Nonlinear domain-wall dynamics in a system of two magnetic layers

The nonlinear dynamics of the magnetization in a spin-valve structure is investigated. Equations describing the dynamics of the magnetization in such a structure are obtained. The stability of the solution corresponding to a motionless flat domain wall is investigated. The nonlinear domain-wall dynamics are investigated in the approximation of a strong exchange interaction between the magnetic layers and in the approximation of a large magnetostatic energy. In the former case the nonlinear dynamical equations are shown to be similar to the equations describing the dynamics of the magnetization in a weak ferromagnet, and in the latter case they are similar to the equations of motion of a magnetic vortex (i.e., a vertical Bloch line) in a domain wall. Zh. éksp. Teor. Fiz. 116, 1365–1374 (October 1999)     

Complex magnetic susceptibility of uniaxial superparamagnetic particles in a strong static magnetic field

The magnetic relaxation of a system of single-domain ferromagnetic particles in the presence of a strong static magnetic field directed at an arbitrary angle relative to the particle anisotropy axis is investigated. A system of linear difference-differential equations for the moments (averaged spherical harmonics) is derived without recourse to the Fokker-Planck equation by averaging Gilbert’s equations with a fluctuating field. An exact solution (in terms of matrix continuous fractions) is found for this system. The relaxation times and spectra of the complex magnetic susceptibility are calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 1642–1649 (September 1998)     

NMR in 55Mn2+ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr₃, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that agree very well with experiment at all fields up to the “collapse” field H _c of all sublattices. The unbiased NMR frequency in CsMnBr₃ is determined to be v _n0=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn²⁺ spins, which according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 3) the presence of two branches of the NMR spectrum in large H _c fields, in which the CsMnBr₃ must be a quasi-one-dimensional antiferromagnetic. Zh. éksp. Teor. Fiz. 113, 352–368 (January 1998) Deceased.     

Instability and breakup of a thick layer of a viscous magnetic fluid in a tilted magnetic field

A linear partial differential equation describing the evolution of an initial disturbance of a flat free surface of a thin layer of a viscous magnetic fluid covering a horizontal plate in the presence of a uniform magnetic field, is derived within a system of ferrohydrodynamic and magnetostatic equations. The effect of magnetizing the plate on the stability of the flat free surface is investigated. An estimate is obtained for the minimum value of the tangential component of the magnetization vector of the fluid sufficient to radically alter the pattern of the final breakup of the continuous layer. Zh. Tekh. Fiz. 69, 14–22 (October 1999)     

Equations of two-fluid hydrodynamics in the Hubbard model

A set of equations is obtained using stationary and nonstationary superconductivity theories in terms of the Hubbard model. The equations near T _c and for a weak magnetic field have the general form of superconductivity equations with a strongly anisotropic Fermi surface. Calculations are performed using a kinetic equation for quasiparticles. The low-temperature collective excitations in a superconductor are studied. An explicit relationship for the temperature dependence of second sound is obtained.     

Influence of the domain structure in a magnetizing field on the high-frequency susceptibility of a ferromagnet

The high-frequency properties of a biaxial domain-containing bulk ferromagnet in an external magnetic field with a modulated amplitude are investigated on the basis of the Landau-Lifshitz equations. The components of the frequency-dependent magnetic susceptibility tensor and the dependence of the resonant frequency of the uniform pulsation modes of a stripe domain on the amplitude of the external magnetic field are determined. Fiz. Tverd. Tela (St. Petersburg) 39, 671–675 (April 1997)     

Critical dynamics of slightly disordered spin systems

A new approach to the analysis of magnetic dipole motion in external magnetic field and fields generated by neighboring magnetic dipoles is suggested, and original general kinetic equations for the dipole density are derived. Special cases of these general equations are the Bloch, Redfield, and Provotorov equations, which are widely used in NMR theory. A comparison between NMR spectra calculated with the new theory and published experimental data also shows good agreement in regions to which the equations listed above do not apply. Zh. éksp. Teor. Fiz. 113, 967–980 (March 1998)     

Semiconductor-metal transition in FeSi in ultrahigh magnetic fields up to 450 T

The magnetic susceptibility and conductivity of single-crystal iron monosilicide are investigated in ultrahigh magnetic fields up to 450 T at a temperature of 77 K. It is found that the conductivity of iron monosilicide increases continuously by two orders of magnitude as the magnetic field increases. The results obtained can be interpreted as a semiconductor-metal transition induced by the magnetic field. The dependence of the conductivity on the magnetic field is described well on the basis of the spin-fluctuation theory. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 326–330 (25 August 1998)     

Ionization of atoms in electric and magnetic fields and the imaginary time method

A semiclassical theory is developed for the ionization of atoms and negative ions in constant, uniform electric and magnetic fields, including the Coulomb interaction between the electron and the atomic core during tunneling. The case of crossed fields (Lorentz ionization) is examined specially, as well as the limit of a strong magnetic field. Analytic equations are derived for arbitrary fields ℰ and ℋ that are weak compared to the characteristic intraatomic fields. The major results of this paper are obtained using the “imaginary time” method (ITM), in which tunneling is described using the classical equations of motion but with purely imaginary “time.” The possibility of generalizing the ITM to the relativistic case, as well as to states with nonzero angular momentum, is pointed out. Zh. éksp. Teor. Fiz. 113, 1579–1605 (May 1998)     

Exact solution for a degenerate Anderson impurity in the limit embedded into a correlated host

We consider the one-dimensional t - J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J = t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity. Received: 31 December 1997 / Accepted: 17 March 1998     

Electromagnetic waves in a round rod in the presence of an arbitrarily directed external magnetic field or anisotropy axis

A rigorous theory of the propagation of electromagnetic waves in round anisotropic and semiconductor rods in the presence of an arbitrarily directed anisotropy axis or external magnetic field is developed. New types of independent waves are discovered. Exact dispersion equations are obtained for them, which define the dependence of their spectral characteristics on the parameters of the semiconductor or anisotropic crystal and on the magnitude and direction of the constant external magnetic field. The results of numerical investigations for rods made from a semiconductor or a uniaxial crystal are presented. Zh. Tekh. Fiz. 67, 86–91 (July 1997)     

Parametric excitation of oscillations of charged particle bunches in a Penning trap

A self-similar solution is obtained for the self-consistent hydrodynamic equations describing the motion of an ellipsoid of charged particles in a Penning trap and in an rf trap. The conditions are determined for which a small periodic variation of the confining magnetic field in the Penning trap drives oscillations of the bunch. Zh. Tekh. Fiz. 67, 27–29 (January 1997)     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Neutron scattering on the strongly correlated electron CeNi Cu system: from non-magnetic behaviour to long-range magnetic order

The ground state of the strongly correlated electron CeNi_1-xCu_x compounds has been investigated by means of neutron scattering experiments. Thus, magnetic diffraction was performed for compounds showing long-range magnetic order (x > 0.2). An evolution from a collinear ferromagnetic structure for x =0.6 to a simple antiferromagnetic one for CeCu takes place through some more complex magnetic structures for intermediate compositions. The magnetic moments are continuously reduced when the Ni content increases reflecting the progressive enhancement of the Kondo screening. The large reduction found for x =0.6 compound is discussed and the existence of a spin glass like component of the magnetic moment cannot be discarded. From the quasielastic spectra, we have obtained the Kondo temperatures which are close to the magnetic ordering ones. The quasielastic line-width evolves from a linear temperature dependence to a T ^1/2 behaviour when approaching the non-magnetic limit. Then, this system provides an interesting example for the evolution of unstable 4 f shell relaxation regimes when modifying the hybridisation strength. Received 22 May 2000     

Nonlinear dynamics of the critical state in hard superconductors and composites based on them

The nonlinear dynamics of the magnetic flux within a superconducting plate in response to the continuous rise in temperature over the course of the entire process of applying the magnetic field is investigated within the critical-state model. The results of numerical simulations based on a method developed to solve the system of Fourier and Maxwell equations with an unknown internal magnetic flux penetration boundary are compared with the corresponding analytical expressions of the isothermal theory. It is shown that the difference between the isothermal and nonisothermal models increases as the heat transfer coefficient decreases and as the rate of increase in the magnetic field strength and the transverse dimensions of the superconductor increase. The errors appearing in the isothermal approximation are very significant in the case of a thermally insulated, massive conductor. Consequently, the calculated values of the thermal losses occurring during the time period preceding the flux jump in the isothermal approximation can be significantly lower than the corresponding nonisothermal values. Zh. Tekh. Fiz. 67, 29–33 (September 1997)     

Dynamical theory of thermal spin fluctuations in metallic ferromagnets

A self-consistent dynamical theory of thermal spin fluctuations is developed which describes their spatial correlation. It is based on the functional integral method and utilizes the quadratic representation for the electron free energy in a fluctuating exchange field with renormalized susceptibilities allowing for the interaction of various spin fluctuation modes. Interpolation between the single-site and homogeneous susceptibilities is used, where these susceptibilities are found self-consistently. The average over fluctuations takes account of both long-wavelength and local excitations. A closed system of equations is formed for both unknown quantities: the magnetization and the mean-square exchange field at a site. The basic characteristics of a specific magnet are the density of electron states and the atomic magnetic moment at T=0. A method is proposed for separating the relatively slow thermal-spin fluctuations from the rapid zero-spin fluctuations forming the ground state of the magnets. At T=0 we have a system of equations of mean field theory. The temperature excites thermal spin fluctuations, which are described by taking account of correlation in time and space. The magnetization, susceptibility, magnitude of the spin fluctuations and their distribution over momenta, and the degree of magnetic short-range order in iron are calculated as functions of the temperature in the ferromagnetic and paramagnetic phases, and also at the transition between them, the Curie temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 90–98 (January 1998)     

Dielectric linear response of magnetized electrons: Drag force on ions

The drag force on ions moving in a magnetized electron plasma is calculated in dielectric linear response. Various representations of the dielectric function ε(k, ω) are investigated for their suitability to display the limits for an infinite and a vanishing magnetic field. While the influence of the magnetic field is negligible in certain regions of k-space, it introduces in other regions a strong oscillatory structure in the dielectric function. This requires a careful treatment of the multidimensional integrations necessary for the drag force. The contributions from oscillatory integrands are treated by the saddle point method. Explicit results are obtained for the dependence of the drag force on the magnetic field, the direction of motion of the ion relative to the magnetic field, the shielding in the electron plasma, its density and the anisotropy of the electron temperature. The importance of the collective response of the electrons is investigated for limiting cases of the magnetic field. The validity of the linearization of the dielectric theory is checked by comparison with results obtained by numerical simulation of the nonlinear Vlasov-Poisson equation. For strong magnetic fields and low ion velocities, the simulations rather agree with the complementary binary collision model than with linear response.     

Role of surface phenomena in the magnetoresistivity of polycrystalline manganites La1−x CaxMnO3

The dc electrical resistivity and magnetoresistivity of polycrystalline manganites La_1−x Ca_xMnO₃ (x=0–0.3) are investigated as functions of the temperature, magnetic field and electric field, along with the microwave surface resistance. The investigations show that the dc electrical resistivity and magnetoresistivity are governed by the surface properties of the intergranular boundaries. The dc electrical resistivity is observed to decrease substantially (tenfold) for a comparatively small electric field (E⋟100 V/cm). Estimates are obtained for the internal electrical resistivity of the granules, the thickness of the contact layer (which depends on the temperature and the magnetic field), and the height of the potential barrier between the interfaces separating the surface layer and inner layer of a granule. Fiz. Tverd. Tela (St. Petersburg) 40, 1881–1884 (October 1998)     

Measurement of the temperature and flow fields of the magnetically stabilized cross-flow N2 arc

A straight, steady-state cross-flow arc is burning in an N₂ wind tunnel. The arc is held in position by the balance of the Lorentz forces produced by an external magnetic field perpendicular to the arc axis and by the viscous forces of the gas flow acting on the arc column. The temperature field in the discharge is determined spectroscopically using the radiation of N I lines. Because of the lack of rotational symmetry an inversion method developed by Maldonado was used to determine the local emission coefficient from the measured integrated spectral intensity distributions across the arc in various directions. For known local temperature the mass flow field inside the arc may be evaluated from the convective term of the energy equation and the continuity equation. This is done by expanding the terms of these two equations around the point of the temperature maximum into Fourier-Taylor series and determining coefficients of the same order and power. The solution of the resulting set of algebraic equations yields the unknown coefficients of the mass flow. The flow field obtained by these calculations shows a relatively strong counterflow through the arc core. In the region for which the series expansion holds a partial structure pertaining to a closed double vortex can be recognized. The terms of the momentum equation are calculated on the basis of these results. In order to obtain a better understanding of the importance attributed to the individual local forces acting on the plasma, a simple model was devised which separates the momentum equation into gradient and curl terms. The discussion shows that the gradient part of the Lorentz force causes mainly the pressure gradient, while the much smaller rotational part of thej×B forces is responsible for propelling the mass flow. The momentum transport inside the arc as well as in its neighbourhood is due to the viscous forces and to the pressure gradient. By contrast, at larger distances from the arc it is essentially the inertial force that determines the momentum transport. It is shown that viscosity as a damping mechanism is necessary for the existence of stationary flow fields as investigated in this work.     

Theory for the calculation of the force characteristics of an electromagnetic suspension of a superconducting body

A theory based on the method of secondary sources is developed for the calculation of the magnetic field and the force characteristics of electromagnetic suspension. This method leads to a system of Fredholm vector integral equations of the second kind in the density of the secondary sources, whose solution gives the surface currents in the superconducting bodies and then by a simple integration, the magnetic field and the force characteristics of the suspension. It is shown how the problem can be reformulated to apply it to the determination of the scalar secondary sources (magnetic charges), leading to integral equations of lower dimension. Examples are given for the calculation of scalar secondary sources for a superconducting half-space and a cylinder. Zh. Tekh. Fiz. 67, 3–9 (January 1997)