The transition of a uniaxial ferromagnetic film to the equilibrium state is studied in the case of a sharp change in the external parameters of the system, such as the temperature and external magnetic field. The motion of the dynamic interface between the paramagnetic phase and strip domain structure near the Curie temperature is discussed in terms of the Landau theory. The profile and typical size of the boundary, as well as the velocity of its stationary displacement, are determined by the envelope method. The results are generalized for the case of uniaxial ferromagnetic films near the second-order orientational phase transition from the uniform state to the strip domain structure.

The martensitic twin and magnetic domain structures of polycrystalline samples of the shape memory Ni2+xMn1-xGa alloys are studied by magnetooptic and Bitter colloid techniques. It is shown, that the analysis of the magnetic domain structure allows one to determine the martensitic structure characteristics, such as symmetry of phases and tetragonal axes orientation. A theoretical model is in good agreement with the experiment.

Behavior of the magnetic susceptibility of quasi-uniaxial ferromagnetic films in the vicinity of the Curie point was studied with allowance for a periodic spatial inhomogeneity in the distribution of the magnetization vector for an arbitrary frequency ω of an external magnetic field. In the presence of a constant bias magnetic field H 0 , the temperature dependence of the real and imaginary parts of the magnetic susceptibility exhibits, in the general case, both frequency-independent extrema and numerous additional extrema having shapes and positions dependent on frequency.

The spectrum of magnetoelastic waves in a periodic structure consisting of alternating ferromagnetic and nonmagnetic layers is studied. In the case of ferromagnetic layers with easy magnetic axes parallel to the layer surfaces, the orientational phase transition induced by a tangent external magnetic field H-0 is discussed. It is predicted that a nonuniform phase with the spatially modulated order parameter must appear, which is caused by a magnetostrictive coupling of magnetization to lattice strains near the interface between the magnetoelastic and elastic media. The phase diagram is obtained, and the existence domains of thermodynamically equilibrium collinear, angular, and domain phases are determined.

The general regularities of the evolution of the spectrum of magnetostatic waves in a periodic system composed of alternating ferromagnetic and nonmagnetic layers are analyzed. The spectrum of electromagnetic waves in an infinite periodic system and the coefficient of reflection of a plane electromagnetic wave from a half-space periodically filled with ferromagnetic and nonmagnetic layers are calculated. The dispersion relation is derived and analyzed for surface magnetostatic waves at the interface between the periodic system of layers and vacuum.

The spectrum of magnetoelastic waves in a periodic structure of alternating ferromagnetic and nonmagnetic layers was studied. In the case of ferromagnetic layers with easy magnetization axes parallel to the layer surfaces, an orientational phase transition induced by an external tangential magnetic field He was considered. The formation of an inhomogeneous phase with a spatially modulated order parameter, which is caused by the magnetization being coupled through magnetostriction to lattice strains near the interfaces separating the magnetoelastic from elastic media, is predicted. It is shown that at a certain critical field in excess of the orientational phase transition field in the system without magnetostriction, a magnetoelastic wave propagating in a direction parallel to the in-plane magnetization vector M becomes unstable at finite values of the wave vector and condenses into a magnetostriction domain structure. A phase diagram in the (L, T, He) coordinates is constructed, and the regions of existence of thermodynamically equilibrium collinear, canted, and domain phases are established (L and T are the thicknesses of the ferromagnetic and nonmagnetic layers, respectively).

The spectrum of magnetoelastic waves in a periodic structure with alternate dielectric ferromagnetic and nonmagnetic layers is studied. The dispersion relation for magnetoelastic shear modes propagating perpendicularly to medium interfaces is obtained. The frequencies of both elastic and spin wave resonances are calculated. The resonance frequency shift caused by interaction of elastic and magnetic subsystems is determined.

A study is made of the magnetoelastic (ME) wave spectrum and ME superstructure nucleation in a system consisting of magnetic/nonmagnetic multilayers. A case of rhombic ferromagnetic layers with the hard magnetization axis (b) over right arrow perpendicular to the layer surface is considered. We show that close to the phase transition associated with the spin reorientation in the layer plane, a ME wave with a horizontal polarization, propagating parallel to the layer plane, becomes unstable. The shear ME wave frequency and group velocity vanish for a finite value of a wave vector, and the wave becomes frozen, forming a ME domain structure localized near the layer interfaces. Existence of a new modulated phase is associated with a ME coupling of the magnetization to lattice deformation on the layer interfaces. The spectra of the surface ME in the homogeneous and modulated phases are calculated. Depending on the magnetic and nonmagnetic layer thickness and temperature, the stability regions of homogeneous collinear ((M) over right arrow parallel to(a) over right arrow) and angular phases and the ME domain structure are found, where (M) over right arrow is the magnetization, and (a) over right arrow is the easy orthorhombic axis direction. The calculation is extended to study the ME wave propagation in the systems consisting of two-sublattice orthoferrite/nonmagnetic multilayers.

We consider a dilute or semidilute polymer solution with localized attracting centers near a flat phase boundary and assume it driven by both stochastic and periodic forces. The attracting inhomogeneities restrict the free motion of macromolecules and play the role of fixed pinning centers. The flat boundary is modeled by a bistable potential whose minima attract the movable polymer segments between neighboring pinning points. We study the motion of these segments. The stochastic forces lead to stochastic oscillations of the polymer parts between the two potential wells near the phase boundary. Application of a small temporal periodic force can synchronize these oscillations and leads to the phenomenon of stochastic resonance for a nonvanishing noise intensity. As an outcome of our theory in agreement with numerical simulations, the resonance is stronger for wider and/or less deep potentials and observed at smaller values of the noise intensity. Additionally, we discuss under what conditions doubly stochastic resonance of the macromolecular motion occurs, that is, if bistability of the potential near the boundary originates in the action of multiplicative noise.

The propagation of electromagnetic waves in a composite medium based on an array of conducting wires in a ferromagnetic nonconducting matrix is discussed. It is demonstrated that, in certain ranges of frequencies and wavelengths, the composite under investigation can possess properties inherent in a “left-handed” medium. The regions of the existence of bulk and surface localized electromagnetic waves are explored. Consideration is given to the dispersion of surface electromagnetic waves in thick layers of the composite.

In Heusler-type alloy Ni2+x−yMn1−xFeyGa, partial substitution of Mn for Ni causes the temperatures of structural (martensitic) TM and magnetic TC (Curie point) phase transitions to converge. Close to the crossover of TM and TC, we have observed the strong strains (Δl/l≈2–4%) induced by the external magnetic field. This effect could be classed with colossal magnetostriction. The system exhibits the magnetic field-controlled one-way shape memory effect at fixed temperature as a result of the magnetic field-induced martensite to austenite structural phase transition.

A model for the motion of an elastic string is studied numerically and analytically. An elastic string in two dimensions and restricted by two pinning centers is considered. We consider two stable configurations (positively or negatively curved) with pinned ends due to the action of a bistable potential. It is further assumed that the string is driven externally by periodic and white noisy forces. The noise enables the string to flip between the two configurations. The small temporally periodic force synchronizes these flippings and the phenomenon of stochastic resonance is observed. The signal-to-noise ratio (SNR) of the output is investigated and shows a maximum for a nonvanishing intensity of the applied noise. Its dependence on the stiffness of the string is studied. The peak of SNR versus the noise intensity D is found to be more pronounced and to be shifted to small values of D with an increase in the effective stiffness of a string. The calculation is extended to stochastic resonance of vortex motion in a type II superconductor. For vortices restricted by two pinning centers, the characteristic time scale τR relevant to the vortex dynamics is shown to depend crucially on the effective vortex stiffness, with the time τR being extremely small for flexible vortices. Therefore, the effects of noise color on the vortex dynamics should be taken into account in many practical situations.

The magnetization and the giant magnetoresistance (GMR) of three-dimensional multilayered lattice of dipolar interacting fine anisotropic magnetic particles embedded in a nonmagnetic metallic matrix are numerically investigated. Using a Monte Carlo method the dependence of the magnetization curve and the GMR effect on the magnetic field H, temperature, the number of layers, the magnetic anisotropy, the separation between neighboring particles, and the particle-size distribution are examined systematically. We found that the enhanced dipolar interaction at high particle densities and for wide particle-size distributions reveals itself as a substantial suppression in the change in the negative MR with the applied field H. For out-of-plane magnetic field orientation, the negative MR is shown to increase more rapidly with H as the number of layers increases. This effect is attributed to the demagnetization field arising from the free magnetic poles forming at the sample surface. The relevance of the present results to the understanding of the magnetic and transport properties in granular composites is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

A reversible field-induced structural phase transition in shape-memory ferromagnetic alloys Ni_{2+x}Mn_{1-x}Ga has been observed at fixed temperature and pressure in magnetic fields about 100 kOe. The theoretical results are in qualitative agreement with experiment.

A ground state of a tangentially magnetized ferromagnetic film of arbitrary thickness L on a massive substrate in the vicinity of the spin-reorientational phase transition (RPT) induced by temperature is studied. A new magnetoelastic phase with spatially modulated order parameter is predicted, the existence of which is associated with the magnetostriction coupling of magnetization to lattice deformation on the crystal interface. It is demonstrated that the RPT from the uniform to the domain phase is the second order one. A domain nucleation close to the RPT temperature is analyzed. The main parameters of a developed domain structure away from the RPT temperature are determined.

Nonlinear self-localized surface waves in a ferroelastic crystal with striction coupling of the order parameter and the lattice deformations are investigated. The frequency spectrum of these waves is shown to be located in the gap between the upper quasioptic and the lower quasiacoustic branches of the harmonic spectrum. A nonlinear Schrodinger equation for the envelope of the surface wave is derived, and its solitonic solutions are obtained. When capillary effects on the surface of the crystal exist, it is found that the nonlinear surface wave excites the volume waves, thus carrying the energy into the crystal interior at the frequency of the fundamental wave. Hence, such a wave is a leaky or quasisurface wave. The conditions for the existence of the nonlinear surface elastic waves and for their self-localization below the quasiacoustic spectrum branch are also discussed. In contrast to the previous case the nonlinear surface waves are stable and thus not leaky.

A study is made of a magnetoelastic (ME) superstructure in a film of a uniaxial ferromagnet with the easy magnetization axis parallel to its surface. The existence of the spatially modulated phase is determined entirely by the magnetostriction coupling of magnetization to lattice deformation at the ferromagnet-substrate interface. In the vicinity of the second-order magnetic orientational phase transition (OPT) induced by an external magnetic field H the static distribution of magnetization and elastic strain in the domain phase is found to be closed to the distribution for the critical Love mode in the homogeneous phase. Apart from the OPT the width of domains increases drastically with H decreasing. It is shown that the singular defects (dislocations and disclinations) appear in the regular domain structure in the vicinity of the OPT, and such defects destroy the long-range order in the ME superstructure. The calculation is extended to ME superstructures in a film of a two-sublattice orthoferrite.

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H c , different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k = k c ≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate.

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni2+xMn1−xGa (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T M and magnetic T C (Curie point) phase transitions to converge. In the region where T C =T M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment.