating rate is obtained. It is shown that Alfvenic surface wave heating, stochastic magnetic pumping, resonant electrodynamic heating, and dynamical dissipation emerge as special cases of a much more general formalism. This generalized theory is applied to solar coronal loops and it is found that active region and large scale loops are underdamped systems. Young active region loops and (possibly) bright points are found to be overdamped systems.
NASA Astrophysics Data System (ADS)
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-01
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires.The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to
Asmat-Uceda, Martin; Buchanan, Kristen S.; Cheng, Xuemei; Wang, Xiao; Clarke, David J.; Tchernyshyov, Oleg
2015-03-28
Magnetostatic interactions between vortices in closely spaced planar structures are important for applications including vortex-based magnonic crystals and spin torque oscillator networks. Analytical theories that include magnetostatic interaction effects have been proposed but have not yet been rigorously tested. Here, we compare micromagnetic simulations of the dynamics of magnetic vortices confined in three disks in an equilateral triangle configuration to analytical theories that include coupling. Micromagnetic simulations show that the magnetostatic coupling between the disks leads to splitting of the gyrotropic resonance into three modes and that the frequency splitting increases with decreasing separation. The temporal profiles of the magnetization depend on the vortex polarities and chiralities; however, the frequencies depend only on the polarity combinations and will fall into one of two categories: all polarities equal or one polarity opposite to the others, where the latter leads to a larger frequency splitting. Although the magnitude of the splitting observed in the simulations is larger than what is expected based on purely dipolar interactions, a simple analytical model that assumes dipole-dipole coupling captures the functional form of the frequency splitting and the motion patterns just as well as more complex models.
The GEM (Gravity-Electro-Magnetism) Theory of Field Unification: Experimental Progress
NASA Astrophysics Data System (ADS)
Brandenburg, J. E.
2006-01-01
Experimental progress on the GEM (Gravity-Electro-Magnetism) unification theory is summarized as applied to human flight and dynamically modified gravity fields and waves. A VBE (``Vacuum Bernoulli Equation'') is derived. This shows Gravitational energy density to be equated to an EM dynamic pressure that is quadratic in the local Poynting Flux: g2/(2π G) + S2/(c2 L)= Constant, where g and S are the local gravity and Poynting vector magnitudes, respectively, and where L is the Lagrangian density of the vacuum EM field. The VBE can be used to understand anomalous weight loss reported in gyroscope experiments and to understand possible gravity modification for human flight. The GEM gravity modification theory is extended to predict a VHE (Vacuum Hall Effect). Methods for creating dynamic gravity fields via VHE for production and detection of high frequency gravity fields involve electric quadrapole fields normal to static magnetic fields. Earlier experiments at 400Hz had seen lifting effects, however, only when a certain field threshold was crossed. An experiment was performed using 60Hz three phase rotating fields but no effects were seen in low frequency fields thus it appears threshold effects in field intensity and frequency may have been seen.
Three-dimensional linear peeling-ballooning theory in magnetic fusion devices
Weyens, T. Sánchez, R.; García, L.; Loarte, A.; Huijsmans, G.
2014-04-15
Ideal magnetohydrodynamics theory is extended to fully 3D magnetic configurations to investigate the linear stability of intermediate to high n peeling-ballooning modes, with n the toroidal mode number. These are thought to be important for the behavior of edge localized modes and for the limit of the size of the pedestal that governs the high confinement H-mode. The end point of the derivation is a set of coupled second order ordinary differential equations with appropriate boundary conditions that minimize the perturbed energy and that can be solved to find the growth rate of the perturbations. This theory allows of the evaluation of 3D effects on edge plasma stability in tokamaks such as those associated with the toroidal ripple due to the finite number of toroidal field coils, the application of external 3D fields for elm control, local modification of the magnetic field in the vicinity of ferromagnetic components such as the test blanket modules in ITER, etc.
NASA Astrophysics Data System (ADS)
Evans, M. W.
1992-07-01
Optical NMR and ESR is a recently introduced technique in which a circularly polarized laser (a "light magnet") is used in an NMR or ESR spectrometer to induce magnetization. The spectral consequencies are developed with a quantum theory similar to the rigorous theory of Zeeman splitting of Russell-Saunders states, a theory which is suitable for atoms and molecules with net electronic angular momentum, and in which the antisymmetric electronic polarizability is finite. The optical NMR and ESR Hamiltonians are developed with the Wigner-Eckhart Theorem. The circularly polarized laser shifts the original NMR or ESR resonance lines, and splits the shifted lines into analytically useful patterns. The theory gives Landé factors which are in agreement with an earlier, simple, semiclassical theory ( J. Phys. Chem.95, 2256-2260 (1991)).
Linear theory for fast collisionless magnetic reconnection in the lower-hybrid frequency range
NASA Astrophysics Data System (ADS)
Jovanović, D.; Shukla, P. K.
2005-05-01
A linear theory is presented for the interplay between the fast collisionless magnetic reconnection and the lower-hybrid waves that has been observed in recent computer simulations [J. F. Drake, M. Swisdak, C. Cattell et al., Science 299, 873 (2003)]. In plasma configurations with a strong guide field and anisotropic electron temperature, the electron dynamics is described within the framework of standard electron magnetohydrodynamic equations, accounting also for the effects of the electron polarization and ion motions in the presence of perpendicular electric fields. In the linear phase, we find two types of instabilities of a thin current sheet with steep edges, corresponding to its filamentation (or tearing) and bending. Using a surface-wave formalism for the perturbations whose wavelength is larger than the thickness of the current sheet, the corresponding growth rates are calculated as the contributions of singularities in the plasma dispersion function. These are governed by the electron inertia and the linear coupling of the reconnecting magnetic field with local plasma modes propagating in the perpendicular direction that are subject to the Buneman instability. The linear surface wave instability may be particularly important as a secondary instability, dissipating the thin current sheets that develop in the course of the fast reconnection in the shear-Alfvén and kinetic-Alfvén regimes, and providing the anomalous resistivity for the growth of magnetic islands beyond the shear-Alfvén and kinetic-Alfvén scales.
Theory of NMR signal behavior in magnetically inhomogeneous tissues: the static dephasing regime.
Yablonskiy, D A; Haacke, E M
1994-12-01
This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium. PMID:7869897
A Hot Big Bang Theory: Magnetic Fields and the Early Evolution of the Protolunar Disk
NASA Astrophysics Data System (ADS)
Gammie, C. F.; Liao, Wei-Ting; Ricker, P. M.
2016-09-01
The leading theory for the formation of Earth’s Moon invokes a collision between a Mars-sized body and the proto-Earth to produce a disk of orbiting material that later condenses to form the Moon. We show that the disk opacity is large, and cooling is therefore inefficient ({t}{cool}{{Ω }}\\gg 1). In this regime, angular momentum transport in the disk leads to steady heating unless α \\lt {({t}{cool}{{Ω }})}-1\\ll 1. Following earlier work by Charnoz and Michaut, and Carballido et al., we show that once the disk is completely vaporized it is well coupled to the magnetic field. We consider a scenario in which turbulence driven by magnetic fields leads to a brief, hot phase where the disk is geometrically thick, with strong turbulent mixing. The disk cools by spreading until it decouples from the field. We point out that approximately half the accretion energy is dissipated in the boundary layer where the disk meets the Earth’s surface. This creates high entropy material close to the Earth, driving convection and mixing. Finally, a hot magnetized disk could drive bipolar outflows that remove mass and angular momentum from the Earth–Moon system.
Theory of magnetic circular dichroism of nonresonant x-ray Raman scattering
NASA Astrophysics Data System (ADS)
Takahashi, Manabu; Hiraoka, Nozomu
2015-09-01
We develop a theory of magnetic circular dichroism (MCD) of hard x-ray Raman scattering (XRS) to analyze the MCD signal at iron L edge from pure ferromagnetic iron. The obtained formula of scattering amplitude has terms corresponding to the charge (Thomson) scattering process, and the orbital and spin scattering processes in the elastic x-ray magnetic scattering. The total scattering intensity is almost independent of incident photon helicity since it is mainly produced by the charge scattering. The weak MCD signals are caused primarily by interference between the charge scattering amplitude and each of the orbital and spin scattering amplitudes. The shape of the MCD spectra depends on angle αM between the wave vector of the incident photon and the magnetization vector. At αM=0∘ , the spin scattering is suppressed so that the MCD spectrum becomes analogous to that observed in the x-ray absorption spectroscopy. At αM=135∘ , the orbital scattering is suppressed, and the spin scattering plays central roles in producing the MCD signal. The magnitude of the MCD signal turns out to be proportional to the spin density of states projected onto the 3 d states in the unoccupied state. Consequently, the value of the integrated MCD signal is proportional to the spin moment in the 3 d states at the scattering site. The calculated MCD spectra with the help of a band structure calculation well reproduce the observed spectra.
NASA Astrophysics Data System (ADS)
Deák, A.; Simon, E.; Balogh, L.; Szunyogh, L.; dos Santos Dias, M.; Staunton, J. B.
2014-06-01
We develop a self-consistent relativistic disordered local moment (RDLM) scheme aimed at describing finite-temperature magnetism of itinerant metals from first principles. Our implementation in terms of the Korringa-Kohn-Rostoker multiple-scattering theory and the coherent potential approximation allows us to relate the orientational distribution of the spins to the electronic structure, thus a self-consistent treatment of the distribution is possible. We present applications for bulk bcc Fe, L10-FePt, and FeRh ordered in the CsCl structure. The calculations for Fe show significant variation of the local moments with temperature, whereas according to the mean-field treatment of the spin fluctuations the Curie temperature is overestimated. The magnetic anisotropy of FePt alloys is found to depend strongly on intermixing between nominally Fe and Pt layers, and it shows a power-law behavior as a function of magnetization for a broad range of chemical disorder. In the case of FeRh we construct a lattice constant vs temperature phase diagram and determine the phase line of metamagnetic transitions based on self-consistent RDLM free-energy curves.
Density-functional-theory calculations of matter in strong magnetic fields. I. Atoms and molecules
Medin, Zach; Lai Dong
2006-12-15
We present calculations of the electronic structure of various atoms and molecules in strong magnetic fields ranging from B=10{sup 12} G to 2x10{sup 15} G, appropriate for radio pulsars and magnetars. For these field strengths, the magnetic forces on the electrons dominate over the Coulomb forces, and to a good approximation the electrons are confined to the ground Landau level. Our calculations are based on the density functional theory, and use a local magnetic exchange-correlation function which is tested to be reliable in the strong field regime. Numerical results of the ground-state energies are given for H{sub N} (up to N=10), He{sub N} (up to N=8), C{sub N} (up to N=5), and Fe{sub N} (up to N=3), as well as for various ionized atoms. Fitting formulae for the B dependence of the energies are also given. In general, as N increases, the binding energy per atom in a molecule, vertical bar E{sub N}|/N, increases and approaches a constant value. For all the field strengths considered in this paper, hydrogen, helium, and carbon molecules are found to be bound relative to individual atoms (although for B less than a few x10{sup 12} G, carbon molecules are very weakly bound relative to individual atoms). Iron molecules are not bound at B < or approx. 10{sup 13} G, but become energetically more favorable than individual atoms at larger field strengths.
Spin-orbit coupled Fermi liquid theory of ultracold magnetic dipolar fermions
NASA Astrophysics Data System (ADS)
Li, Yi; Wu, Congjun
2012-05-01
We investigate Fermi liquid states of the ultracold magnetic dipolar Fermi gases in the simplest two-component case including both thermodynamic instabilities and collective excitations. The magnetic dipolar interaction is invariant under the simultaneous spin-orbit rotation but not under either the spin or the orbit one. Therefore, the corresponding Fermi liquid theory is intrinsically spin-orbit coupled. This is a fundamental feature of magnetic dipolar Fermi gases different from electric dipolar ones. The Landau interaction matrix is calculated and is diagonalized in terms of the spin-orbit coupled partial-wave channels of the total angular momentum J. The leading thermodynamic instabilities lie in the channels of ferromagnetism hybridized with the ferronematic order with J=1+ and the spin-current mode with J=1-, where + and - represent even and odd parities, respectively. An exotic propagating collective mode is identified as spin-orbit coupled Fermi surface oscillations in which spin distribution on the Fermi surface exhibits a topologically nontrivial hedgehog configuration.
Magnetic properties of f-electron systems in spin-polarized relativistic density functional theory
NASA Astrophysics Data System (ADS)
Yamagami, H.; Mavromaras, A.; Kübler, J.
1997-12-01
The magnetic ground state of the series of lanthanide and actinide trivalent ions is investigated by means of spin-polarized relativistic spin-density functional theory. In the local density functional approximation (LDA) an internal effective magnetic field due to exchange and correlation couples to the spin degrees of freedom. The resulting set of coupled Dirac equations yields ground-state multiplets that obey the well-known Hund's rules. This remarkable result comes about by the coupling of the j = l + 1/2 with the j = l - 1/2 states due to the exchange - correlation potential that is, as usual, the functional derivative of the exchange - correlation energy with respect to the spin magnetic moment. The effect of the coupling is shown to depend on the varying relative strengths of spin - orbit coupling and exchange splitting within the f series. Since in the f levels the internal exchange splitting dominates rather than the spin - orbit splitting, the energy level scheme is that of the Paschen - Back effect, and thus features of the Russell - Saunders coupling persist in spite of relativistic effects.
Magnetic shielding of a laboratory Hall thruster. I. Theory and validation
Mikellides, Ioannis G. Katz, Ira; Hofer, Richard R.; Goebel, Dan M.
2014-01-28
We demonstrate a technique by which erosion of the acceleration channel in Hall thrusters can be reduced by at least a few orders of magnitude. The first principles of the technique, now known as “magnetic shielding,” have been derived based on the findings of 2-D numerical simulations. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster to test the theory and are the main subject of this Part I article. Part II expands on the results of the experiments. Near the walls of the magnetically shielded (MS) thruster theory and experiment agree that (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered compared to the unshielded thruster. Erosion rates deduced directly from the wall probes show reductions of at least ∼3 orders of magnitude at the MS inner wall when an ion energy threshold of 30.5 V is used in the sputtering yield model of the channel material. At the outer wall the probes reveal that the ion energy was below the assumed threshold. Using a threshold of 25 V, the simulations predict a minimum reduction of ∼600 at the MS inner wall. At the MS outer wall ion energies are found to be below 25 V. When a 50-V threshold is used the computed ion energies are below the threshold at both sides of the channel. Uncertainties, sensitivities, and differences between theory and experiment are also discussed. The elimination of wall erosion in Hall thrusters solves a problem that has remained unsettled for more than five decades.
Gyrotropic guiding-center fluid theory for turbulent inhomogeneous magnetized plasma
Jasperse, John R.; Basu, Bamandas; Lund, Eric J.; Bouhram, Mehdi
2006-07-15
In this paper, a new fluid theory is given in the guiding-center and gyrotropic approximation which is derivable from the Vlasov-Maxwell equations. The theory includes the effect of wave-particle interactions for the weakly turbulent, weakly inhomogeneous, nonuniformly magnetized plasma, and it is applicable to a variety of space and laboratory plasmas. It is assumed that the turbulence is random and electrostatic, and that the velocity-space Fokker-Planck operator can be used to calculate the correlation functions that describe the wave-particle interactions. Conservation laws are derived that relate the low-order velocity moments of the particle distributions to the turbulence. The theory is based on the work of Hubbard [Proc. R. Soc. London, Ser. A 260, 114 (1961)] and Ichimaru and Rosenbluth [Phys. Fluids 13, 2778 (1970)]. In the work presented here, the idea is proposed that the fluid equations can be solved (1) by using measurements of the turbulence to specify the electric-field fluctuations; and (2) by using measurements of the low-order velocity moments to specify the initial and boundary conditions.
W.M. Tang
2005-01-03
The present lecture provides an introduction to the subject of gyrokinetic theory with applications in the area of magnetic confinement research in plasma physics--the research arena from which this formalism was originally developed. It was presented as a component of the ''Short Course in Kinetic Theory within the Thematic Program in Partial Differential Equations'' held at the Fields Institute for Research in Mathematical Science (24 March 2004). This lecture also discusses the connection between the gyrokinetic formalism and powerful modern numerical simulations. Indeed, simulation, which provides a natural bridge between theory and experiment, is an essential modern tool for understanding complex plasma behavior. Progress has been stimulated in particular by the exponential growth of computer speed along with significant improvements in computer technology. The advances in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics have produced increasingly good agreement between experimental observations and computational modeling. This was enabled by two key factors: (i) innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales and (ii) access to powerful new computational resources.
Prediction of d^0 magnetism in self-interaction corrected density functional theory
NASA Astrophysics Data System (ADS)
Das Pemmaraju, Chaitanya
2010-03-01
Over the past couple of years, the phenomenon of ``d^0 magnetism'' has greatly intrigued the magnetism community [1]. Unlike conventional magnetic materials, ``d^0 magnets'' lack any magnetic ions with open d or f shells but surprisingly, exhibit signatures of ferromagnetism often with a Curie temperature exceeding 300 K. Current research in the field is geared towards trying to understand the mechanism underlying this observed ferromagnetism which is difficult to explain within the conventional m-J paradigm [1]. The most widely studied class of d^0 materials are un-doped and light element doped wide gap Oxides such as HfO2, MgO, ZnO, TiO2 all of which have been put forward as possible d0 ferromagnets. General experimental trends suggest that the magnetism is a feature of highly defective samples leading to the expectation that the phenomenon must be defect related. In particular, based on density functional theory (DFT) calculations acceptor defects formed from the O-2p states in these Oxides have been proposed as being responsible for the ferromagnetism [2,3]. However. predicting magnetism originating from 2p orbitals is a delicate problem, which depends on the subtle interplay between covalency and Hund's coupling. DFT calculations based on semi-local functionals such as the local spin-density approximation (LSDA) can lead to qualitative failures on several fronts. On one hand the excessive delocalization of spin-polarized holes leads to half-metallic ground states and the expectation of room-temperature ferromagnetism. On the other hand, in some cases a magnetic ground state may not be predicted at all as the Hund's coupling might be under estimated. Furthermore, polaronic distortions which are often a feature of acceptor defects in Oxides are not predicted [4,5]. In this presentation, we argue that the self interaction error (SIE) inherent to semi-local functionals is responsible for the failures of LSDA and demonstrate through various examples that beyond
Kinetic theory of a two-dimensional magnetized plasma. II - Balescu-Lenard limit.
NASA Technical Reports Server (NTRS)
Vahala, G.
1972-01-01
The kinetic theory of a two-dimensional one-species plasma in a uniform dc magnetic field is investigated in the small plasma parameter limit. The plasma consists of charged rods interacting through the logarithmic Coulomb potential. Vahala and Montgomery earlier (1971) derived a Fokker-Planck equation for this system, but it contained a divergent integral, which had to be cut off on physical grounds. This cutoff is compared to the standard cutoff introduced in the two-dimensional unmagnetized Fokker-Planck equation. In the small plasma parameter limit, it is shown that the Balescu-Lenard collision term is zero in the long time average limit if only two-body interactions are considered. The energy transfer from a test particle to an equilibrium plasma is discussed and is also shown to be zero in the long time average limit. This supports the unexpected result of zero Balescu-Lenard collision term.
The expansion of polarization charge layers into magnetized vacuum - Theory and computer simulations
NASA Technical Reports Server (NTRS)
Galvez, Miguel; Borovsky, Joseph E.
1991-01-01
The formation and evolution of polarization charge layers on cylindrical plasma streams moving in vacuum are investigated using analytic theory and 2D electrostatic particle-in-cell computer simulations. It is shown that the behavior of the electron charge layer goes through three stages. An early time expansion is driven by electrostatic repulsion of electrons in the charge layer. At the intermediate stage, the simulations show that the electron-charge-layer expansion is halted by the positively charged plasma stream. Electrons close to the stream are pulled back to the stream and a second electron expansion follows in time. At the late stage, the expansion of the ion charge layer along the magnetic field lines accompanies the electron expansion to form an ambipolar expansion. It is found that the velocities of these electron-ion expansions greatly exceed the velocities of ambipolar expansions which are driven by plasma temperatures.
Exact kinetic theory for the instability of an electron beam in a hot magnetized plasma
Timofeev, I. V.; Annenkov, V. V.
2013-09-15
Efficiency of collective beam-plasma interaction strongly depends on the growth rates of dominant instabilities excited in the system. Nevertheless, exact calculations of the full unstable spectrum in the framework of relativistic kinetic theory for arbitrary magnetic fields and particle distributions were unknown until now. In this paper, we give an example of such a calculation answering the question whether the finite thermal spreads of plasma electrons are able to suppress the fastest growing modes in the beam-plasma system. It is shown that nonrelativistic temperatures of Maxwellian plasmas can stabilize only the oblique instabilities of relativistic beam. On the contrary, non-Maxwellian tails typically found in laboratory beam-plasma experiments are able to substantially reduce the growth rate of the dominant longitudinal modes affecting the efficiency of turbulent plasma heating.
Statistical-mechanical theory of the overall magnetic properties of mesocrystals
NASA Astrophysics Data System (ADS)
Huang, J. P.
2004-10-01
The mesocrystal showing both electrorheological and magnetorheological effects is called electro-magnetorheological (EMR) solids. Prediction of the overall magnetic properties of the EMR solids is a challenging task due to the coexistence of the uniaxially anisotropic behavior and structural transition as well as long-range interaction between the suspended particles. To consider the uniaxial anisotropy effect, we present an anisotropic Kirkwood-Fröhlich equation for calculating the effective permeabilities by adopting an explicit characteristic spheroid rather than a characteristic sphere used in the derivation of the usual Kirkwood-Fröhlich equation. Further, by applying an Ewald-Kornfeld formulation we are able to investigate the effective permeability by including the structural transition and long-range interaction explicitly. Our theory can reduce to the usual Kirkwood-Fröhlich equation and Onsager equation naturally. To this end, the numerical simulation shows the validity of monitoring the structure of EMR solids by detecting their effective permeabilities.
NASA Astrophysics Data System (ADS)
Simons, F. J.; Beggan, C.; Saarimaki, J.; Whaler, K. A.; Lewis, K. W.; Plattner, A.
2012-12-01
We elucidate "myths and truths concerning the estimation of power spectra", to paraphrase a paper written by G. Efstathiou. A complete theory and software are available to construct and interpret localized power spectral densities from noisy and incomplete data observed on a spherical surface, but such knowledge has not made it into mainstream geophysical practice. Key are the doubly-orthogonal spherical Slepian functions, and their Cartesian and vectorial relatives. There is confusion in the community on whether to use the Slepian functions as a basis for data approximation, as windows for regularization of power spectral estimates, or hybridly. Each of these choices is legitimate but carries consequences to be characterized statistically. We review the options and present them tutorially, to motivate the scientific analysis of the terrestrial magnetic power spectrum, and that of Mars, which follows. The power spectral density of the lithospheric magnetic field on Earth is broken down in to portions that are geographically limited to the oceans, the continents, and various portions thereof. The different overall power levels and varying character of the power spectra contain much diagnostic information about the generation and preservation of magnetic signatures in the Earth's crust. On Mars, we conducted a parameterized inversion for magnetization strength and decorrelation depth, and compared this to independent, gravity-based estimates for crustal thickness. Both the terrestrial and martian examples are different manifestations of how Slepian functions can be used for power-spectral localization, and in both cases, the uncertainty of the spectral estimates, both on the horizontal axis (degree resolution) and the vertical axis (estimation variance), is easily computed via exact and also approximate relationships. The analytical expressions that are involved may look off-puttingly complex, but the result of the theoretical analysis is a set of simple routines
Solovyova, Anna Y; Goldina, Olga A; Ivanov, Alexey O; Lebedev, Aleksandr V; Elfimova, Ekaterina A
2016-08-28
Temperature dependencies of the static initial magnetic susceptibility for ferrofluids at various concentrations are studied using experiment and statistical-mechanical theories. Magnetic susceptibility measurements are carried out for twelve samples of magnetite-based fluids stabilized with oleic acid over a wide range of temperatures (210 K ≲T ≲ 390 K); all samples have the same granulometric composition but different volume ferroparticle concentrations (0.2 ≲ φ ≲ 0.5). Experimental results are analyzed using three theories: the second-order modified mean-field theory (MMF2) [A. O. Ivanov and O. B. Kuznetsova, Phys. Rev. E 64, 41405 (2001)]; its correction for polydisperse ferrofluids arising from Mayer-type cluster expansion and taking into account the first terms of the polydisperse second virial coefficient [A. O. Ivanov and E. A. Elfimova, J. Magn. Magn. Mater 374, 327 (2015)]; and a new theory based on MMF2 combined with the first terms of the polydisperse second and third virial contributions to susceptibility. It turns out that the applicability of each theory depends on the experimental sample density. If twelve ferrofluid samples are split into three groups of strong, moderate, and low concentrated fluids, the temperature dependences of the initial magnetic susceptibility in each group are very precisely described by one of the three theories mentioned above. The determination of a universal formula predicting a ferrofluid susceptibility over a broad range of concentrations and temperatures remains as a challenge. PMID:27586948
Electrostatic drift waves in a 2D magnetic current sheet - a new kinetic theory
NASA Astrophysics Data System (ADS)
Fruit, G.; Louarn, P.; Tur, A.
2015-12-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, a kinetic model for electromagnetic instabilities in resonant interaction with trapped bouncing electrons has been proposed for several years. Fruit et al. 2013 already used it to investigate the possibilities for electrostatic instabilities. Tur et al. 2014 generalizes the model for full electromagnetic perturbations.It turns out that some corrections should be added to the electrostatic version of Fruit et al. 2013. We propose to revist the theory in this present paper.Starting with a modified 2D Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electrostatic fluctuations with period of the order of the electron bounce period (a few seconds). The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electrostatic modes is finally obtained through the quasineutrality condition.The new feature of the present model is the inclusion of diamagnetic drift effects due to the density gradient in the tail. It is well known in MHD theory that drift waves are driven unstable through collisions or other dissipative effects. Here electrostatic drift waves are revisited in this more complete kinetic model including bouncing electrons and finite Larmor radius effects. A new mode has been found with original propagation proprieties. It is moreover mildly unstable due to electron or ion damping (dissipative instability).
SL(2,R) duality-symmetric action for electromagnetic theory with electric and magnetic sources
Lee, Choonkyu; Min, Hyunsoo
2013-12-15
For the SL(2,R) duality-invariant generalization of Maxwell electrodynamics in the presence of both electric and magnetic sources, we formulate a local, manifestly duality-symmetric, Zwanziger-type action by introducing a pair of four-potentials A{sup μ} and B{sup μ} in a judicious way. On the two potentials A{sup μ} and B{sup μ} the SL(2,R) duality transformation acts in a simple linear manner. In quantum theory including charged source fields, this action can be recast as a SL(2,Z)-invariant action. Also given is a Zwanziger-type action for SL(2,R) duality-invariant Born–Infeld electrodynamics which can be important for D-brane dynamics in string theory. -- Highlights: •We formulate a local, manifestly duality-symmetric, Zwanziger-type action. •Maxwell electrodynamics is generalized to include dilaton and axion fields. •SL(2,R) symmetry is manifest. •We formulate a local, manifestly duality-symmetric, nonlinear Born–Infeld action with SL(2,R) symmetry.
Nonuniqueness of magnetic fields and energy derivatives in spin-polarized density functional theory
NASA Astrophysics Data System (ADS)
Gál, T.; Ayers, P. W.; De Proft, F.; Geerlings, P.
2009-10-01
The effect of the recently uncovered nonuniqueness of the external magnetic field B(r⃑) corresponding to a given pair of density n(r⃑) and spin density ns(r⃑) on the derivative of the energy functional of spin-polarized density functional theory, and its implications for the definition of chemical reactivity descriptors, is examined. For ground states, the nonuniqueness of B(r⃑) implies the nondifferentiability of the energy functional Ev,B[n,ns] with respect to ns(r⃑). It is shown, on the other hand, that this nonuniqueness allows the existence of the one-sided derivatives of Ev,B[n,ns] with respect to ns(r⃑). Although the N-electron ground state can always be obtained from the minimization of Ev,B[n,ns] without any constraint on the spin number Ns=∫ns(r⃑)dr⃑, the Lagrange multiplier μs associated with the fixation of Ns does not vanish even for ground states. μs is identified as the left- or right-side derivative of the total energy with respect to Ns, which justifies the interpretation of μs as a (spin) chemical potential. This is relevant not only for the spin-polarized generalization of conceptual density functional theory, the spin chemical potential being one of the elementary reactivity descriptors, but also for the extension of the thermodynamical analogy of density functional theory for the spin-polarized case. For higher-order reactivity indices, B(r⃑)'s nonuniqueness has similar implications as for μs, leading to a split of the indices with respect to Ns into one-sided reactivity descriptors.
Sukstanskii, Alexander L; Yablonskiy, Dmitriy A
2003-08-01
A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant deltaR2. In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay lnS approximately -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc. PMID:12914839
NASA Astrophysics Data System (ADS)
Kalaee, Mohammad Javad; Katoh, Yuto
2016-05-01
In the beaming angle theory, the magnetic field direction is assumed perpendicular to the normal boundary, and the prediction of this theory, from beaming angle is base on the Jones' formula. We investigate the effect of deviation the magnetic field direction respect to normal boundary direction. In this study, we present new conditions that under these conditions two modes, extraordinary and ordinary modes waves can match. Also, we show for these cases the beaming angle does not correspond to Jones' formula. This effect leads to the angles larger and smaller than the angle estimated by Jones' formula. This effect on the mode conversion process becomes important in a case where local fluctuations in the direction of the density gradient vector or the magnetic field direction are observed. By comparing the beaming angle from observations with the beaming angles resulting from different ∆ Φ , we showed a ∆ Φ about 3 to 5° are necessary in consistence with observation.
NASA Astrophysics Data System (ADS)
Kh., Lotfy
2012-06-01
In the present paper, we introduce the coupled theory (CD), Lord-Schulman (LS) theory, and Green-Lindsay (GL) theory to study the influences of a magnetic field and rotation on a two-dimensional problem of fibre-reinforced thermoelasticity. The material is a homogeneous isotropic elastic half-space. The method applied here is to use normal mode analysis to solve a thermal shock problem. Some particular cases are also discussed in the context of the problem. Deformation of a body depends on the nature of the force applied as well as the type of boundary conditions. Numerical results for the temperature, displacement, and thermal stress components are given and illustrated graphically in the absence and the presence of the magnetic field and rotation.
NASA Astrophysics Data System (ADS)
Balakin, Alexander B.; Lemos, José P. S.; Zayats, Alexei E.
2016-04-01
Alternative theories of gravity and their solutions are of considerable importance since, at some fundamental level, the world can reveal new features. Indeed, it is suspected that the gravitational field might be nonminimally coupled to the other fields at scales not yet probed, bringing into the forefront nonminimally coupled theories. In this mode, we consider a nonminimal Einstein-Yang-Mills theory with a cosmological constant. Imposing spherical symmetry and staticity for the spacetime and a magnetic Wu-Yang ansatz for the Yang-Mills field, we find expressions for the solutions of the theory. Further imposing constraints on the nonminimal parameters, we find a family of exact solutions of the theory depending on five parameters—two nonminimal parameters, the cosmological constant, the magnetic charge, and the mass. These solutions represent magnetic monopoles and black holes in magnetic monopoles with de Sitter, Minkowskian, and anti-de Sitter asymptotics, depending on the sign and value of the cosmological constant Λ . We classify completely the family of solutions with respect to the number and the type of horizons and show that the spacetime solutions can have, at most, four horizons. For particular sets of the parameters, these horizons can become double, triple, and quadruple. For instance, for a positive cosmological constant Λ , there is a critical Λc for which the solution admits a quadruple horizon, evocative of the Λc that appears for a given energy density in both the Einstein static and Eddington-Lemaître dynamical universes. As an example of our classification, we analyze solutions in the Drummond-Hathrell nonminimal theory that describe nonminimal black holes. Another application is with a set of regular black holes previously treated.
Magnetic impurities in nanotubes: From density functional theory to Kondo many-body effects
NASA Astrophysics Data System (ADS)
Baruselli, P. P.; Fabrizio, M.; Smogunov, A.; Requist, R.; Tosatti, E.
2013-12-01
Low-temperature electronic conductance in nanocontacts, scanning tunneling microscopy (STM), and metal break junctions involving magnetic atoms or molecules is a growing area with important unsolved theoretical problems. While the detailed relationship between contact geometry and electronic structure requires a quantitative ab initio approach such as density functional theory (DFT), the Kondo many-body effects ensuing from the coupling of the impurity spin with metal electrons are most properly addressed by formulating a generalized Anderson impurity model to be solved with, for example, the numerical renormalization group (NRG) method. Since there is at present no seamless scheme that can accurately carry out that program, we have in recent years designed a systematic method for semiquantitatively joining DFT and NRG. We apply this DFT-NRG scheme to the ideal conductance of single wall (4,4) and (8,8) nanotubes with magnetic adatoms (Co and Fe), both inside and outside the nanotube, and with a single carbon atom vacancy. A rich scenario emerges, with Kondo temperatures generally in the Kelvin range, and conductance anomalies ranging from a single channel maximum to destructive Fano interference with cancellation of two channels out of the total four. The configuration yielding the highest Kondo temperature (tens of Kelvins) and a measurable zero-bias anomaly is that of a Co or Fe impurity inside the narrowest nanotube. The single atom vacancy has a spin, but a very low Kondo temperature is predicted. The geometric, electronic, and symmetry factors influencing this variability are all accessible, which makes this approach methodologically instructive and highlights many delicate and difficult points in the first-principles modeling of the Kondo effect in nanocontacts.
NASA Astrophysics Data System (ADS)
Pham, Hung Tan; Cuong, Ngo Tuan; Tam, Nguyen Minh; Lam, Vu Dinh; Tung, Nguyen Thanh
2016-01-01
We study CoxCryOm+ (x + y = 2, 3 and 1 ≤ m ≤ 4) clusters by means of density-functional-theory calculations. It is found that the clusters grow preferentially through maximizing the number of metal-oxygen bonds with a favor on Cr sites. The size- and composition-dependent magnetic behavior is discussed in relation with the local atomic magnetic moments. While doped species show an oscillatory magnetic behavior, the total magnetic moment of pure cobalt and chromium oxide clusters tends to enhance or reduce as increasing the oxygen content, respectively. The dissociation energies for different evaporation channels are also calculated to suggest the stable patterns, as fingerprints for future photofragmentation experiments.
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory.
Cheng, Mu-Jeng; Nielsen, Robert J; Tahir-Kheli, Jamil; Goddard, William A
2011-05-28
We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)(2)P(2)O(7), VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J(OPO) = -156.8 K and J(O) = -68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J(layer) = 19.2 K and J(chain) = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J(OPO) originates from a super-exchange interaction through the bridging -O-P-O- unit. In contrast, J(O) results from a direct overlap of 3d(x(2)-y(2)) orbitals on two vanadium atoms in the same V(2)O(8) motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO)(2)-V are covalent and rigid, whereas the bonds of V-(O)(2)-V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J(O), changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J(O) to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O(2) can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride. PMID:21503330
On unsteady-motion theory of magnetic force for maglev systems.
Chen, S. S.; Zhu, S.; Cai, Y.; Energy Technology
1995-12-14
Motion-dependent magnetic forces are the key elements in the study of magnetically levitated vehicle (maglev) system dynamics. This paper presents an experimental and analytical study that will enhance our understanding of the role of unsteady-motion-dependent magnetic forces and demonstrate an experimental technique that can be used to measure those unsteady magnetic forces directly. The experimental technique is a useful tool for measuring motion-dependent magnetic forces for the prediction and control of maglev systems.
On the unsteady-motion theory of magnetic forces for maglev
Chen, S.S.; Zhu, S.; Cai, Y.
1996-02-01
Motion-dependent magnetic forces are the key elements in the study of magnetically levitated vehicle (maglev) system dynamics. This paper presents an experimental and analytical study that will enhance their understanding of the role of unsteady-motion-dependent magnetic forces and demonstrate an experimental technique that can be used to measure those unsteady magnetic forces directly. The experimental technique provides a useful tool to measure motion-dependent magnetic forces for the prediction and control of maglev systems.
The Ginzburg-Landau theory for a thin superconducting loop in a large magnetic field
NASA Astrophysics Data System (ADS)
Shieh, Tien-Tsan
When a temperature is lower than a certain critical value, a superconducting sample undergoes a phase transition from a normal state to a superconducting state. This onset process of superconductivity can be studied as a Rayleigh quotient under the framework of the Ginzburg-Landau theory. In particular, I study the onset problem for a thin superconducting loop in a large magnetic field. This double limit problem was first carried out by Richardson and Rubinstein by using formal asymptotic expansions. I rigorously show that a one-dimensional Rayleigh quotient in the spirit of Gamma-convergence. The full Gamma-convergence of the Ginzburg-Landau functional for a thin domain and a large field is also obtained. The rigorous analysis in this thesis shows the validity of Richardson and Rubinstein's formal results. It is also shown that the Rayleigh quotient related to this onset problem has a periodic variation with a parabolic background. The parabolic background effect can be explained by a non-ignorable effect if finite-width cross-section of a thin superconducting sample. This illustrate the observation of the Little-Parks experiment.
Three-dimensional brain magnetic resonance imaging segmentation via knowledge-driven decision theory
Verma, Nishant; Muralidhar, Gautam S.; Bovik, Alan C.; Cowperthwaite, Matthew C.; Burnett, Mark G.; Markey, Mia K.
2014-01-01
Abstract. Brain tissue segmentation on magnetic resonance (MR) imaging is a difficult task because of significant intensity overlap between the tissue classes. We present a new knowledge-driven decision theory (KDT) approach that incorporates prior information of the relative extents of intensity overlap between tissue class pairs for volumetric MR tissue segmentation. The proposed approach better handles intensity overlap between tissues without explicitly employing methods for removal of MR image corruptions (such as bias field). Adaptive tissue class priors are employed that combine probabilistic atlas maps with spatial contextual information obtained from Markov random fields to guide tissue segmentation. The energy function is minimized using a variational level-set-based framework, which has shown great promise for MR image analysis. We evaluate the proposed method on two well-established real MR datasets with expert ground-truth segmentations and compare our approach against existing segmentation methods. KDT has low-computational complexity and shows better segmentation performance than other segmentation methods evaluated using these MR datasets. PMID:26158060
Two-dimensional Kagome phosphorus and its edge magnetism: a density functional theory study.
Yu, Guodong; Jiang, Liwei; Zheng, Yisong
2015-07-01
By means of density functional theory calculations, we predict a new two-dimensional phosphorus allotrope with the Kagome-like lattice(Kagome-P). It is an indirect gap semiconductor with a band gap of 1.64 eV. The gap decreases sensitively with the compressive strain. In particular, shrinking the lattice beyond 13% can drive it into metallic state. In addition, both the AA and AB stacked Kagome-P multi-layer structures exhibit a bandgap much smaller than 1.64 eV. Edges in the Kagome-P monolayer probably suffer from the edge reconstruction. An isolated zigzag edge can induce antiferromagnetic (AF) ordering with a magnetic transition temperature of 23 K. More importantly, when applying a stretching strain beyond 4%, such an edge turns to possess a ferromagnetic ground state. A very narrow zigzag-edged Kagome-P ribbon displays the spin moment distribution similar to the zigzag-edged graphene nanoribbon because of the coupling between the opposites edges. But the inter-edge coupling in the Kagome-P ribbon vanishes more rapidly as the ribbon width increases. These properties make it a promising material in spintronics. PMID:26020446
On a theory of an FEL amplifier with circular waveguide and guiding magnetic field
Saldin, E.L.; Schneidmiller, E.A.; Yurkov, M.V.
1995-12-31
We consider an FEL amplifier with an axisymmetric electron beam, circular waveguide, helical undulator and guiding magnetic field. The presented nonlinear theory of the FEL amplifier is based on Hamiltonian description of particle motion and radiation field representation with Green function method. The space charge fields, energy spread and diffraction effects are taken into consideration. Such an FEL amplifier configuration possesses some peculiarities when it operates in a regime with the negative longitudinal mass (i.e. when{mu}{sup -1}{proportional_to}dv{sub z}/dE < 0). It is shown that in the presence of strong space charge fields, the so-called {open_quotes}negative mass{close_quotes} instability may influence significantly on the FEL amplifier operation resulting in a significant increase in the FEL amplifier efficiency. It is proposed in the presented paper to use the effect of the {open_quotes}negative mass instability{close_quotes} to achieve an effective bunching of the CERN Linear Collider (LIC) driving beam.
Benjamin A. Frandsen; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-11
Here, we present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominatedmore » by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. Furthermore, the Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.« less
NASA Astrophysics Data System (ADS)
Frandsen, Benjamin A.; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-01
We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ˜1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.
Frandsen, Benjamin A; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J; Staunton, Julie B; Billinge, Simon J L
2016-05-13
We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ∼1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory. PMID:27232042
Theory of spin-orbit enhanced electric-field control of magnetism in multiferroic BiFeO3.
de Sousa, Rogério; Allen, Marc; Cazayous, Maximilien
2013-06-28
We present a microscopic theory that shows the importance of spin-orbit coupling in perovskite compounds with heavy ions. In BiFeO3 (BFO) the spin-orbit coupling at the bismuth ion sites results in a special kind of magnetic anisotropy that is linear in the applied E field. This interaction can convert the cycloid ground state into a homogeneous antiferromagnet, with a weak ferromagnetic moment whose orientation can be controlled by the E-field direction. Remarkably, the E-field control of magnetism occurs without poling the ferroelectric moment, providing a pathway for reduced energy dissipation in spin-based devices made of insulators. PMID:23848915
NASA Astrophysics Data System (ADS)
Cassak, P. A.; Shay, M. A.
2009-03-01
The comment by Semenov et al. has called into question our derivation of the outflow velocity in asymmetric magnetic reconnection. We present three reasons that the analysis presented in the comment is incorrect. Most importantly, the authors of the comment have incorrectly applied results from one-dimensional shock theory to the problem of conservation through a two-dimensional dissipation region. For completeness, we compare their predictions to numerical simulation results, finding that their theory does not describe the data. We conclude the analysis in the comment is without merit.
Destabilization of 2D magnetic current sheets by resonance with bouncing electron - a new theory
NASA Astrophysics Data System (ADS)
Fruit, Gabriel; Louarn, Philippe; Tur, Anatoly
2016-07-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, we propose a kinetic model for electromagnetic instabilities in resonant interaction with trapped bouncing electrons. The geometry is clearly 2D and uses Harris sheet profile. Fruit et al. 2013 already used this model to investigate the possibilities of electrostatic instabilities. Tur et al. 2014 generalizes the model for full electromagnetic perturbations. Starting with a modified Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electromagnetic fluctuations with period of the order of the electron bounce period (a few seconds). The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electromagnetic modes is finally obtained through the quasi neutrality condition and the Ampere's law for the current density. The present talk will focus on the main results of this theory. The electrostatic version of the model may be applied to the near-Earth environment (8-12 R_{E}) where beta is rather low. It is showed that inclusion of bouncing electron motion may enhance strongly the growth rate of the classical drift wave instability. This model could thus explain the generation of strong parallel electric fields in the ionosphere and the formation of aurora beads with wavelength of a few hundreds of km. In the electromagnetic version, it is found that for mildly stretched current sheet (B_{z} > 0.1 B _{lobes}) undamped modes oscillate at typical electron bounce frequency with wavelength of the order of the plasma sheet thickness. As the stretching of the plasma sheet becomes more intense, the frequency of these normal modes decreases and beyond a certain threshold in B_{z}/B _{lobes}, the mode becomes explosive (pure imaginary frequency) with typical growing rate of a few
NASA Astrophysics Data System (ADS)
Petit, L.; Paudyal, D.; Mudryk, Y.; Gschneidner, K. A.; Pecharsky, V. K.; Lüders, M.; Szotek, Z.; Banerjee, R.; Staunton, J. B.
2015-11-01
We explain a profound complexity of magnetic interactions of some technologically relevant gadolinium intermetallics using an ab initio electronic structure theory which includes disordered local moments and strong f -electron correlations. The theory correctly finds GdZn and GdCd to be simple ferromagnets and predicts a remarkably large increase of Curie temperature with a pressure of +1.5 K kbar-1 for GdCd confirmed by our experimental measurements of +1.6 K kbar-1 . Moreover, we find the origin of a ferromagnetic-antiferromagnetic competition in GdMg manifested by noncollinear, canted magnetic order at low temperatures. Replacing 35% of the Mg atoms with Zn removes this transition, in excellent agreement with long-standing experimental data.
NASA Astrophysics Data System (ADS)
Gao, Junming; Cui, Zhan; Hao, Bailin
1990-08-01
A knowldge-based project, the GRAPE system(Group Representation and Application in Physics Environment), is described in this paper. The GRAPE system is designed to provide physicists with a group theoretical environment to help them solve problems in group theory and representation. The user can communicate with GRAPE in plain English. At the present stage, it contains the knowledge of crystallography point groups, space groups as well as magnetic space groups both in group structure and group representations. The GRAPE system consists of five modules besides the knowledge base and the data base: a natural language interface, a computation module, a tutprial module, a bibliography module, and a program library. Group theoretical analysis for the Landau theory of continuous phase transitions has been the first application of the GRAPE system. The calculation for determining directions of phase transition at the Γ point for 230 space groups, 230 grey space groups and 674 black and white magnetic space groups has been performed.
Clemens, Benjamin; Regenbogen, Christina; Koch, Kathrin; Backes, Volker; Romanczuk-Seiferth, Nina; Pauly, Katharina; Shah, N. Jon; Schneider, Frank; Habel, Ute; Kellermann, Thilo
2015-01-01
In functional magnetic resonance imaging (fMRI) studies that apply a “subsequent memory” approach, successful encoding is indicated by increased fMRI activity during the encoding phase for hits vs. misses, in areas underlying memory encoding such as the hippocampal formation. Signal-detection theory (SDT) can be used to analyze memory-related fMRI activity as a function of the participant’s memory trace strength (d′). The goal of the present study was to use SDT to examine the relationship between fMRI activity during incidental encoding and participants’ recognition performance. To implement a new approach, post-experimental group assignment into High- or Low Performers (HP or LP) was based on 29 healthy participants’ recognition performance, assessed with SDT. The analyses focused on the interaction between the factors group (HP vs. LP) and recognition performance (hits vs. misses). A whole-brain analysis revealed increased activation for HP vs. LP during incidental encoding for remembered vs. forgotten items (hits > misses) in the insula/temporo-parietal junction (TPJ) and the fusiform gyrus (FFG). Parameter estimates in these regions exhibited a significant positive correlation with d′. As these brain regions are highly relevant for salience detection (insula), stimulus-driven attention (TPJ), and content-specific processing of mnemonic stimuli (FFG), we suggest that HPs’ elevated memory performance was associated with enhanced attentional and content-specific sensory processing during the encoding phase. We provide first correlative evidence that encoding-related activity in content-specific sensory areas and content-independent attention and salience detection areas influences memory performance in a task with incidental encoding of facial stimuli. Based on our findings, we discuss whether the aforementioned group differences in brain activity during incidental encoding might constitute the basis of general differences in memory performance
NASA Astrophysics Data System (ADS)
Singh, Gurpreet; Mohanty, B. P.; Saini, G. S. S.
2016-02-01
Structure, vibrational and nuclear magnetic resonance spectra, and antioxidant action of ascorbic acid towards hydroxyl radicals have been studied computationally and in vitro by ultraviolet-visible, nuclear magnetic resonance and vibrational spectroscopic techniques. Time dependant density functional theory calculations have been employed to specify various electronic transitions in ultraviolet-visible spectra. Observed chemical shifts and vibrational bands in nuclear magnetic resonance and vibrational spectra, respectively have been assigned with the help of calculations. Changes in the structure of ascorbic acid in aqueous phase have been examined computationally and experimentally by recording Raman spectra in aqueous medium. Theoretical calculations of the interaction between ascorbic acid molecule and hydroxyl radical predicted the formation of dehydroascorbic acid as first product, which has been confirmed by comparing its simulated spectra with the corresponding spectra of ascorbic acid in presence of hydrogen peroxide.
NASA Astrophysics Data System (ADS)
Maruyama, Tomoyuki; Cheoun, Myung-Ki; Kajino, Toshitaka; Mathews, Grant J.
2016-06-01
We study pion production by proton synchrotron radiation in the presence of a strong magnetic field when the Landau numbers of the initial and final protons are ni,f ∼104-105. We find in our relativistic field theory calculations that the pion decay width depends only on the field strength parameter which previously was only conjectured based upon semi-classical arguments. Moreover, we also find new results that the decay width satisfies a robust scaling relation, and that the polar angular distribution of emitted pion momenta is very narrow and can be easily obtained. This scaling implies that one can infer the decay width in more realistic magnetic fields of 1015 G, where ni,f ∼1012-1013, from the results for ni,f ∼104-105. The resultant pion intensity and angular distributions for realistic magnetic field strengths are presented and their physical implications discussed.
NASA Astrophysics Data System (ADS)
Maruyama, Tomoyuki; Cheoun, Myung-Ki; Kajino, Toshitaka; Mathews, Grant J.
2016-06-01
We study pion production by proton synchrotron radiation in the presence of a strong magnetic field when the Landau numbers of the initial and final protons are ni,f ∼104-105. We find in our relativistic field theory calculations that the pion decay width depends only on the field strength parameter which previously was only conjectured based upon semi-classical arguments. Moreover, we also find new results that the decay width satisfies a robust scaling relation, and that the polar angular distribution of emitted pion momenta is very narrow and can be easily obtained. This scaling implies that one can infer the decay width in more realistic magnetic fields of 1015 G, where ni,f ∼1012-1013, from the results for ni,f ∼104-105. The resultant pion intensity and angular distributions for realistic magnetic field strengths are presented and their physical implications discussed.
ERIC Educational Resources Information Center
Wida, Sam
1992-01-01
Uses extremely strong neodymium magnets to demonstrate several principles of physics including electromagnetic induction, Lenz's Law, domain theory, demagnetization, the Curie point, and magnetic flux lines. (MDH)
Relativistic generation of vortex and magnetic field
Mahajan, S. M.; Yoshida, Z.
2011-05-15
The implications of the recently demonstrated relativistic mechanism for generating generalized vorticity in purely ideal dynamics [Mahajan and Yoshida, Phys. Rev. Lett. 105, 095005 (2010)] are worked out. The said mechanism has its origin in the space-time distortion caused by the demands of special relativity; these distortions break the topological constraint (conservation of generalized helicity) forbidding the emergence of magnetic field (a generalized vorticity) in an ideal nonrelativistic dynamics. After delineating the steps in the ''evolution'' of vortex dynamics, as the physical system goes from a nonrelativistic to a relativistically fast and hot plasma, a simple theory is developed to disentangle the two distinct components comprising the generalized vorticity--the magnetic field and the thermal-kinetic vorticity. The ''strength'' of the new universal mechanism is, then, estimated for a few representative cases; in particular, the level of seed fields, created in the cosmic setting of the early hot universe filled with relativistic particle-antiparticle pairs (up to the end of the electron-positron era), are computed. Possible applications of the mechanism in intense laser produced plasmas are also explored. It is suggested that highly relativistic laser plasma could provide a laboratory for testing the essence of the relativistic drive.
NASA Astrophysics Data System (ADS)
Wahlberg, C.; Graves, J. P.
2016-07-01
Ideal magnetohydrodynamic (MHD) theory is used to investigate some of the fundamental properties of the geodesic acoustic continuum modes (GAMs) in tokamaks, including their global structure, their associated magnetic components both inside and outside the plasma, and effects of a non-circular cross section of the plasma. In addition to the well-known m=1 side-bands in the perturbed density and pressure of the (electrostatic) GAM, the MHD continuum GAM also includes a m=1 side-band in the perturbed toroidal magnetic field as well as m=2 side-bands in the perturbed density, pressure, poloidal flow and in the magnetic components δ {{B}r} and δ {{B}θ} (m is the poloidal mode number). These m=2 side-bands exist within the whole plasma and the magnetic components also outside the plasma, and the magnitudes of these components in the vacuum region are calculated in the paper. It is shown that, for plasmas with a conducting wall not too far from the plasma surface, the perturbed magnetic field in the vacuum region is dominated by its poloidal component δ {{B}θ} , with poloidal dependence \\sin 2θ , in agreement with experiments. Aspects of the plasma equilibrium that affect the magnitude of the perturbed magnetic field in the vacuum region are discussed in the paper. Furthermore, the influence of a non-circular plasma cross section on the GAM frequency and on the spectrum of the global, perturbed magnetic field is analysed. It is found that the only significant effect of a non-circular cross section on the GAM frequency comes from elongation and its variation across the plasma radius. However, higher-order shaping effects, as well as finite aspect ratio, induce other Fourier components than m=2 in the magnetic halo that surrounds the GAM surface.
Zhang, Yachao; Yang, Yang; Jiang, Hong
2013-12-12
The 3d-4f exchange interaction plays an important role in many lanthanide based molecular magnetic materials such as single-molecule magnets and magnetic refrigerants. In this work, we study the 3d-4f magnetic exchange interactions in a series of Cu(II)-Gd(III) (3d(9)-4f(7)) dinuclear complexes based on the numerical atomic basis-norm-conserving pseudopotential method and density functional theory plus the Hubbard U correction approach (DFT+U). We obtain improved description of the 4f electrons by including the semicore 5s5p states in the valence part of the Gd-pseudopotential. The Hubbard U correction is employed to treat the strongly correlated Cu-3d and Gd-4f electrons, which significantly improve the agreement of the predicted exchange constants, J, with experiment, indicating the importance of accurate description of the local Coulomb correlation. The high efficiency of the DFT+U approach enables us to perform calculations with molecular crystals, which in general improve the agreement between theory and experiment, achieving a mean absolute error smaller than 2 cm(-1). In addition, through analyzing the physical effects of U, we identify two magnetic exchange pathways. One is ferromagnetic and involves an interaction between the Cu-3d, O-2p (bridge ligand), and the majority-spin Gd-5d orbitals. The other one is antiferromagnetic and involves Cu-3d, O-2p, and the empty minority-spin Gd-4f orbitals, which is suppressed by the planar Cu-O-O-Gd structure. This study demonstrates the accuracy of the DFT+U method for evaluating the 3d-4f exchange interactions, provides a better understanding of the exchange mechanism in the Cu(II)-Gd(III) complexes, and paves the way for exploiting the magnetic properties of the 3d-4f compounds containing lanthanides other than Gd. PMID:24274078
Parity nonconservation and the origin of cosmic magnetic fields
NASA Technical Reports Server (NTRS)
Vilenkin, A.; Leahy, D. A.
1982-01-01
Three mechanisms of cosmic magnetic field generation are discussed: (1) asymmetric decay of particles emitted by rotating black holes; (2) asymmetric proton emission by black holes due to weak radiative corrections, and (3) equilibrium parity-violating currents. It is shown that all three mechanisms can produce a seed field sufficiently strong to account for the present galactic fields.
NASA Astrophysics Data System (ADS)
Nicolas, G.; Dorantes-Dávila, J.; Pastor, G. M.
2006-07-01
The effects of orbital polarizations on the magnetic properties of transition-metal nanostructures are investigated in the framework of a self-consistent tight-binding theory. Three different approximations to the intra-atomic two-center Coulomb interactions are considered: (i) full orbital dependence of the direct and exchange Coulomb interactions Umm' and Jmm' as given by atomic symmetry, (ii) orbital independent interactions U=Umm'¯ and J=Jmm'¯ , and (iii) orbital polarization (OP) approximation of the form HOP=-(B/2)∑iLi2 , where Li refers to the orbital momentum operator at atom i and B to the Racah coefficient. Results are given for the local orbital magnetic moments ⟨Liδ⟩ along high-symmetry magnetization directions δ and for the corresponding magnetic anisotropy energies ΔEδγ of surfaces, films, and clusters of Fe, Co, and Ni. The quantitative differences between the approximations allow us to quantify the effects of orbital polarizations on ⟨Liδ⟩ and ΔEδγ . One observes that, with an appropriate choice of B , the OP ansatz yields a very good agreement with the rigorous orbital dependent calculations. The simplest orbital independent approach underestimates ⟨Liδ⟩ and ΔEδγ systematically. However, it provides a good qualitative description of the main general trends as a function of dimensionality, local environment, and d -band filling. Advantages and limitations of the various approaches are discussed.
Johnson, David Linton; Schwartz, Lawrence M
2015-06-01
Two-dimensional (2D) nuclear magnetic resonance (NMR) experiments involve a sequence of longitudinal (T(1)) and transverse (T(2)) measurements. In a previous paper we showed that if each of these 1D measurements can be represented by two exponential decays then there can be an accurate analytic solution for the 2D measurements with no additional information. In this paper we extend the theory to the case where there are three decay channels for the 1D measurements. The resulting analytic theory introduces a single free parameter, which is a rotation angle in the vector space spanned by the normal modes. Our predictions agree quite well with numerical results based on the microporous grain consolidation (μGC) model. The theory allows one to deduce information about decay modes in situations in which they may not be measurable in a conventional 1D measurement because the amplitude of that mode is too small. PMID:26172724
Theory of light-induced effective magnetic field in Rashba ferromagnets
NASA Astrophysics Data System (ADS)
Qaiumzadeh, Alireza; Titov, Mikhail
2016-07-01
Motivated by recent experiments on all-optical magnetization reversal in conductive ferromagnetic thin films we use nonequilibrium formalism to calculate the effective magnetic field induced in a Rashba ferromagnet by a short laser pulse. The main contribution to the effect originates in the direct optical transitions between spin-split subbands. The resulting effective magnetic field is inversely proportional to the impurity scattering rate and can reach the amplitude of a few Tesla in the systems like Co/Pt bilayers. We show that the total light-induced effective magnetic field in ferromagnetic systems is the sum of two contributions: a helicity dependent term, which is an even function of magnetization, and a helicity independent term, which is an odd function of magnetization. The primary role of the spin-orbit interaction is to widen the frequency range for direct optical transitions.
Theory of magnetic-field-induced polarization flop in spin-spiral multiferroics
NASA Astrophysics Data System (ADS)
Mochizuki, Masahito
2015-12-01
The magnetic-field-induced 90∘ flop of ferroelectric polarization P in a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a microscopic spin model. I find that the direction of the P flop or the choice of +Pa or -Pa after the flop is governed by magnetic torques produced by the applied magnetic field H acting on the Mn spins and thus is selected in a deterministic way, in contradistinction to the naively anticipated probabilistic flop. This mechanism resolves a puzzle of the previously reported memory effect in the P direction depending on the history of the magnetic-field sweep, and enables controlled switching of multiferroic domains by externally applied magnetic fields. My Monte-Carlo analysis also uncovers that the magnetic structure in the P ∥a phase under H ∥b is not a previously anticipated simple a b -plane spin cycloid but a conical spin structure.
Chaotic structures of nonlinear magnetic fields. I - Theory. II - Numerical results
NASA Technical Reports Server (NTRS)
Lee, Nam C.; Parks, George K.
1992-01-01
A study of the evolutionary properties of nonlinear magnetic fields in flowing MHD plasmas is presented to illustrate that nonlinear magnetic fields may involve chaotic dynamics. It is shown how a suitable transformation of the coupled equations leads to Duffing's form, suggesting that the behavior of the general solution can also be chaotic. Numerical solutions of the nonlinear magnetic field equations that have been cast in the form of Duffing's equation are presented.
NASA Astrophysics Data System (ADS)
Halpern, Paul
In 1943, Schrödinger presented several papers to the Royal Irish Academy outlining his efforts to unify electromagnetism, gravitation, and what is now known as the strong force in a Unitary Field Theory. To furnish experimental proof for his ideas, he suggested that variations in Earth's magnetic field might be understood through his theoretical notion that electromagnetism is attenuated by a kind of cosmological constant. We'll explore the nature and context of his proposal and examine reactions to his assertions.
NASA Astrophysics Data System (ADS)
Garibay-Alonso, R.; Dorantes-Dávila, J.; Pastor, G. M.
2015-05-01
A local electronic theory of transition-metal magnetism at finite temperatures is presented, which takes into account longitudinal and transverse spin fluctuations on the same footing. The magnetic properties are determined in the framework of a rotational-invariant d -band model Hamiltonian by applying a four-field Hubbard-Stratonovich functional-integral method in the static approximation. The role of transverse spin excitations on the temperature-dependent magnetic properties is investigated by performing alloy averages in the single-site virtual crystal approximation. Bulk Fe is considered as the representative example for the applications. Results are given for the average magnetization M , for the spin-excitation energies, and for the transverse and longitudinal contributions to the local magnetic moments μl at atom l . The importance of noncollinear spin excitations is quantified by comparison with the corresponding collinear calculations. An important reduction of about 33% of the calculated Curie temperature TC is obtained, which now amounts to 1250 K and is thus relatively close to the experimental value. The longitudinal (transverse) components of μl are found to decrease (increase) as a function of temperature until the full rotational symmetry is reached at TC. This reflects the increasing importance of the transverse spin fluctuations. The origin of the temperature dependence of M and μl is analyzed in terms of the local spin-fluctuation energies.
NASA Astrophysics Data System (ADS)
Sutter, Kiplangat
This thesis illustrates the utilization of Density functional theory (DFT) in calculations of gas and solution phase Nuclear Magnetic Resonance (NMR) properties of light and heavy nuclei. Computing NMR properties is still a challenge and there are many unknown factors that are still being explored. For instance, influence of hydrogen-bonding; thermal motion; vibration; rotation and solvent effects. In one of the theoretical studies of 195Pt NMR chemical shift in cisplatin and its derivatives illustrated in Chapter 2 and 3 of this thesis. The importance of representing explicit solvent molecules explicitly around the Pt center in cisplatin complexes was outlined. In the same complexes, solvent effect contributed about half of the J(Pt-N) coupling constant. Indicating the significance of considering the surrounding solvent molecules in elucidating the NMR measurements of cisplatin binding to DNA. In chapter 4, we explore the Spin-Orbit (SO) effects on the 29Si and 13C chemical shifts induced by surrounding metal and ligands. The unusual Ni, Pd, Pt trends in SO effects to the 29Si in metallasilatrane complexes X-Si-(mu-mt)4-M-Y was interpreted based on electronic and relativistic effects rather than by structural differences between the complexes. In addition, we develop a non-linear model for predicting NMR SO effects in a series of organics bonded to heavy nuclei halides. In chapter 5, we extend the idea of "Chemist's orbitals" LMO analysis to the quantum chemical proton NMR computation of systems with internal resonance-assisted hydrogen bonds. Consequently, we explicitly link the relationship between the NMR parameters related to H-bonded systems and intuitive picture of a chemical bond from quantum calculations. The analysis shows how NMR signatures characteristic of H-bond can be explained by local bonding and electron delocalization concepts. One shortcoming of some of the anti-cancer agents like cisplatin is that they are toxic and researchers are looking for
Magnetism in NiFeMo disordered alloys: Experiment and theory
NASA Astrophysics Data System (ADS)
Banerjee, Mitali; Banerjee, Rudra; Majumdar, A. K.; Mookerjee, Abhijit; Sanyal, Biplab; Nigam, A. K.
2010-10-01
In this communication we carry out experimental investigation of the behavior of magnetization with temperature and magnetic field of six samples at different compositions of the disordered ternary alloy NiFeMo. We analyze the data using a fist-principles density functional based electronic structure method and a mean-field phase diagram study.
Theory of long period magnetic pulsations, 3. Local field line oscillations
Hasegawa, A.; Tsui, K.H.; Assis, A.S.
1983-08-01
The local magnetic field is shown to oscillate at its Alfven resonance frequency (ies) in response to a wide band source whose frequency range covers the resonance frequency (ies). The proposed mechanism explains certain observations of magnetic pulsations where the frequency is found to vary continuously as a function of latitude for a given event.
Magnetic Control of Solutal Buoyancy-driven Convection. Part 1; Theory and Experiments
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Leslie, F. W.
2003-01-01
Experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented.
The magnetic field of a single axon. A comparison of theory and experiment.
Roth, B J; Wikswo, J P
1985-01-01
The magnetic field and the transmembrane action potential of a single nerve axon were measured simultaneously. The volume conductor model was used to calculate the magnetic field from the measured action potential, allowing comparison of the model predictions with the experimental data. After analyzing the experiment for all systematic errors, we conclude that the shape of the magnetic field can be accurately predicted from the transmembrane potential and, more importantly, the shape of the transmembrane potential can be calculated from the magnetic field. The data are used to determine ri, the internal resistance per unit length of the axon, to be 19.3 +/- 1.9 k omega mm-1, implying a value for the internal conductivity of 1.44 +/- 0.33 omega -1 m-1. Magnetic measurements are compared with standard bioelectric techniques for studying nerve axons. PMID:4016213
NASA Astrophysics Data System (ADS)
Rappazzo, A. F.
2015-12-01
Coronal loops threaded by a strong axial magnetic field, with its footpoints shuffled by photospheric motions, develop magnetically dominated turbulent dynamics where energy cascades from large to small scales, forming strong current sheets that can account for a large fraction of the observed heating. The conditions for the formation of current sheets will be briefly examined. Nevertheless such current sheets and the associated magnetic reconnection cannot be observed directly, since the predicted spatial scale at which magnetic energy is released as thermal energy in nanoflares is well below the resolution limits of present instrumentation for observing the corona. Therefore in order to constrain this model, advance our understanding ofcoronal heating and correctly interpret observations it is necessary to produce simulated "observables" that can be compared with observations. The propertied of the radiative emission computed from fully compressible simulations, and the observational implications of the topology developed by the turbulent magnetic field will be discussed.
NASA Astrophysics Data System (ADS)
Vélez, Ederley; Alberola, Antonio; Polo, Víctor
2009-10-01
The magnetic exchange coupling constants between two Mn(II) centers for a set of five inverse crown structures have been investigated by means of a methodology based on broken-symmetry unrestricted density functional theory. These novel and highly unstable compounds present superexchange interactions between two Mn centers, each one with S = 5/2 through anionic "guests" such as oxygen, benzene, or hydrides or through the cationic ring formed by amide ligands and alkali metals (Na, Li). Magnetic exchange couplings calculated at B3LYP/6-31G(d,p) level yield strong antiferromagnetic couplings for compounds linked via an oxygen atom or hydride and very small antiferromagnetic couplings for those linked via a benzene molecule, deprotonated in either 1,4- or 1,3- positions. Analysis of the magnetic orbitals and spin polarization maps provide an understanding of the exchange mechanism between the Mn centers. The dependence of J with respect to 10 different density functional theory potentials employed and the basis set has been analyzed.
Kirschvink, Joseph L.; Winklhofer, Michael; Walker, Michael M.
2010-01-01
The first demonstrations of magnetic effects on the behaviour of migratory birds and homing pigeons in laboratory and field experiments, respectively, provided evidence for the longstanding hypothesis that animals such as birds that migrate and home over long distances would benefit from possession of a magnetic sense. Subsequent identification of at least two plausible biophysical mechanisms for magnetoreception in animals, one based on biogenic magnetite and another on radical-pair biochemical reactions, led to major efforts over recent decades to test predictions of the two models, as well as efforts to understand the ultrastructure and function of the possible magnetoreceptor cells. Unfortunately, progress in understanding the magnetic sense has been challenged by: (i) the availability of a relatively small number of techniques for analysing behavioural responses to magnetic fields by animals; (ii) difficulty in achieving reproducible results using the techniques; and (iii) difficulty in development and implementation of new techniques that might bring greater experimental power. As a consequence, laboratory and field techniques used to study the magnetic sense today remain substantially unchanged, despite the huge developments in technology and instrumentation since the techniques were developed in the 1950s. New methods developed for behavioural study of the magnetic sense over the last 30 years include the use of laboratory conditioning techniques and tracking devices based on transmission of radio signals to and from satellites. Here we consider methodological developments in the study of the magnetic sense and present suggestions for increasing the reproducibility and ease of interpretation of experimental studies. We recommend that future experiments invest more effort in automating control of experiments and data capture, control of stimulation and full blinding of experiments in the rare cases where automation is impossible. We also propose new
Theory of the pulse response from a small antenna in a magnetized plasma
NASA Technical Reports Server (NTRS)
Grabbe, Crockett L.
1989-01-01
The electrostatic plasma response to a small pulsed antenna in a magnetic field is analyzed. The ringing of the plasma at three discrete frequencies--the upper-hybrid frequency and two resonance cone branch frequencies--is evidenced, and the amplitudes of these frequency responses is determined as a function of the characteristic plasma frequencies, the angle of observation with respect to the magnetic field, and the pulse length. Applications to plasma diagnostics are discussed. It is shown that the upper hybrid response and the response at either of the resonance cone branch frequencies is adequate information to determine the plasma density, and the magnetic field magnitude and angle.
NASA Astrophysics Data System (ADS)
Williams, P. Stephen; Carpino, Francesca; Zborowski, Maciej
2009-05-01
Quadrupole magnetic field-flow fractionation (QMgFFF) is a separation and characterization technique for magnetic nanoparticles such as those used for cell labeling and for targeted drug therapy. A helical separation channel is used to efficiently exploit the quadrupole magnetic field. The fluid and sample components therefore have angular and longitudinal components to their motion in the thin annular space occupied by the helical channel. The retention ratio is defined as the ratio of the times for non-retained and a retained material to pass through the channel. Equations are derived for the respective angular and longitudinal components to retention ratio.
NASA Astrophysics Data System (ADS)
Luo, Huijuan; Li, Hejun; Fu, Qiangang; Chu, Yanhui; Cao, Xiaoyu; Sun, Can; Yuan, Xiaoyan; Liu, Lei
2013-12-01
Understanding the interactions between graphene and biomolecules is of fundamental relevance to the area of nanobiotechnology. Herein, we take l-cysteine as the probe biomolecule and investigate its adsorption on pristine graphene and B-, N-, Al-, Ni-, Ga-, Pd-doped graphene using density functional theory calculations. Three kinds of upright adsorption configurations, via unprotonated functional groups (-SH, -NH2, -COOH), are considered. The calculations reveal pristine graphene physically adsorbs l-cysteine. N-doped graphene shows physisorption towards the S-end and N-end l-cysteine, and chemisorption towards the O-end radical. Strong chemisorption, with site-specific preference, occurs on Al-, Ni-, Ga- and Pd-doped graphene, accompanied by severe structural changes. Spin polarization with an unusual mirror symmetry on Ni- and Pd-doped graphene is induced by chemisorption of unprotonated l-cysteine, except for O-end adsorption on Pd-doped graphene. The magnetization arises mainly from spin polarization of the C 2pz orbital, with a minor magnetism located on Ni or Pd. The influence of van der Waals forces is also evaluated. A thorough analysis of the adsorption stability and magnetism of these systems would be beneficial to facilitate applications in graphene-based biosensing, biomolecule immobilization, magnetic bio-separation and other fields in bionanotechnology.
NASA Astrophysics Data System (ADS)
Reid, J.-Ph.; O'Neill, Chris; Walker, Alex; Lithgow, Calum; Abdul-Jabbar, Gino; Yelland, Edward; Sokolov, Dmitry A.; Huxley, Andrew D.
The ferromagnet PrPtAl is unlike any other. At the phase boundary between paramagnetism and ferromagnetism the fluctuations of the order parameter are so strong that energetically favourable phases of novel modulated magnetism emerge. In fact, it's the lack of order (the 'disorder') that is pivotal to promote a new 'order'. This mechanism is referred to as 'order by disorder' and is the centre of numerous theoretical studies. In this seminar, following an introduction on the topic of ferromagnetic materials, I will show how we can use both electrical and thermal conductivities to learn everything about these phases of modulated magnetism and to validate the predictions of the 'order by disorder' theory.
Xie, L; Wang, P; Pan, X Q
2014-08-01
The novel discovery of electron vortices carrying quantized orbital angular momentum motivated intensive research of their basic properties as well as applications, e.g. structural characterization of magnetic materials. In this paper, the fundamental interactions of electron vortices within infinitely long atomic-column-like electromagnetic fields are studied based on the relativistically corrected Pauli-Schrödinger equation and the perturbation theory. The relative strengths of three fundamental interactions, i.e. the electron-electric potential interaction, the electron-magnetic potential/field interaction and the spin-orbit coupling are discussed. The results suggest that the perturbation energies of the last two interactions are in an order of 10(3)-10(4) smaller than that of the first one for electron vortices. In addition, it is also found that the strengths of these interactions are strongly dependant on the spatial distributions of the electromagnetic field as well as the electron vortices. PMID:24690540
NASA Astrophysics Data System (ADS)
Baireuther, P.; Hutasoit, J. A.; Tworzydło, J.; Beenakker, C. W. J.
2016-04-01
We formulate a linear response theory of the chiral magnetic effect in a finite Weyl semimetal, expressing the electrical current density j induced by a slowly oscillating magnetic field B or chiral chemical potential μ in terms of the scattering matrix of Weyl fermions at the Fermi level. Surface conduction can be neglected in the infinite-system limit for δ j/δ μ , but not for δ j/δ B: the chirally circulating surface Fermi arcs give a comparable contribution to the bulk Weyl cones no matter how large the system is, because their smaller number is compensated by an increased flux sensitivity. The Fermi arc contribution to {μ }-1δ j/δ B has the universal value {(e/h)}2, protected by chirality against impurity scattering—unlike the bulk contribution of opposite sign.
NASA Astrophysics Data System (ADS)
Kurian, P.; Verzegnassi, C.
2016-01-01
We consider in a quantum field theory framework the effects of a classical magnetic field on the spin and orbital angular momentum (OAM) of a free electron. We derive formulae for the changes in the spin and OAM due to the introduction of a general classical background field. We consider then a constant magnetic field, in which case the relevant expressions of the effects become much simpler and conversions between spin and OAM become readily apparent. An estimate of the expectation values for a realistic electron state is also given. Our findings may be of interest to researchers in spintronics and the field of quantum biology, where electron spin has been implicated on macroscopic time and energy scales.
Rusz, Ján; Idrobo, Juan Carlos
2016-03-24
It was recently proposed that electron magnetic circular dichroism (EMCD) can be measured in scanning transmission electron microscopy (STEM) with atomic resolution by tuning the phase distribution of a electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. Here we present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.
NASA Astrophysics Data System (ADS)
Rusz, Ján; Idrobo, Juan Carlos
2016-03-01
It was recently proposed that electron magnetic circular dichroism can be measured in scanning transmission electron microscopy with atomic resolution by tuning the phase distribution of an electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. We present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.
Backhopping effect in magnetic tunnel junctions: Comparison between theory and experiment
Skowroński, Witold Wrona, Jerzy; Stobiecki, Tomasz; Ogrodnik, Piotr; Świrkowicz, Renata; Barnaś, Józef; Reiss, Günter; Dijken, Sebastiaan van
2013-12-21
We report on magnetic switching and backhopping effects due to spin-transfer-torque in magnetic tunnel junctions. Experimental data on current-induced switching in junctions with a MgO tunnel barrier reveal random back-and-forth switching between magnetization states, which appears when the current direction favors the parallel magnetic configuration. The effect depends on the barrier thickness t{sub b} and is not observed in tunnel junctions with very thin MgO tunnel barriers, t{sub b} < 0.95 nm. The switching dependence on bias voltage and barrier thickness is explained in terms of the macrospin model, with the magnetization dynamics described by the modified Landau-Lifshitz-Gilbert equation. Numerical simulations indicate that the competition between in-plane and out-of-plane torque components can result in a non-deterministic switching behavior at high bias voltages, in agreement with experimental observations. When the barrier thickness is reduced, the overall coupling between the magnetic layers across the barrier becomes ferromagnetic, which suppresses the backhopping effect.
Discuss on using Jiles-Atherton theory for charactering magnetic memory effect
NASA Astrophysics Data System (ADS)
Xu, Mingxiu; Xu, Minqiang; Li, Jianwei; Ma, Songshan; Xing, Haiyan
2012-11-01
The characterization of magnetic memory effect is the primary task of the metal magnetic memory (MMM) technique used in quantitative evaluation, and it has been characterized by the Jiles-Atherton (J-A) model in recent papers. The feasibility of using the MMM field to characterize magnetization intensity M and the feasibility of using the J-A model in MMM detection are discussed in this paper to clarify the magnetic memory effect in the elastic stage. According to analysis with the J-A model and the results of a rotating bending fatigue experiment, the MMM field follows the law that the state approaches equilibrium under the action of cyclic stress. This is similar to what the J-A model shows, only with replacement of the global equilibrium state Man in the J-A model with the local equilibrium state M0. The expression of M0 is obtained by considering the energy balance in the process of magnetization. M0 changes with the pinning resistance, external field, and type of stress cycle. Additionally, the M0-σ curve is a loop around the Man-σ curve, which is consistent with experimental results. How the modified J-A model describes the variation in MMM field in the process of fatigue is also discussed. This paper shows that the J-A model can be used to analyze the magnetic memory effect in the process of fatigue after replacing Man with M0.
NASA Technical Reports Server (NTRS)
Mikellides, Ioannis G.; Katz, Ira; Hofer, Richard R.; Goebel, Dan M.
2012-01-01
A proof-of-principle effort to demonstrate a technique by which erosion of the acceleration channel in Hall thrusters of the magnetic-layer type can be eliminated has been completed. The first principles of the technique, now known as "magnetic shielding," were derived based on the findings of numerical simulations in 2-D axisymmetric geometry. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster. This magnetically shielded (MS) thruster was then built and tested. Because neither theory nor experiment alone can validate fully the first principles of the technique, the objective of the 2-yr effort was twofold: (1) to demonstrate in the laboratory that the erosion rates can be reduced by >order of magnitude, and (2) to demonstrate that the near-wall plasma properties can be altered according to the theoretical predictions. This paper concludes the demonstration of magnetic shielding by reporting on a wide range of comparisons between results from numerical simulations and laboratory diagnostics. Collectively, we find that the comparisons validate the theory. Near the walls of the MS thruster, theory and experiment agree: (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered by at least 2.5-3 times compared to the unshielded (US) thruster. Also, based on carbon deposition measurements, the erosion rates at the inner and outer walls of the MS thruster are found to be lower by at least 2300 and 1875 times, respectively. Erosion was so low along these walls that the rates were below the resolution of the profilometer. Using a sputtering yield model with an energy threshold of 25 V, the simulations predict a reduction of 600 at the MS inner wall. At the outer wall ion energies are computed to be below 25 V, for which case we set the erosion to zero in the simulations. When a 50-V threshold is used the computed ion energies are below the threshold at both
Gao Zhifeng; Zhang Jing
2009-04-15
In this paper, we use the method of calculus of variations to establish the existence of energy-minimizing radially symmetric magnetic monopole solutions in the general (4p-1)-dimensional Yang-Mills gauge field theory developed recently by Radu and Tchrakian. We also show that these solutions are either self-dual or anti-self-dual and, hence, unique. Our study extends the existence work of Belavin, Polyakov, Schwartz, and Tyupin and the equivalence and uniqueness work of Maison in three dimensions and the work of Yang in seven dimensions to the situation of arbitrary (4p-1) dimensions.
Bammer, Roland; Stollberger, Rudolf
2012-01-01
Counterexamples are used to motivate the revision of the established theory of tracer transport. Then dynamic contrast enhanced magnetic resonance imaging in particular is conceptualized in terms of a fully distributed convection–diffusion model from which a widely used convolution model is derived using, alternatively, compartmental discretizations or semigroup theory. On this basis, applications and limitations of the convolution model are identified. For instance, it is proved that perfusion and tissue exchange states cannot be identified on the basis of a single convolution equation alone. Yet under certain assumptions, particularly that flux is purely convective at the boundary of a tissue region, physiological parameters such as mean transit time, effective volume fraction, and volumetric flow rate per unit tissue volume can be deduced from the kernel. PMID:17429633
NASA Astrophysics Data System (ADS)
de Andrade, L. C. G.
2016-01-01
A generalized dynamo equation in the first order torsion Garcia de Andrade L C (2012 Phys. Lett. B 711 143) has previously been derived. From this equation it is shown that for the 10 kpc scale, torsion gravity is not able to help seed galactic dynamos since the dynamo time is not long enough to take into account structure formation. In this paper, the dynamo equation is extended to second-order torsion terms—but unfortunately, the situation is even worse and the torsion does not seem to help dynamo efficiency. Nevertheless, in the intergalactic magnetic field scale of 1 mpc, the efficiency of the self-induction equation with torsion changes, and even in the first-order torsion case, one obtains large-scale magnetic fields with 109 yr dynamo efficiency. Dynamo efficiency in the case of interstellar matter (ISM) reaches a diffusion time of 1013 yr. This seems to be in contrast with a recent investigation by Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08) where they obtained, from another type of torsion theory called teleparallelism (A Einstein, Math Annalen (1922)), a large scale intergalactic magnetic field of 10-9 G. If this is not a model-dependent result, there is an apparent contradiction that has to be addressed. It is shown that for dynamo efficiency in astrophysical flow without shear, a strong seed field of 10-11 G is obtained, which is suitable for seeding galactic dynamos. As an example of a non-parity-violating dynamo equation, a magnetic field of the order of 10-27G is obtained as a seed field for the galactic dynamo from the theory of Einstein’s unified teleparallelism. This shows that in certain gravity models, torsion is able to enhance cosmological magnetic fields in view of obtaining better dynamo efficiency. To better compare our work with Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08), we consider the slow decay of magnetic fields in the teleparallel model. This observation is due to an anonymous referee who
On the origin of cosmic magnetic fields
NASA Astrophysics Data System (ADS)
Kulsrud, Russell M.; Zweibel, Ellen G.
2008-04-01
and most popular mechanism is the α-Ω mean field dynamo theory developed by a number of people in the late sixties. This theory and its application to galactic magnetic fields is discussed in considerable detail in this review. We point out certain difficulties with this theory that make it seem unlikely that this is the whole story. The main difficulty with this as the only such amplification mechanism is rooted in the fact that, on galactic scales, flux is constant and is frozen in the interstellar medium. This implies that flux must be removed from the galactic discs, as is well recognized by the standard theory. For our Galaxy this turns out to be a major problem, since unless the flux and the interstellar mass are somehow separated, some interstellar mass must also be removed from the deep galactic gravitational well. This is very difficult. It is pointed out that unless the field has a substantial field strength, much larger than that of the seed fields, this separation can hardly happen. And of course, it must if the α-Ω dynamo is to start from the ultra weak seed field. (It is our philosophy, expressed in this review, that if an origin theory is unable to create the magnetic field in our Galaxy it is essentially incomplete.) Thus, it is more reasonable for the first and largest amplification to occur before the Galaxy forms, and the matter embedded in the field is gravitationally trapped. Two such mechanisms are discussed for such a pregalactic origin; (1) they are generated in the turbulence of the protogalaxy and (2) the fields come from giant radio jets. Several arguments against a primordial origin are also discussed, as are ways around them. Our conclusion as to the most likely origin of cosmic magnetic fields is that they are first produced at moderate field strengths by primordial mechanisms and then changed and their strength increased to their present value and structure by a galactic disc dynamo. The primordial mechanisms have not yet been
NASA Technical Reports Server (NTRS)
Gary, S. P.; Tokar, R. L.
1985-01-01
The present investigation is concerned with the application of a second-order theory for electromagnetic instabilities in a collisionless plasma to two modes which resonate with hot ion beams. The application of the theory is strictly limited to the linear growth phase. However, the application of the theory may be extended to obtain a description of the beam at postsaturation if the wave-beam resonance is sufficiently broad in velocity space. Under the considered limitations, it is shown that, as in the cold beam case, the fluctuating fields do not gain appreciable momentum and that the primary exchange of momentum is between the beam and main component.
Nonisothermal theory of the positive column of an electric discharge in the axial magnetic field
Ul'yanov, D. K. Ul'yanov, K. N.
2013-01-15
A nonisothermal model of the positive column allowing for electron energy balance is analyzed. The influence of the axial magnetic field on the characteristics of the cylindrical positive column of a low-pressure discharge is investigated in the hydrodynamic approximation. It is shown that the magnetic field affects the plasma density distribution, plasma velocity, and electron energies. The radial dependences of the plasma density, electron energy, and plasma velocity, as well as the azimuthal velocities of electrons and ions, are calculated for helium at different values of the magnetic field strength. It is established that inertia should be taken into account in the equations for the azimuthal motion of electrons and ions. The results obtained in the hydrodynamic approximation differ significantly from those obtained in the framework of the common diffusion model of the positive column in the axial magnetic field. It is shown that the distributions of the plasma density and radial plasma velocity in the greater part of the positive column tend to those obtained in the diffusion approximation at higher values of the axial magnetic field and gas density, although substantial differences remain in the near-wall region.
NASA Technical Reports Server (NTRS)
Stepinski, Tomasz F.; Reyes-Ruiz, Mauricio; Vanhala, Harri A. T.
1993-01-01
A hydromagnetic dynamo provides the best mechanism for contemporaneously producing magnetic fields in a turbulent solar nebula. We investigate the solar nebula in the framework of a steady-state accretion disk model and establish the criteria for a viable nebular dynamo. We have found that typically a magnetic gap exists in the nebula, the region where the degree of ionization is too small for the magnetic field to couple to the gas. The location and width of this gap depend on the particular model; the supposition is that gaps cover different parts of the nebula at different evolutionary stages. We have found, from several dynamical constraints, that the generated magnetic field is likely to saturate at a strength equal to equipartition with the kinetic energy of turbulence. Maxwell stress arising from a large-scale magnetic field may significantly influence nebular structure, and Maxwell stress due to small-scale fields can actually dominate other stresses in the inner parts of the nebula. We also argue that the bulk of nebular gas, within the scale height from the midplane, is stable against Balbus-Hawley instability.
On the theory of magnetic field generation by relativistically strong laser radiation
Berezhiani, V.I.; Shatashvili, N.L.; Mahajan, S.M. |
1996-07-01
The authors consider the interaction of subpicosecond relativistically strong short laser pulses with an underdense cold unmagnetized electron plasma. It is shown that the strong plasma inhomogeneity caused by laser pulses results in the generation of a low frequency (quasistatic) magnetic field. Since the electron density distribution is determined completely by the pump wave intensity, the generated magnetic field is negligibly small for nonrelativistic laser pulses but increases rapidly in the ultrarelativistic case. Due to the possibility of electron cavitation (complete expulsion of electrons from the central region) for narrow and intense beams, the increase in the generated magnetic field slows down as the beam intensity is increased. The structure of the magnetic field closely resembles that of the field produced by a solenoid; the field is maximum and uniform in the cavitation region, then it falls, changes polarity and vanishes. In extremely dense plasmas, highly intense laser pulses in the self-channeling regime can generate magnetic fields {approximately} 100 Mg and greater.
Difficulties in learning the introductory magnetic field theory in the first years of university
NASA Astrophysics Data System (ADS)
Guisasola, Jenaro; Almudí, José M.; Zubimendi, José L.
2004-05-01
This study examined university engineering and physical science students' misconceptions of the nature of magnetic field. It is assumed that a significant knowledge of the sources of magnetic field is a basic prerequisite when students have to think about electromagnetic phenomena. To analyze students' conceptions, we have taken into account the fact that individuals build mental representations to help them understand the functioning of a physical system. These mental representations include different explanatory categories of reality in one same individual, depending on the context and the contents concerned. Therefore, we have designed an interview and an open-question questionnaire with an emphasis on explanations, so as to analyze the students' reasoning. We found that most of the students failed to identify the source of the magnetic field and they confuse magnetic force and magnetic field. It is concluded that although the questionnaire and interviews involved a wide range of phenomena, the misconceptions identified fall into four main categories of explanations which can inform curriculum development by identifying the strengths and weaknesses of the students' conceptions.
Theory of electronic and magnetic properties of weak antiferromagnetic TiAu
NASA Astrophysics Data System (ADS)
Goh, Wen Fong; Pickett, Warren
To date, only a few itinerant magnetic compounds have been found, viz. ZrZn2, TiBe2 and Sr3In, all comprised of nonmagnetic elements. TiAu, a newly synthesized itinerant weak antiferromagnet, orders antiferromagnetically below 36 K. Neutron diffraction reveals an ordered local moment of only 0.15 μB/Ti at a wave vector Q=(0, π/b,0). Hole doping, viz. Ti1-xScxAu, causes the magnetic moment to disappear at a quantum critical point xSc=0.13. We present results of an extensive study of the electronic and magnetic properties of TiAu. DFT calculations reveal van Hove singularities at (0,0.45 π/b,0.49 π/c), 4 meV above the Fermi level. Several types of analysis will be discussed: fixed spin moment studies and Stoner enchancement; magnetic energies; magnetism versus doping; Fermi surface nesting; corrections for spin fluctuations. Supported by Grant NSF DMREF DMR 1534719.
Theory of spin wave modes in tangentially magnetized thin cylindrical dots: A variational approach
NASA Astrophysics Data System (ADS)
Zivieri, R.; Stamps, R. L.
2006-04-01
We present a theoretical study of the quantized spin wave spectrum in tangentially magnetized cylindrical thin magnetic dots. Low-energy spin waves in magnetic dots may be subdivided into four families: Damon-Eshbach like, backward like, mixed, and end modes. Frequencies and mode profiles are found using a variational approach based on carefully chosen trial functions. The variational method has the advantage that it can be used for large dots that are not practical to treat using numerical finite-element methods. Results for small dots generated using the variational method compare well with micromagnetic results. The variational method is demonstrated with an analysis of data obtained from experimental Brillouin light scattering data from saturated thin cylindrical Permalloy dots. Our approach allows for the definition of parameters describing important contributions to the spin wave energies. As an example, we show that a variational parameter γ provides a measure of spin wave localization near the dot border for one class of modes.
Theory of magnetic field line random walk in noisy reduced magnetohydrodynamic turbulence
Ruffolo, D.; Matthaeus, W. H.
2013-01-15
When a magnetic field consists of a mean part and fluctuations, the stochastic wandering of its field lines is often treated as a diffusive process. Under suitable conditions, a stable value is found for the mean square transverse displacement per unit parallel displacement relative to the mean field. Here, we compute the associated field line diffusion coefficient for a highly anisotropic 'noisy' reduced magnetohydrodynamic model of the magnetic field, which is useful in describing low frequency turbulence in the presence of a strong applied DC mean magnetic field, as may be found, for example, in the solar corona, or in certain laboratory devices. Our approach is nonperturbative, based on Corrsin's independence hypothesis, and makes use of recent advances in understanding factors that control decorrelation over a range of parameters described by the Kubo number. Both Bohm and quasilinear regimes are identified.
Extended gyrokinetic field theory for time-dependent magnetic confinement fields
Sugama, H.; Watanabe, T.-H.; Nunami, M.
2014-01-15
A gyrokinetic system of equations for turbulent toroidal plasmas in time-dependent axisymmetric background magnetic fields is derived from the variational principle. Besides governing equations for gyrocenter distribution functions and turbulent electromagnetic fields, the conditions which self-consistently determine the background magnetic fields varying on a transport time scale are obtained by using the Lagrangian, which includes the constraint on the background fields. Conservation laws for energy and toroidal angular momentum of the whole system in the time-dependent background magnetic fields are naturally derived by applying Noether's theorem. It is shown that the ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work agree with the results from the conventional recursive formulation with the WKB representation except that collisional effects are disregarded here.
Ab initio theory for ultrafast magnetization dynamics with a dynamic band structure
NASA Astrophysics Data System (ADS)
Mueller, B. Y.; Haag, M.; Fähnle, M.
2016-09-01
Laser-induced modifications of magnetic materials on very small spatial dimensions and ultrashort timescales are a promising field for novel storage and spintronic devices. Therefore, the contribution of electron-electron spin-flip scattering to the ultrafast demagnetization of ferromagnets after an ultrashort laser excitation is investigated. In this work, the dynamical change of the band structure resulting from the change of the magnetization in time is taken into account on an ab initio level. We find a large influence of the dynamical band structure on the magnetization dynamics and we illustrate the thermalization and relaxation process after laser irradiation. Treating the dynamical band structure yields a demagnetization comparable to the experimental one.
Kinetic theory of alpha particles production in a dense and strongly magnetized plasma
NASA Astrophysics Data System (ADS)
Cereceda, Carlo; Deutsch, Claude; De Peretti, Michel; Sabatier, Michel; Basko, Mikhail M.; Kemp, Andreas; Meyer-ter-Vehn, Jurgend
2000-11-01
In connection with fundamental issues relevant to magnetized target fusion, the distribution function of thermonuclear alpha particles produced in situ in a dense, hot, and strongly magnetized hydrogenic plasma considered fully ionized in a cylindrical geometry is investigated. The latter is assumed in local thermodynamic equilibrium with Maxwellian charged particles. The approach is based on the Fokker-Planck equation with isotropic source S and loss s terms, which may be taken arbitrarily under the proviso that they remain compatible with a steady state. A novel and general expression is then proposed for the isotropic and stationary distribution f(v). Its time-dependent extension is worked out numerically. The solutions are valid for any particle velocity v and plasma temperature T. Higher order magnetic and collisional corrections are also obtained for electron gyroradius larger than Debye length. f(v) moments provide particle diffusion coefficient and heat thermal conductivity. Their scaling on collision time departs from Braginski's.