Dynamics of domain walls in ultrathin magnetic films

The domain walls in ultrathin ferromagnetic films with uniaxial magnetic anisotropy are investigated theoretically. It is shown that taking account of the magnetodipole and magnetoelastic interactions leads to the appearance of an effective anisotropy with respect to the direction of the normal to the plane of the wall. The existence of a new type of domain walls—“corner” walls, at which the magnetization vector is rotated in the plane making a certain angle, which depends on the film parameters, with the plane of the domain wall and the static and dynamic properties of these walls are investigated. The dependence of the limiting velocity of the domain walls on the film thickness is found. Zh. éksp. Teor. Fiz. 112, 1476–1489 (October 1997)     

Spectrum of flexural oscillations of a 180-degree domain wall in yttrium iron garnet

The long-wave asymptotic behavior of the spectrum of flexural oscillations of a monopolar domain wall in a cubic ferromagnet is found. The experimental results, numerical calculations, and theoretical results obtained for uniaxial ferromagnets are compared. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 10, 797–802 (25 May 1996)     

Different types of nonlinear convective oscillations in a multilayer system under the joint action of buoyancy and thermocapillary effect

The nonlinear development of oscillatory instability under the joint action of buoyant and thermocapillary effects in a multilayer system, is investigated. The nonlinear convective regimes are studied by the finite difference method. Two different types of boundary conditions – periodic boundary conditions and rigid heat-insulated lateral walls, are considered. It is found that in the case of periodic boundary conditions, the competition of both mechanisms of instability may lead to the development of specific types of flow: buoyant-thermocapillary traveling wave and pulsating traveling wave. In the case of rigid heat-insulated boundaries, various types of nonlinear flows – symmetric and asymmetric oscillations, have been found.     

Static and dynamic properties of an isolated strip domain in a thin ferromagnetic film

A perturbation theory is developed for the integrodifferential Landau-Lifshits equation that describes the state of 2π-domain walls in ferromagnetic films. The static and dynamic parameters of a 2π-domain wall are determined, taking into account its micromagnetic structure. The limits of applicability of geometric domain wall models are indicated. Fiz. Tverd. Tela (St. Petersburg) 40, 269–273 (February 1998)     

Field-induced phase transitions in magnetooptical films with a small positive anisotropy constant

We study phase transitions induced by a static magnetic field in magnetically uniaxial films with a small positive anisotropy constant. The phase diagram of these objects is determined in the H _∥-H _⊥ plane, where H _∥ and H _⊥ are, respectively, the components of the magnetizing field along and perpendicular to the surface normal. The stability boundary is located for all of the main types of domain configurations observed: a simple stripe domain structure, a stripe domain structure with periodic bending by surface distortions in the profile of the domain walls, and hexagonal lattices of cylindrical magnetic bubbles. Zh. éksp. Teor. Fiz. 111, 283–297 (January 1997)     

Effect of the micromagnetic structure of domain walls on the properties of an isolated domain in a thin magnetic film

The effect of the inner structure of domain walls on the time-independent parameters of an isolated stripe domain in a thin ferromagnetic film is studied. The adopted variant of the perturbation theory makes it possible to account, within a unified approach, for the contributions of the magnetostatic and exchange interactions. Zh. éksp. Teor. Fiz. 115, 1315–1325 (April 1999)     

Effect of the inner structure of domain walls on the stability of an isolated stripe domain in a pulsed field

This paper studies the effect of the inner structure of domain walls on the stability of an isolated stripe domain localized in a thin ferromagnetic film against a pulse of magnetic field applied perpendicularly to the film surface. It is found that the value of the critical amplitude of the pulsed signal strongly depends on the value of the magnetizing field in which the system was initially placed. It is also established that the difference on stability of domains with unipolar and bipolar walls in pulsed fields diminishes as the amplitude of the magnetizing field decreases. Finally, the dependence of the region of stability in a pulse field on the parameters of the system is determined for various domain types. Zh. éksp. Teor. Fiz. 116, 1694–1705 (November 1999)     

Dynamics of a pendulum with a quasiperiodic perturbation

The dynamics of a damped pendulum with a quasiperiodic external perturbation is investigated. It is shown that in contrast to a pendulum with a periodic perturbation, a quasiperiodic perturbation leads to chaos in the weakly nonlinear limit when the peak-to-peak amplitude of the oscillations of the pendulum is small. This effect is attributed to the appearance of saddle states induced by the external perturbation. The analytical conditions for the appearance of chaotic oscillations are obtained by the method of running Lyapunov exponents and by the repeated-averaging technique. Zh. Tekh. Fiz. 67, 1–7 (October 1997)     

“Unusual” domain walls in multilayer systems: Ferromagnet + layered antiferromagnet

The structure and conditions for the onset of a new type of domain wall in multilayer systems comprising a ferromagnet and a layered antiferromagnet is investigated by numerical simulation. Domain walls occur as the result of frustrations produced by interface roughness, i.e., by the existence of atomic steps on them. The domain walls are investigated both in a ferromagnetic film on a layered antiferromagnetic substrate and in multilayer structures. It is shown that a domain wall broadens with increasing distance from the interface; this trend is attributed to the nontrivial dependence of the wall energy on the thickness of the layer. The structure of the domain walls in multilayer ferromagnet-layered antiferromagnet systems varies dramatically as a function of the energies of interlayer and in-layer exchange interactions between adjacent layers. Zh. éksp. Teor. Fiz. 114, 1817–1826 (November 1998)     

The effect of friction on nonlinear oscillations of interacting domain walls in an external periodic field

The effect of dissipation on nonlinear oscillations in a system of domain walls experiencing an external harmonic field is studied numerically. The problem is formulated for uniaxial ferromagnet films, with the easy magnetic axis being perpendicular to the surface and with the harmonic field being aligned with the axis. Account is taken of the dynamic redistribution of magnetic poles on the film surface, which enables one to derive, in a natural way, an expression for a restoring force acting on the domain walls. The force is a nonlinear function of domain-wall displacement from the equilibrium position. It is found that the domain walls may execute complicated steady-state quasi-periodic oscillations and long-term chaotic oscillations. Attractors in the phase space of the system are determined.     

Nonlinear Waves in Carbon Nanotubes under Conditions of Electron-Phonon Bonding

Relative simplicity of the atomic structure of carbon nanotubes being hollow cylinders with walls formed by rings of six carbon atoms (generally, the walls can be multilayered) enables the researchers to use this class of substances as model one to reveal the basic mechanisms of the dynamics of quasi-one—dimensional systems. The present work studies the nonlinear properties of carbon nanotubes with strong electron interactions described by the Hubbard Hamiltonian. A microscopic Hamiltonian describing electrons in carbon nanotubes with allowance for the electron mobility, Coulomb repulsion of electrons in one site of carbon nanotubes, and changes in spacing of the neighboring sites caused by acoustic oscillations is suggested. An effective nonlinear system of equations describing the dynamics of electron wave functions within the framework of the suggested Hamiltonian is derived. The existence of nonlinear stable periodic oscillations of electron wave functions in the examined model, in particular, corresponding to acoustic oscillations with different polarization states is established. The influence of the problem parameters on the character of nonlinear wave stability is revealed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 76–81, June, 2005.     

The interaction energy of parallel Bloch walls

The paper gives an exact calculation of the interaction energy of parallel 180° Bloch walls of an unbounded uniaxial ferromagnetic as a function of their distance s and of the external field. The general relation, valid for a periodic domain structure, is specialised for the case of two Bloch walls.The author wishes to thank F. Kroupa and V. Janovec, of the Institute, for their valuable suggestions.     

Influence of antiferromagnetic ordering on the de Haas-van Alphen effect in a semimetal

The distinctive characteristics of the de Haas-van Alphen effect in semimetals with antiferromagnetic long-range order are investigated theoretically. It is shown that the transition of the subsystem of localized spins from the canted antiferromagnetic phase to the ferromagnetic phase is accompanied by an abrupt change in the “frequency” of the magnetization oscillations of band carriers M _∼. In the below-critical range of magnetic fields, M _∼ is not a function periodic in 1/H. Significantly, the additional contribution to the phase of the oscillatory factors is proportional to H ² and is determined entirely by quantum fluctuations in the antiferromagnetic subsystem. Fiz. Tverd. Tela (St. Petersburg) 39, 204–210 (February 1997)     

Macroscopic quantum resonance tunneling of domain walls

Results are presented of a theoretical investigation of the tunneling of magnetic domain walls, taking account of the interaction of the walls with the thermal system of the crystal. It is shown that thermal stimulation increases considerably the transmittance of the potential barriers during propagation of walls through a crystal. Fiz. Tverd. Tela (St. Petersburg) 40, 1855–1860 (October 1998)     

Nonlinear diffraction in spontaneous three-wave and coherent four-wave light scattering by polaritons

Spontaneous three-wave and coherent four-wave scattering by polaritons in crystalline media with periodically modulated nonlinear quadratic susceptibility have been studied both theoretically and experimentally. Phase matching conditions and an expression for the scattering spectral line shape with due account of cascade processes in nonlinear diffraction for coherent Stokes polariton light scattering have been derived. Measurements of the light intensity distribution due to the three-and four-wave scattering in a LiNbO₃:Nd:Mg crystal with a periodic domain structure are in good agreement with theoretical results. The prospects for using the effects of nonlinear diffraction in spectroscopic studies of media with periodic distributions of nonlinear optical parameters, specifically, in precision measurements of the IR refractive index dispersion and determination of the period and profile of the quadratic susceptibility distribution are discussed. Zh. éksp. Teor. Fiz. 112, 2001–2015 (December 1997)     

Microinhomogeneities in the distribution of oxygen concentration in ferrite-garnet films

Artificial microinhomogeneities were created in iron-garnet (TmBiCa)₃(FeGa)₅O_12−γ films with various degrees of oxygen nonstoichiometry, changes in the individual magnetic characteristics of these inhomogeneities were tracked during chemical oxidation and reduction, and the interactions of these inhomogeneities with planar domain walls and spiral domains were investigated. The resulting experimental data were interpreted by analyzing how the saturation magnetization, domain-wall surface-energy density, and uniaxial anisotropy constants depended on the concentrations of different-valency iron ions, cationic vacancies, and anionic vacancies. Fiz. Tverd. Tela (St. Petersburg) 41, 2016–2019 (November 1999)     

“Switching” phase transition of the nanodomain state of the frustrated ferromagnet-antiferromagnet system

The phenomenon of switching of the domain walls generated by frustrations in a two-layer ferromagnet-antiferromagnet nanostructure has been studied theoretically taking into account the energy of the uniaxial anisotropy beyond the exchange approximation. This phenomenon manifests itself in the fact that, as the magnetic field increases, the ferromagnetic layer divided into nanodomains by domain walls perpendicular to the layer plane becomes single-domain, and the antiferromagnetic layer that is uniform in weak fields is divided into 180° domains by the domain walls perpendicular to the layer. The phase diagram of the two-layer nanostructure has been constructed in the variables “the magnetic field-the characteristic distance between atomic step edges at the interface between the layers.”     

Stochastic resonance in a nonequilibrium electron-hole plasma

A photogenerated electron-hole plasma, heated in the process of Auger recombination, is studied. It is shown that in the plasma near the threshold for the appearance of uniform relaxational self-excited oscillations, weak noise transforms into a stochastic sequence of large-amplitude spikes. An additional optical periodic signal with amplitude approximately five times smaller than the noise variance, depending on the form of this signal, transforms these stochastic oscillations into low-amplitude quasiharmonic oscillations or into periodic spike self-excited oscillations of enormous amplitude. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 422–427 (10 October 1999)     

Nonlinear laser-induced regime of surface-electromagnetic-wave generation and submicron periodic relief during liquid-phase photochemical etching of n-III–V semiconductors

This paper is devoted to an experimental study of the physical processes underlying the phenomenon of laser-induced generation of periodic relief on the surface of n-III–V semiconductors during liquid-phase photochemical or photoelectrochemical etching accompanying the resonance interaction of surface electromagnetic waves (SEWs). The increments of the exponentially increasing amplitudes of the dominant Fourier harmonics of the relief have been measured at the initial (linear) stage of the time evolution of the surface profile. It is proven by comparing the theoretical and experimental results that the mechanism for forming periodic structures that we have proposed is adequate. Ways of monochromatizing the generated relief and controlling the line shape of the surface grating are studied. It is experimentally detected for the first time that the nonlinear stage of the time evolution of the relief is characterized, in accordance with the predictions of the theory developed by the authors, by amplitude and phase oscillations of the first and second Fourier harmonics of the surface profile. It is shown to be possible to generate relief that suppresses specular reflection from the surface. A new nonmasked laser method is developed for forming high-quality submicron relief diffraction gratings, combining a holographic method and a method involving laser-induced relief generation during resonance excitation of SEWs. Diffraction gratings with a period of d=0.24–0.54 μm and a depth of h=0.1–0.2 μm over an area of 0.5×31 cm have been created on an n-InP surface. Zh. éksp. Teor. Fiz. 111, 174–198 (January 1997)     

Giant domain walls in a ferromagnet

Giant domain walls with a width of ∼7 μm are observed on the surface of a ferromagnet — an amorphous magnetically soft alloy. A magnetooptic investigation shows that the walls have a Néel structure in the subsurface region. The subsurface structure of these walls differs substantially from that of the narrower walls previously observed in iron, Permalloy, and amorphous materials. According to the theoretical model of Scheinfein and co-workers, which relates the width of an asymmetric Bloch wall in the bulk with the width at the surface, the width of the wall in the bulk is estimated to be 3–4 μm. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 528–530 (25 October 1999)