Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

On the effect of an elastic shear stress upon the magnetization of ferromagnetic sheets

When an elastic shear stress and a cyclical magnetic field, parallel to each other, are applied in the plane of a ferromagnetic sheet, magnetization changes perpendicular to the field are induced in the sample. “Transverse” hysteresis loops, i.e. transverse magnetization plotted as a function of the longitudinal field, were studied in various materials. The characteristic shape of the loop and the differences in sign and magnitude of the “transverse” magnetization have been qualitatively explained in terms of magnetic domain theory. Work supported by G.N.S.M. (CNR).     

Effect of an arbitrarily directed permanent magnetic field on the dynamic susceptibility of a superparamagnetic particle under the conditions of stochastic resonance

Theoretically, based on the approximation of discrete orientations, the effect of thermal stochastic resonance is considered for a superparamagnetic particle with magnetic anisotropy of the “easy axis” type in view of an additional external permanent magnetic field applied at an arbitrary angle relative to the easy magnetization axis. The dynamic susceptibility has been calculated for a single-domain iron particle under the conditions of its modulation by a weak external radio-frequency field at various absolute values and directions of the additional permanent magnetic field vector. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 67–71, March, 2006.     

Photoinduced changes in the space-modulated magnetic order of a FeBO<Subscript>3</Subscript>:Mg single crystal

An effect of nonpolarized white light on the modulated magnetic structure of a FeBO₃:Mg single crystal, which arises in this light-plane weak ferromagnet in the low temperature range during technical magnetization, has been revealed. It has been found that the degree of the light action on the magnetic state of FeBO₃:Mg depends both on its duration and on the orientation of the spontaneous magnetization vector M of the crystal during illumination. Interpretation of the results obtained has been performed in the context of the “magnetic ripple” theory on the assumption that the absorbed light induces additional uniaxial magnetic anisotropy in the easy plane of the crystal and that the anisotropy axis is collinear to the vector M during illumination.     

Thermodynamics of the HMF model with a magnetic field

We study the thermodynamics of the Hamiltonian mean field (HMF) model with an external potential playing the role of a “magnetic field”. If we consider only fully stable states, the caloric curve does not present any phase transition. However, if we take into account metastable states (for a restricted class of perturbations), we find a very rich phenomenology. In particular, the caloric curve displays a region of negative specific heat in the microcanonical ensemble in which the temperature decreases as the energy increases. This leads to ensembles inequivalence and to zeroth order phase transitions similar to the “gravothermal catastrophe” and to the “isothermal collapse” of self-gravitating systems. In the present case, they correspond to the reorganization of the system from an “anti-aligned” phase (magnetization pointing in the direction opposite to the magnetic field) to an “aligned” phase (magnetization pointing in the same direction as the magnetic field). We also find that the magnetic susceptibility can be negative in the microcanonical ensemble so that the magnetization decreases as the magnetic field increases. The magnetic curves can take various shapes depending on the values of energy or temperature. We describe first order phase transitions and hysteretic cycles involving positive or negative susceptibilities. We also show that this model exhibits gaps in the magnetization at fixed energy, resulting in ergodicity breaking.     

“Left-handed” state and polarization characteristics of waves in “semiconductor-magnet” superlattices

Dispersion properties of circularly polarized eigenwaves propagating in the “semiconductor-magnet” layered periodic structure along the axis of its periodicity and external magnetic field have been considered. The possibility of controlling the effective material parameters of the structure and the feasibility of negative refractive index for the wave with right (resonant) circular polarization has been shown. High magneto-optical activity of this gyrotropic structure has been ascertained, which leads to large Faraday rotation angles if the structure is in the state of “left-handed” medium.     

Periodic domain structures obtained by growth of LiNbO<Subscript>3</Subscript> crystals doped with gadolinium

The growth of a periodic domain structure in LiNbO₃ crystals doped with gadolinium (Gd, 0.44 wt %) was investigated by special electron beam surface charging in a scanning electron microscope and on a atomic force microscope. Ferroelectric domain boundaries with “tail-to-tail” and “head-to-head” P _s orientations were comparatively analyzed. The domain boundaries had different forms and differed by their physical properties. Micron-size surface crystal regions close to the “tail-to-tail” boundaries were charged slower by electron irradiation and had modified elastic properties. This region was detected by using the lateral force mode of atomic force microscopy. The observed morphology and property features were supposed to be due to different concentration gradients of the Gd impurity and different width of its distribution close to the domain boundaries of different types.     

Interactions in Noncommutative Dynamics

A mathematical notion of interaction is introduced for noncommutative dynamical systems, i.e., for one parameter groups of *-automorphisms of endowed with a certain causal structure. With any interaction there is a well-defined “state of the past” and a well-defined “state of the future”. We describe the construction of many interactions involving cocycle perturbations of the CAR/CCR flows and show that they are nontrivial. The proof of nontriviality is based on a new inequality, relating the eigenvalue lists of the “past” and “future” states to the norm of a linear functional on a certain C ^*-algebra. To the memory of Irving Segal Received: 12 October 1999 / Accepted: 21 October 1999     

Effect of infrared illumination on the modulated magnetic structure of the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The effect of unpolarized white light on the period and conditions of the existence of the modulated magnetic structure of the FeBO₃: Mg single crystal, which is formed in this easy-plane weak ferromagnet in the low-temperature range during technical magnetization, has been investigated experimentally. It has been revealed that the degree of light action on the magnetic state of the crystal depends on both the irradiation duration and the orientation of the ferromagnetic vector during illumination. It has been established that light with wavelengths in the range 0.8 μm < λ < 0.9 μm has a maximum effect on the parameters of the modulated magnetic structure formed in the FeBO₃: Mg single crystal. The results obtained have been interpreted in the framework of the “magnetic ripple” theory using the model of photosensitivity of anisotropic magnetic centers associated with the presence of Mg impurity ions in the composition of the crystal.     

Optical vortices in the scattering field of magnetic domain holograms

Experimental investigations and theoretical-model analyses have been made of the magnetooptic diffraction of light at ferrite garnet magnetic films with a banded domain structure which includes isolated magnetic grating defects in the form of “forks” and “breaks.” An analysis of the magnetic grating structure and the light diffraction field shows that in terms of its action on laser radiation, a banded domain grating is similar to a computer-synthesized phase hologram of an isolated pure screw-wavefront dislocation. It is shown that as a result of magnetooptic diffraction at a magnetic hologram, optical vortices may be reconstructed with a helicoidal wavefront carrying the topological charge l=±1,±2. Zh. Tekh. Fiz. 68, 54–58 (December 1998)     

Exchange reduction of the magnetization modulus in the vicinity of a Bloch point

The structure of the Bloch point (BP) core (magnetization “hedgehog”) is determined with regard to the exchange-induced reduction of the length of the magnetization vector. The magnetization modulus vanishes at the center of a small sphere embracing the BP. The value of energy of such a BP in the vicinity of the Curie ferromagnetic point in the approximation of the second-order Landau phase transition is lower than the corresponding energy value calculated in the constant modulus approximation. The stability of the BP core to radial pulsations is demonstrated.     

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)     

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.     

Small-angle neutron scattering study of the magnetoplastic effect in the beryllium bronze aged in magnetic fields

The magnetoplastic effect induced in the beryllium bronze BrB-2 aged in magnetic fields of up to 1 T has been studied using the microhardness method. “Positive” and “negative” magnetoplastic effects have been shown to exist. The possibility of applying the small-angle polarized-neutron scattering method to the investigation of the effect of a magnetic field on the kinetics of phase formation in Cu-Be alloys has been analyzed. Models of the structure of possible scattering centers have been proposed.     

Longitudinal critical current in a ferrite-type-II superconductor structure

The effect of the interaction of Abrikosov vortices with the magnetization on the longitudinal vortical instability in a layered ferromagnet-type-II superconductor structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse magnetic susceptibility is large, the magnitude of the longitudinal critical current in the structure can be almost 1.5 times smaller than in the isolated superconductor. The reason for this is compensation of stray field sources outside the superconductor by “magnetic charges” arising from a jump in the transverse magnetization on the surface of the magnet. A structure is considered in which the thickness of the superconductor significantly exceeds the London penetration depth of the magnetic field and the wavelength of the critical mode. For this reason (in light of the absence of high-quality bulk high-temperature superconductors), to experimentally study the described phenomenon it is necessary to use conventional low-temperature superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 231–235 (February 1997)     

Phase diagram of a thin film of the Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloy with the “bulk” or “planar” magnetoelastic interaction

Concentration-induced reorientation phase transitions in thin magnetic films of FeCo alloys have been investigated taking into account “planar” or “bulk” magnetoelastic interaction. The critical concentrations of Co corresponding to the phase transition points, as well as the types of the phase transitions, have been determined. The phase diagrams have been plotted.     

Role of exchange and magnetostatic interactions in the effect of thermal magnetization of fine-crystalline alloys of highly anisotropic magnetics

The theory of micromagnetism has been applied to a system consisting of plane-parallel layers of a monoaxial magnetic which differ by the orientation of easy magnetization axes to numerically investigate the effect of thermal magnetization which was simulated by varying the anisotropy constant. It has been revealed that the effect of thermal magnetization does not show up if the exchange interaction between the layers is disregarded. An enhancement of magnetostatic interaction, which was simulated by increasing the degree of noncomplanarity of the easy magnetization axes in the system layers results in a reduction of the effect. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 8–11, August, 2007.     

ESR and longitudinal response in metals containing localized paramagnetic centers with spin<Emphasis Type="Italic">S</Emphasis>>1/2

The theory describing electron spin resonance (ESR) and the longitudinal magnetization response of coupled spin systems in a metal containing both delocalized conduction electrons (“espins”) and localized paramagnetic centers (“s-spins”) is generalized to the case of arbitrary half-integer spin value,S>1/2, of the s-spins. The consideration is based on the Bloch-Hasegawa equations supplemented by taking into account the coupled evolution of the longitudinal magnetization components and the effect of weak ESR saturation by the microwave field. The ESR transversal susceptibility and longitudinal magnetization response are worked out in terms of normal modes related to the coupled s- and e-spin oscillators taking into account the ESR fine structure (FS) of the s-spins. These modes are characterized by effective (renormalized) frequencies and relaxation rates (decays) which differ from the partial ones. In the specific cases of a well-resolved FS (in the isothermal limit) and of the relaxational collapse of the FS due to strong exchange coupling between the s- and e-spins (in both the isothermal and bottlenecked limits), the analytical expressions are derived which are relevant to the modulation technique of measuring extremely fast spin-lattice relaxation times in metals.     

Electron spectrum and wave functions of icosahedral quasicrystals

Electron spectra and wave functions of icosahedral quasicrystals have been investigated in the tight-binding approximation using the two-fragment structural model (the Amman-MacKay network) with “central” decoration. A quasicrystal has been considered as a limiting structure in a set of optimal cubic approximants with increasing lattice constants. The method of level statistics indicates that the energy spectrum of an icosahedral quasicrystal contains a singular (nonsmooth) component. The density of electron states has been calculated for the first four optimal cubic approximants of the icosahedral quasicrystal, and the respective Lebesgue measures of energy spectra of these approximants have been obtained. Unlike the case of a one-dimensional quasiperiodic structure, the energy spectrum of an icosahedral quasicrystal does not contain a hierarchical gap structure typical of the Cantor set of measure zero in a one-dimensional quasicrystal. Localization of wave functions in an icosahedral quasicrystal has been studied, and their “critical” behavior has been detected. The effect of disorder due to substitutional impurities on electron properties of icosahedral quasicrystals has been investigated. This disorder makes the electron spectrum “smoother” and leads to a tendency to localization of wave functions. Zh. éksp. Teor. Fiz. 113, 1009–1025 (March 1998)     

Composite boson many-body theory for Frenkel excitons

We present a many-body theory for Frenkel excitons which takes into account their composite nature exactly. Our approach is based on four commutators similar to the ones we previously proposed for Wannier excitons. They allow us to calculate any physical quantity dealing with N excitons in terms of “Pauli scatterings” for carrier exchange in the absence of carrier interaction and “interaction scatterings” for carrier interactions in the absence of carrier exchange. We show that Frenkel excitons have a novel “transfer assisted exchange scattering”, specific to these excitons. It comes from indirect Coulomb processes between localized atomic states. These indirect processes, commonly called “electron-hole exchange” in the case of Wannier excitons and most often neglected, are crucial for Frenkel excitons, as they are the only ones responsible for the excitation transfer. We also show that in spite of the fact that Frenkel excitons are made of electrons and holes on the same atomic site, so that we could naively see them as elementary particles, they definitely are composite objects, their composite nature appearing through various properties, not always easy to guess. The present many-body theory for Frenkel excitons is thus going to appear as highly valuable to securely tackle their many-body physics, as in the case of nonlinear optical effects in organic semiconductors.