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)     

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)     

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.     

Drift of vertical Bloch lines in a domain wall with a noninterchangeable spectrum in perpendicularly magnetized films

The dynamics of vertical Bloch lines in variable external magnetic fields is examined with allowance for the magnetostatic noninterchangeability of the spectrum of the domain wall. The drift velocity of the translational motion of vertical Bloch lines is calculated and is found to be nonzero in second order in the weak oscillating field. Zh. éksp. Teor. Fiz. 112, 2169–2177 (December 1997)     

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)     

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)     

Anharmonicity of Bloch oscillations in GaAs/AlAs superlattices in the case of inhomogeneously broadened excitonic states in a weak electric field

The variation of the spectral shape of the four-wave mixing signal in GaAs/AlGaAs superlattices was studied experimentally and theoretically. It was shown that in an external electric field, leading to anti-crossing of the levels of heavy and light excitons, sawtooth oscillations of the energy position of the levels are observed. These oscillations are due to the inhomogeneous broadening of the states participating in the Bloch oscillations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 820–825 (25 May 1998)     

Electron motion in a Holstein molecular chain in an electric field

A charge motion in an electric field in a Holstein molecular chain is modeled in the absence of dissipation. It is shown that in a weak electric field a Holstein polaron moves uniformly experiencing small oscillations of its shape. These oscillations are associated with the chain’s discreteness and caused by the presence of Peierls-Nabarro potential there. The critical value of the electric field intensity at which the moving polaron starts oscillating at Bloch frequency is found. It is shown that the polaron can demonstrate Bloch oscillations retaining its shape. It is also shown that a breathing mode of Bloch oscillations can arise. In all cases the polaron motion along the chain is infinite.     

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)     

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)     

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)     

Critical behavior of the wave functions of icosahedral quasicrystals

The character of the localization of the wave functions of an icosahedral quasicrystal is investigated in the tight-binding approximation. It is found that the wave functions exhibit “critical behavior”: they are neither localized, as in the case of Anderson localization, nor delocalized, as in the case of Bloch states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 8, 559–563 (25 October 1996)     

Magnetic moment of a quasi-one-dimensional nanostructure in an inclined magnetic field

Shubnikov-de Haas oscillations in quasi-one-dimensional nanostructures (carbon nanotubes and quantum channels) are investigated. It is shown that two types of aperiodic oscillations arise in such systems: oscillations involving a change in the strength of the magnetic field, and oscillations involving a change in the angle of inclination of the field with respect to the symmetry axis of the system. It is found that the monotonic part of the magnetic moment lies in the plane of size confinement of the system and that the oscillating part has both longitudinal and transverse components. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 549–552 (10 April 1996)     

Oscillations of the thermodynamic properties of a one-dimensional mesoscopic ring caused by Zeeman splitting

It is shown that the interrelationship of spin splitting in a magnetic field and spatial quantization in a one-dimensional ballistic ring coupled with a reservoir results in mesoscopic oscillations of a new type which vanish with increasing temperature. The period of such oscillations is inversely proportional to the density of states in the spin subsystem in the ring. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 595–600 (10 November 1999)     

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.     

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)     

On the nature of the oscillations of cyclotron absorption in InAs/GaSb quantum wells

The mechanism of oscillations of the half-width and intensity of the cyclotron resonance (CR) line of electrons in a semimetal quantum well based on an InAs/AlSb/GaSb heterostructure is investigated experimentally and theoretically. It is shown that the oscillations of the CR spectrum are due to mixing of states of the spatially separated two-dimensional electrons and holes. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 753–758 (25 November 1998)     

Undamped self-oscillations and dynamical chaos under conditions of impurity breakdown of a semiconductor in the fixed-current regime

It is shown theoretically that chaotic oscillations of the Hall field, the Hall constant, and the magnetoresistance appear in a compensated semiconductor under impurity breakdown conditions. It is shown that the transition to chaos occurs via the Feigenbaum scenario. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 340–343 (10 March 1998)     

Coherent suppression of the EPR-nutation signal in quartz

Bloch equations are used to derive an analytic expression for the integral nutation-signal decay rate in a two-level quantum system. It is found that when inhomogeneous line broadening is taken into account, the temporal decay of the response is due not only to coherent suppression but also to additional decay whose rate depends on the ratio of the Rabi frequency to the inhomogeneous linewidth. The results are used to explain the anomalous (Rabi-frequency-dependent) EPR-nutation decay detected in quartz in the experiments of R. Boscaino, F. M. Gelardi, and J. P. Corb [Phys. Rev. B 48, 7077 (1993)]. Contrary to the statement made by these researchers that this decay is non-Bloch, it is found that the process can be described by Bloch equations without any modification. Zh. éksp. Teor. Fiz. 114, 484–491 (August 1998)     

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.