Generation of mesoscopic magnetic structures by local laser action

New effects are observed wherein the internal structure of the domain walls in a thin magnetic iron garnet film are modified by the action of focused laser radiation. A single laser pulse with increasing power gives rise to the following: 1) displacement of vertical Bloch lines in a domain wall; 2) generation of a pair of vertical Bloch lines on initially line-free walls; and, 3) an irreversible change in shape of a domain wall and the domain structure as a whole. The mechanism leading to the generation and displacement of Bloch lines is connected with the motion of domain walls which is induced by a local change in the distribution of demagnetizing fields as a result of a heating-induced decrease of the magnetization in the focal spot of the laser radiation. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 398–402 (25 September 1997)     

Solitary flexural waves on a supersonic domain wall in yttrium orthoferrite

Solitary flexural waves on a supersonic domain wall in yttrium orthoferrite are observed and investigated. These waves have a sharp leading edge and a protracted trailing edge, reminiscent of the waves accompanying moving vertical Bloch lines in iron garnet films. The total velocity of the solitary flexural waves in yttrium orthoferrites for all observed amplitudes equals the maximum velocity of the domain walls. Two solitary waves with identical amplitudes colliding head-on are annihilated. The waves possess topological charges, and they move and form dynamic profiles under the influence of gyroscopic forces. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 760–765 (25 May 1997)     

Spectrum of flexural oscillations of a domain wall with drifting Bloch lines

It is observed that in single-crystalline yttrium iron garnet the amplitude of characteristic flexural oscillations of a 180° domain wall containing Bloch lines increases sharply when drift of the Bloch lines is excited. The resonance frequencies of these oscillations are virtually identical to those of flexural oscillations of a monopolar wall. It is shown experimentally that this phenomenon is most likely caused by a magnetic aftereffect. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 72–75 (10 January 1998)     

Dynamics of domain walls with drifting Bloch lines in single crystals of yttrium iron garnet

Induction and magnetic methods are used to study the effect of drifting Bloch lines on the wall velocity in a single crystal sample of yttrium iron garnet cut in the form of a long prism with only one 180-degree domain wall. A sharp increase in the velocity and in resonance bending vibrations of the wall are observed when Bloch line drift is initiated. The character of the wall motion is investigated under these conditions. An analysis of the experimental data shows that the effective reduction in the influence of drifting Bloch lines on the characteristics of the wall motion may be related to a magnetic aftereffect phenomenon. Zh. éksp. Teor. Fiz. 115, 1377–1385 (April 1999)     

Transition micromagnetic structures in asymmetric vortexlike domain walls (Static solutions and dynamic reconstructions)

The possible types of transition structures with a three-dimensional magnetization distribution over regions in the vortex asymmetric domain walls that exist in magnetically uniaxial soft magnetic films with in-plane anisotropy are studied by computer simulation in terms of a micromagnetic approach. It is shown that the possible structure types include both the type of vertical Bloch lines that was discussed earlier in other works and new types, namely, singular (Bloch) points and clusters consisting of vertical Bloch lines and Bloch points. The spatial configurations of the transition structures are calculated and their topological properties are found. The numerical simulation of the dynamics of closely spaced substructure regions reveals various scenarios of their interaction, including annihilation accompanied by energy release and the excitation of nonlinear waves.     

Effects of Bloch lines and Bloch points on resonances of magnetic bubbles

Resonant spectra of 180° domain-wall oscillations in isolated magnetic bubbles are obtained with swept rf sinusoidal excitation and magnetooptic detection. The resonant frequencies are found to depend on the domain wall state. A quantitative micromagnetic theory involving contributions to the wall mass from already-present vertical Bloch lines with and without Bloch points explains the resonant frequencies, including their field dependence, and relates them to the wall states. It is found that for small in-plane fields, Bloch lines provide most of the effective wall mass. Moreover, if a Bloch point is present on a Bloch line, it tends to decrease the Bloch-line mass contribution.     

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)     

Modification of the domain wall structure and generation of submicron magnetic formations by local optical irradiation

Experimental and theoretical investigations are made of the generation of vertical Bloch lines in a magnetic iron garnet film exposed to pulsed optical radiation. High-speed photography and anisotropic dark-field microscopy are used to study characteristic features of the generation of Bloch lines and domain structure relaxation processes after the local action of a laser pulse. Optimum optical irradiation parameters to ensure the controlled generation of Bloch lines are established. A theoretical model is developed which links the generation of Bloch lines to the migration of domain walls induced by local changes in the distribution of the degaussing fields caused by a reduction in magnetization with temperature at the optical radiation focusing point. The experimental results indicate that the controlled formation of magnetic structures smaller than or of the order of 0.1 μm by local optical irradiation is quite feasible.     

Néel lines in ferrimagnetic garnet epilayers with orthorhombic anisotropy and canted magnetization

Néel lines in (TbY)₃(FeAl)₅O₁₂ epitaxial layers with orthorhombic anisotropy and canted magnetization are optically observed by the presence of a kink of the wall plane. The intrinsically asymmetrical character of Bloch walls is demonstrated by wall widening experiments. It follows that wall kinking may either be a consequence of the anisotropy specific to the samples, and/or be the result of magnetostatic interactions. A relaxation type numerical computation of the Néel line configuration indicates that anisotropy is the main source of wall kinking, demagnetizing field effects contributing to an amplification of the kink. The structure of those Néel lines is shown to bear two characters, that of a half-circular line characteristic of samples with in-plane anisotropy and that of a splay line, well known in bubble materials. Further, the charge distribution appears basically dipolar and lines contract when the saturation magnetization increases. They are therefore magnetostatistically analogous to vertical Néel (most generally called Bloch) lines in garnet epilayers supporting bubbles.     

Generation, dynamics, and collisions of bending waves at domain boundaries in yttrium orthoferrite

Solitary bending waves have been observed on domain boundaries of Néel type in wafers of yttrium orthoferrite, having a very sharp leading edge and an extended trailing edge and offset as a whole from the domain boundary and moving with high speeds close to the limiting velocity. Head-on collisions of two such waves of the same amplitude lead to their complete annihilation. Analogous collisions of two such waves, but of different amplitudes, lead to the appearance of a wave with the difference amplitude moving in the same direction as the wave of larger amplitude. The solitary bending waves investigated in this study appear to move under the action of gyroscopic forces acting on magnetic vortices on domain boundaries in yttrium orthoferrite, analogous to vertical Bloch lines with departure of the magnetization vector from the ac plane. From equality of the gyroscopic force with the friction force acting on the leading edge of the solitary bending wave we have estimated the amplitudes of these waves and the magnitudes of the topological charges of the magnetic vortices. Zh. éksp. Teor. Fiz. 115, 2160–2169 (June 1999)     

Mikromagnetisch singuläre punkte in bubbles

The magnetization pattern of a vertical Bloch line in a bubble wall containing a singular point in its center is investigated by a variational method. Introducing a hypothetical intermediate structure, the energy of such a configuration can be separated into two parts: one which describes a local embedding energy for the singular point, and one which may be derived from the structure of the domain wall only. The results indicate that above a critical film thickness Bloch lines containing a singular point represent, in agreement with a prediction by Slonczewski, the energetically most favourable configuration.     

On the possible internal structure of the ferroelectric Ising lines in BaTiO3

ABSTRACT

This article is devoted to a detailed analysis of the inner structure of the Ising line, a topological defect separating two parts of a ferroelectric Bloch domain wall with opposite helicity, in the framework of the phenomenological Ginzburg–Landau–Devonshire model. Results performed for Ising lines in a ?211?-oriented 180-degree domain wall of rhombohedral BaTiO₃ suggest remarkably strong dependence of the polarization profiles on the form of gradient terms. Profiles of the skyrmion density, vorticity and divergence of the polarization field were calculated numerically in the few-nm vicinity of the Ising line defect.     

Polarization as a Berry Phase

The macroscopic polarization of a crystalline dielectric is best defined as a Berry phase of the electronic Bloch wavefunctions, a feature that is best exemplified by the spontaneous polarization of a ferroelectric crystal for which a microscopic theory was previously unavailable.     

石榴石磁泡膜中3类硬磁畴的形成方法

研究石榴石磁泡膜中硬磁畴畴壁中垂直布洛赫线的稳定性可为研制布洛赫线存储器提供有益的帮助 .3类硬磁畴的形成是研究硬磁畴稳定性的前提 .本文综述了在石榴石磁泡膜上形成硬磁畴的 2类方法———“脉冲偏场法”和“低直流偏场法” .结合文献中的典型样品 ,对用“脉冲偏场法”和“低直流偏场法”形成 3类硬磁畴的过程进行了简单介绍 .     

Symmetry theory of the flexomagnetoelectric effect in the Bloch lines

It was shown that there are 48 magnetic point groups of the Bloch lines including 22 (11 time-invariant and 11 time-noninvariant) enantiomorphic and 26 non-enantiomorphic groups. The Bloch lines with the time-noninvariant enantiomorphism have identical types (parities) of the magnetization and polarization dependences. The list of soliton-like Bloch lines is derived from the symmetry classification. The tip electrode method of the creation of these Bloch lines is suggested for the potential applications in the magnetoelectric memory devices. The method of the experimental determination of the flexomagnetoelectric properties of the Bloch lines carried by the Bloch domain wall has been suggested. New type of the flexomagnetoelectric coupling, which is determined by the spatial derivatives of the electric polarization, can be found in the vicinity of the Curie temperature or compensation point of the ferrimagnets. The multi-state Bloch line magnetoelectric/multiferroic memory is proposed. It can be considered as a concept of the magnetoelectric enhancement of existing Bloch line memory invention.     

Topological structure of the fiber bundle of fermion eigenvectors on a lattice in a magnetic field

It is shown that for an arbitrary lattice Hamiltonian of fermions in a uniform magnetic field the Chern number of the complete fiber bundle of the magnetic Bloch states of this Hamiltonian is equal to zero. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 367–368 (10 March 1996)     

Influence of Temperature on Equilibrium Separation Between Vertical Bloch Lines in OHBs in Garnet Bubble Films

The diameters of the ordinary hard bubbles （OHBs） and soft bubbles in epitaxial garnet films are measured under the microscope at various temperatures. It is found that the bubble diameters of OHBs increase with temperature, and it is concluded that the equilibrium separation between two neighbouring vertical Bloch lines （VBLs） Seq is widened with increasing temperature. At the same time, the results can be understood simply as that there are more VBLs in the domain walls of the first dumbbell domains （IDs） than those in walls of OHBs at the same temperature.     

Numerical simulation of wall dynamics in (111)-oriented garnet films in the presence of an in-plane magnetic field

The domain wall motion in the presence of an in-plane magnetic field H_y perpendicular to the wall is simulated using a fall implicit numerical scheme. Calculations are performed for the drive fields 0 Oe<H_z<15 Oe and in-plane fields -210 Oe?H_y?210 Oe. The relation between the average wall velocity $\text{v}$ and the drive field H_z is discussed considering the wall structure. It was found that an in-plane field increases the peak velocity of the wall and extends the range of the drive fields, where the linear mobility relation is valid. A dynamical Bloch line stacking was found for sufficiently large drives. The influence of an in-plane field on the angular span of horizontal Bloch lines is discussed also. In particular the occurrence of 2π-horizontal Bloch lines is described. Numerical results obtained with a full implicit method are compared with the experimental observations of bubble motion and good agreement is found for |H_y|≤100 Oe.     

Dependence of the domain-wall velocity on the driving field in garnet films

The dependence of the domain-wall velocity on the amplitude of the driving magnetic field pulses is investigated in an iron garnet film of the (YSmCa)₃(FeGe)₅O₁₂ system with a (111) orientation. The results obtained are analyzed from the standpoint of existing theory. A maximum corresponding to the disruption of steady-state motion is observed on the dependence. Thereafter, the velocity at first decreases sharply and then increases. It is theorized that a process involving the periodic generation, propagation, and annihilation of horizontal Bloch lines occurs in the wall in this period. Data are obtained for the velocity saturation region, which confirm a previously proposed empirical formula and a theoretical model, according to which the saturation regime corresponds to a state of chaos. Fiz. Tverd. Tela (St. Petersburg) 39, 660–663 (April 1997)     

Kronig-Penney problem for a biparabolic potential: Quasifree particle motion

The Bloch states of a particle in a one-dimensional, periodic, biparabolic field are determined. The solutions are expressed in terms of confluent hypergeometric functions. It is shown that the first term of the Tricomi expansion for confluent hypergeometric functions represents the quasifree above-barrier motion of a particle and is identical to the exact Kronig-Penney solution for a square potential. Fiz. Tverd. Tela (St. Petersburg) 41, 1317–1319 (July 1999)