On the limiting velocity and forced motion of ferromagnetic domain walls in an external field perpendicular to the easy-magnetization axis

A theory is constructed for the dynamics and braking of domain walls in ferromagnets when a magnetic field is applied perpendicular to the axis of easy magnetization (i.e., a transverse field H _⊥). The theory is valid for velocities v up to the limiting domain wall velocity v c. The Landau-Lifshitz equations in the dissipationless approximation are used to investigate the motion of domain walls and the change in the character of the wall motion as its velocity v approaches v _c. The force acting on a domain wall due to viscous friction is calculated within the framework of generalized relaxation theory, and the dependence of the domain wall velocity v on the forcing field H _z is investigated. Calculations of the braking force show that the contributions of various dissipation mechanisms to the friction force have different dependences on the domain wall velocity, which affects the form of the function v=v(H _z). The shapes of the curves v(H _z) differ very markedly from one another for different values of the field H _⊥. The theory developed here can be used to describe the experimental results, in particular the almost linear behavior of v=v(H _z) for small H _⊥ and its strongly nonlinear behavior when H _⊥∼H _a, whereas these data cannot be reconciled within the standard theory based on relaxation terms of Hilbert type. Zh. éksp. Teor. Fiz. 112, 953–974 (September 1997)     

Soliton-magnon scattering in a two-dimensional isotropic magnetic material

We use the generalized σ-model to analytically study the solution of the problem of magnon scattering in two-dimensional isotropic ferromagnets and antiferromagnets in the presence of a Belavin-Polyakov soliton. We obtain the exact analytical solution to this problem for the partial mode with the azimuthal quantum number m=1. The scattering amplitude for other values of m (i.e., values not equal to unity) are studied analytically in the long-and short-wavelength approximations and also numerically for an arbitrary value of the wave number. We establish the general laws governing the soliton-magnon interaction. For a magnetic material of finite dimensions we calculate the frequencies of the magnon modes. We also use the data on local modes to derive the equations of motion of the soliton. Finally, we calculate the low-temperature (long-wavelength) asymptotic behavior of the magnon density of states due to the soliton-magnon interaction. Zh. éksp. Teor. Fiz. 116, 1091–1114 (September 1999)     

Effect of weak disorder on the magnetic properties of highly correlated electron systems

The effect of weak disorder on the magnetic properties of highly correlated electron systems is studied by the diagrammatic technique with X operators within the t-J-A model, which combines the t-J with the Anderson model. The generalized random-phase approximation (GRPA) is used to calculate the dynamic magnetic susceptibility, and the condition of its divergence is employed to determine the criteria of paramagnetic phase instability with respect to ferromagnetic and antiferromagnetic ordering. Magnetic phase diagrams are constructed, and the Curie and Néel temperatures are calculated. It is shown that weak disorder in a highly correlated electron system suppresses ferromagnetism and extends antiferromagnetism. Fiz. Tverd. Tela (St. Petersburg) 39, 1609–1615 (September 1997)     

Electronic,structural and magnetic properties of TbO under pressure: FP-LAPW study

ABSTRACT

Using the framework of the density functional theory, we calculated electronic, magnetic and structural properties of terbium oxide (TbO) in rocksalt (RS), cesium chloride (CsCl) and zincblende (ZB). Full potential linearized augmented plane wave (FP-LAPW) method within the local spin density approximation (LSDA) and generalized gradient (PBE-GGA) approximations are used. Magnetic and non-magnetic calculations are performed and a modified version of Becke and Johnson (mBJ) exchange potential has been used to calculate the band gaps. We found that, although TbO is stable in a ferromagnetic state, it is stable in RS phase at ambient condition. Both LSDA and PBE-GGA calculations revealed that the three structures are metallic. However, using the mBJ calculation, it is clear that RS and CsCl phases of TbO compound are metallic, while ZB phase is found to be an insulator in the spin-up case and a semiconductor in the spin-down case at ambient pressure.     

Generation of electromagnetic radiation in the motion of vortices in magnetically coupled superconducting films

We examine theoretically the generation of electromagnetic radiation in the relative motion of vortex lattices in magnetically coupled films in the dc transformer geometry. We establish the conditions under which the force of mutual pinning of the vortex lattices varies according to a harmonic law as a function of the relative displacement of the vortices in the films within a given range of magnetic field inductions. In this case the equation describing the viscous flow of vortex lattices in magnetically coupled films is the same as the equation of the resistively shunted Josephson junction model. We show that magnetically coupled superconductors exhibit the properties of a Josephson element without any restrictions on the geometrical size of such a system imposed by the coherence length ξ. The frequency f of the electromagnetic radiation generated by the relative motion of vortex lattices in magnetically coupled superconductors depends on the spatial period of the vortex lattices and the velocity of relative vortex motion, which means that the frequency of the radiation can be tuned by applying a magnetic field or a current. Zh. éksp. Teor. Fiz. 113, 1319–1338 (April 1998)     

Dense quark matter conductivity in ultra-intense magnetic field

Heavy-ion collisions generate a huge magnetic field of the order of 10¹⁸ G for a duration of about 0.2 fm/c. This time may become an order of magnitude longer if the electrical conductivity of quark matter is large. We calculate the conductivity in the regime of high density and show that contrary to naive expectations it only weakly depends on the magnetic field.     

Effects of a non-standardW ± magnetic moment inW ± production via deep inelastice −P scattering

We calculate the production of charged bosons in deep inelastice ^?P scattering in the context of an electroweak model in which the vector boson self interactions may be different from those prescribed by the electroweak standard model. We present results which show the dependence of the cross section on the anomalous magnetic dipole moment κ of theW ^±. We find for energies available at HERA that even small deviations from the standard model value of κ imply observable deviations in theW ^± production rates. We also show that the contributions from heavy boson exchange diagrams are very important.     

A general model for power dissipation by domain wall movement in thin ferromagnetic films

In this work we present a model for magnetic domain wall (DW) dynamics in metallic ferromagnets with uniaxial anisotropy. The model allows us to consider both viscous and eddy current dampings. While the latter mechanism involves only one degree of freedom — the coordinate of the DW, the former involves all degrees of freedom of the spins in magnetic domains and in DW’s. The structure of the DW itself must then be considered, the conjecture used allowing one to describe the spin precessions which may appear in low quality factor Q materials. The model built is nonlinear, allowing one to deal with the set of complex susceptibilities characterizing the response to an ac drive field in strongly nonlinear circumstances, and has a general character, due to the normalized quantities used. In this work we analyze the susceptibility and the dissipated power spectra as functions of a dc transversal field, which can be treated only by this kind of models. We show that the viscous damping can become comparable to eddy current damping when precessions are excited.     

Dynamos at extreme magnetic Prandtl numbers: insights from shell models

We present a magnetohydrodynamic (MHD) shell model suitable for computation of various energy fluxes of MHD turbulence for very small and very large magnetic Prandtl numbers Pm; such computations are inaccessible to direct numerical simulations. For small Pm, we observe that both kinetic and magnetic energy spectra scale as k^?5/3 in the inertial range, but the dissipative magnetic energy scales as k^?11/3exp?(? k/k_η). Here the kinetic energy at large length scale feeds the large-scale magnetic field that cascades to small-scale magnetic field, which gets dissipated by Joule heating. The large-Pm dynamo has a similar behaviour except that the dissipative kinetic energy scales as k^?13/3. For this case, the large-scale velocity field transfers energy to the large-scale magnetic field, which gets transferred to small-scale velocity and magnetic fields; the energy of the small-scale magnetic field also gets transferred to the small-scale velocity field, and the energy thus accumulated is dissipated by the viscous force.     

Gradientless propulsion of magnetic bubble domains

We find that some types of magnetic bubbles can be propelled by homogeneous fields: a modulated bias field superimposed on a steady in-plane field. Characteristic velocity vectors are observed which can discriminate among wall configurations having a common winding number s. Two particular complementary bubbles having s = 1 move in opposite directions nearly orthogonal to the in-plane field because the translational coercive force couples to motions of the wall radius and two Bloch lines.     

On the magnetic excitations in nickel

By comparing measured and calculated versions of the generalized magnetic response function of nickel over a wide range of temperatures it is shown that the much-discussed simple model of an itinerant-electron ferromagnet based on a random-phase-approximation treatment of the Hubbard Hamiltonian accounts, in its essentials, for the magnetism of that substance. Thermal neutron inelastic scattering techniques were used to explore the whole Brillouin zone of wave vectors K in nickel up to energies ▄ω of some 2k _B T _C over the range of temperatures up to 2T _C; the computations were based on direct evaluation of the Lindhard expression for the generalized susceptibility, using a tight-binding band structure, followed by exchange enhancement of the spin part as per Izuyama, Kim and Kubo. The contribution to the neutron scattering from orbital motion of the electrons was estimated to be small, and in a special experimental investigation a satisfactorily low upper limit on this quantity for our present purposes was observed.

In the course of the article we attempt to illuminate with our computational results the theoretical content of the random-phase approximation, and to show how it accounts for spin waves, paramagnons and a variety of other phenomena. For instance at low temperatures, where the collective or spin-wave mode is the most prominent feature, the Stoner-mode states are so strongly mixed with the spin-wave states by the electron-electron interaction that little trace is left of the ‘band of Stoner modes’ often referred to in elementary accounts of this phenomenon. With increase of temperature, the severe broadening of the spin wave and the characteristic style of evolution of the magnetic response function through the critical temperature are correctly predicted, outside a narrow range of conditions about the critical point. The dramatic divergence of the susceptibility characteristic of the phase transition is confined much more closely towards the origin of K,ω space than could be accounted for by free-electron gas theory, and this is explained. On the other hand what appears to be a case of breakdown of the random-phase approximation is detected in the ferromagnetic state, where in a certain set of conditions at large K and small ω an excess of longitudinal spin correlation was measured over that expected theoretically.

Aside from the aforementioned aberrant region, however, absolute agreement between experiment and theory essentially within the statistical error is obtainable by assuming that the effective Coulomb interaction parameter I _eff(K) falls off gently between K=0 and K _max in a manner consistent with the predictions of Singwi et al. ; with increase of temperature, I _eff(0) declines gently and the amount of fall-off with K increases. It is shown that the theory of the Heisenberg ferromagnet fails entirely to account for the observations—indeed, fails to correlate any of the data on nickel—and that at least three-quarters of the known magnetic stiffness of nickel must be due to band-theory effects rather than to Heisenberg-type exchange integrals.     

QED in arbitrary linear media: Amplifying media

Recently, we have developed a unified approach to QED in arbitrary linearly responding media in equilibrium—media that give rise to absorption [Phys. Rev. A 75, 053813 (2007)]. In the present paper we show that, under appropriate conditions, the theory can be quite naturally generalized to amplifying media the effect of which is described within the framework of linear response theory. We discuss the limits of validity of the generalized theory and make contact with earlier quantization schemes suggested for the case of linearly and locally responding amplifying dielectric-type media. To illustrate the theory, we present the electromagnetic-field correlation functions that determine the Casimir force in the presence of amplifying media.     

Nonmonotonic effects of perpendicular magnetic anisotropy on current-driven vortex wall motions in magnetic nanostripes

In a magnetic nanostripe, the effects of perpendicular magnetic anisotropy(PMA) on the current-driven horizontal motion of vortex wall along the stripe and the vertical motion of the vortex core are studied by micromagnetic simulations.The results show that the horizontal and vertical motion can generally be monotonously enhanced by PMA. However, when the current is small, a nonmonotonic phenomenon for the horizontal motion is found. Namely, the velocity of the horizontal motion firstly decreases and then increases with the increase of the PMA. We find that the reason for this is that the PMA can firstly increase and then decrease the confining force induced by the confining potential energy. In addition, the PMA always enhances the driving force induced by the current.     

Analytical results of the generalized mean-field theory for diluted magnetic semiconductors with RKKY interaction

Analytical results have been obtained in the framework of the generalized mean-field theory for diluted semiconductors with RKKY interaction. That theory accounts for the non-equivalency of different lattice sites by introducing the distribution function of local effective magnetic fields for non-regular (random) systems with magnetic interaction. The procedure is described that permits to deduce the analytical expression for that function. Corresponding improvement of the traditional mean-field theory could be observed by comparing results of such a generalized analytical model with exact results known for some simple cases, with numerical results of different authors considering the disorder of magnetic impurities’ arrangement, and with experimental data, as well.     

Reduction of magnetic noise in atom chips by material optimization

We discuss the influence of the material type in metal wires to the electromagnetic fluctuations in magnetic microtraps close to the surface of an atom chip. We show that significant reduction of the magnetic noise can be achieved by replacing the pure noble metal wires with their dilute alloys. The alloy composition provides an additional degree of freedom which enables a controlled reduction of both magnetic noise and resistivity if the atom chip is cooled. In addition, we provide a careful re-analysis of the magnetically induced trap loss observed by Yu-Ju Lin et al. [Phys. Rev. Lett. 92, 050404 (2004)] and find good agreement with an improved theory.     

Numerical investigation of subcooled boiling characteristics of magnetic nanofluid under the effect of quadrupole magnetic field

This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic nanofluid (refrigerant-113 and 4 vol% Fe₃O₄) in a vertical annulus, which is exposed to a nonuniform transverse magnetic field generated by the quadrupole magnet. A control volume technique and SIMPLEC algorithm have been used for discretizing the governing equations and pressure-velocity coupling, respectively. The two-fluid model has been used to simulate subcooled flow boiling of the refrigerant-113. The results indicate that subcooled flow boiling characteristics change not only by using nanofluid as the working fluid, but also by applying the nonuniform transverse magnetic field. In the presence of the aforementioned magnetic field due to the Kelvin force, the fluid attracted to the outer wall. This leads to higher bubble detachment frequency so that the heat pumping is increased and the void fraction on the heated wall is decreased. Thus, the critical heat flux as one of the most important parameters in boiling processes will be increased.     

Gyroscopic force acting on the magnetic vortex in a weak ferromagnet

The gyroscopic (Magnus) force in weak ferromagnets acting on magnetic vortices when the domain walls move in the external magnetic field has been investigated. The general expressions for the gyroscopic force in weak ferromagnets are obtained. The particular calculation of the gyroscopic force in rhombic weak ferromagnets is performed using the method that allows one to calculate it neglecting the internal structure of a vortex in the domain wall. It is shown that the gyroscopic force for most types of domain walls is nonzero and is determined by the mean sublattice magnetization, the Dzyaloshinsky interaction constant, and the constant of the exchange interaction between the sublattices.     

Negative differential viscosity in magnetic suspensions

Recent experiments have shown that the dependence of the macroscopic viscous stress on the mean velocity gradient during the Couette flow of concentrated magnetic suspensions in an external magnetic field is N-shaped. As the field strength is decreased, the amplitude of the N-shaped curve decreases and in the absence of the field, the stress monotonically increases with the shear velocity. A model is proposed to explain the shape of the rheological curve. The model assumes that the magnetic field initiates the formation of dense aggregates in a suspension, which connect the opposite walls of a measurement cell. In the Couette flow, the friction of aggregates on the cell walls causes their deviation from the applied magnetic field through an angle determined by the velocity of the relative motion of the walls. For large enough velocities, the aggregates are detached from the wall and are destroyed by viscous forces. It is shown that the friction of aggregates on cell walls results in the initial increasing and decreasing part of the N-shaped rheogram, while the flow after the detachment of aggregates corresponds to its right increasing part.     

A theory of the magnetic susceptibility of composites

The absorption of ac magnetic field energy by nonconducting composites made with fillers consisting of microscopic magnetic inclusions with various shapes is investigated over a wide range of frequencies and temperatures. It is predicted that the temperature dependence of the susceptibility χ(ω,T) of these structures is nonstandard only when the interaction between particles of the finely dispersed phase is included. The effect of the magnetic particles is taken into account by introducing a stochastic force into the Boltzmann equation, and using the resulting equation to calculate the susceptibility χ, which is a complicated function of the concentration p of the added dispersed phase. It is shown that the susceptibility should have a singularity near the point p=p _cr. Fiz. Tverd. Tela (St. Petersburg) 39, 1622–1627 (September 1997)     

Constant-cutoff approach to magnetic moments of hyperons

We calculate the magnetic moments of hyperons in the simplified CHK model, with the stabilizing term proportional toe ^–2 omitted, based on the constant-cutoff limit of the cutoff quantization method developed by Balakrishna, Sanyuk, Schechter, and Subbaraman, which avoids the difficulties with the usual soliton boundary conditions pointed out by Iwasaki and Ohyama. Thus we show that there is qualitative agreement with the experimental values and the accuracy is similar to that obtained with the complete CHK model.