Dynamics of antiferromagnetic vortices in yttrium orthoferrite domain walls

The total velocity of solitary flexural waves nonlinearly increases with an increase in the velocity of domain walls and becomes saturated at a level of 20 km s^?1; the smaller the wave amplitude, the more rapidly saturation occurs. Counter collisions of solitary flexural waves lead to the formation of a single wave with a difference amplitude moving in the same direction as the wave with a larger amplitude. The experimental results confirm that solitary flexural waves accompany antiferromagnetic vortices at domain walls in yttrium orthoferrite.     

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)     

Gyroscopic dynamics of antiferromagnetic vortices in domain boundaries of yttrium orthoferrite

It is established experimentally that the magnetic field directed along the b axis has little effect on the velocities of antiferromagnetic vortices in the domain boundary (DB) of yttrium orthoferrite and fails to explain the presence of an appreciable gyroscopic force acting on these vortices. This force is induced by the dynamic canting of magnetic sublattices proportional to the DB velocity. Due to the canting, the velocities of antiferromagnetic vortices depend initially quadratically on the DB velocity, as was experimentally found in this work. The dynamics of antiferromagnetic vortices in the yttrium orthoferrite DBs is gyroscopic and quasi-relativistic, with the limiting velocity of 20 km/s equal to the velocity of spin waves at the linear portion of their dispersion curve.     

Generation of pairs of antiferromagnetic vortices and their dynamics at a domain wall in yttrium orthoferrite

The stable generation of pairs of antiferromagnetic vortices at a domain wall moving at a velocity of 12 km/s is investigated at the instant it passes through a defect in a thin plate of yttrium orthoferrite. The velocities of a vortex and an antivortex moving in opposite directions along the domain wall and being accompanied by solitary flexural waves are ±16 km/s. The total velocity of antiferromagnetic vortices is close to the maximum velocity of the domain wall, 20 km/s. Such a high velocity can only be due to the action of a quite large gyroscopic force. An external dc magnetic field (±400 Oe) applied along the b axis of the orthoferrite affects this velocity insignificantly. The effective magnetic field that violates the Lorentz invariance of the dynamics considerably exceeds this value.     

Solitary deflection waves on the supersonic domain wall in yttrium orthoferrite

The moving antiferromagnetic vortices are accompanied by solitary deflection waves. These waves allow to investigate generation and nonlinear dynamics of the antiferromagnetic vortices on the moving domain wall with the help of the two- and three-fold digital high speed photography. On the quasi-relativistic domain wall the vortex dynamics is quasi-relativistic with the limiting velocity c=20 km/s, which is equal to the spin-wave velocity. The solitary deflection waves dynamics can be explained assuming existence of the gyroscopic force. A theory for the gyroscopic force in the orthoferrite domain wall is elaborating by A.K. Zvezdin et al. currently. We present a comparison of the theoretical and experimental results on the dynamics of the solitary deflection waves, which accompany the antiferromagnetic vortices in the domain wall of orthoferrites.     

Nonlinear magnetoacoustic interactions in weak ferromagnets

The pinning and interaction of a single domain wall with normal magnetoelastic waves excited during its motion in a single-crystal yttrium orthoferrite plate, were discovered and investigated by a method based on the magneto-optical Faraday effect. The dependences of the bending wave amplitude and the spectra of shear waves, which can be excited by a moving domain wall, were calculated. The results obtained are interpreted with allowance for the interactions of excited oscillations in both the magnetic and elastic subsystems of the orthoferrite.     

Gyroscopic quasi-relativistic dynamics of antiferromagnetic vortex in domain boundary of yttrium orthoferrite

It is demonstrated that the quasi-relativistic dynamics of antiferromagnetic vortices in a quasi-relativistic domain boundary of yttrium orthoferrite are caused by the unusually strong gyroscopic force.     

Reflection of antiferromagnetic vortices on a supersonic domain wall in yttrium orthoferrite

Reflection of solitary flexural waves propagating in a supersonic domain wall of yttrium orthoferrite from the domain wall part moving with the transverse-sound velocity is observed experimentally. This observation confirms that such a reflection of a solitary flexural wave leads to a change in the sign of the topological charge of the antiferromagnetic vortex accompanied by this wave, which proves a direct relationship between these two objects.     

Interaction of a moving domain wall with surface magnetoelastic waves in yttrium orthoferrite

The amplitude-frequency characteristics of magnetoelastic surface waves excited by moving domain walls in a lamellar yttrium orthoferrite samples are discovered and measured. The results of analysis of the effect of magnetoelastic surface waves on the dynamics of domain walls in this orthoferrite are considered. The nonlinear interaction between magnetoelastic surface waves accompanying a moving domain wall is analyzed.     

Gyroscopic force acting on the magnetic vortex in a weak ferromagnet

The gyroscopic (Magnus) force in weak ferromagnets acting on magnetic vortices when the domain walls move in the external magnetic field has been investigated. The general expressions for the gyroscopic force in weak ferromagnets are obtained. The particular calculation of the gyroscopic force in rhombic weak ferromagnets is performed using the method that allows one to calculate it neglecting the internal structure of a vortex in the domain wall. It is shown that the gyroscopic force for most types of domain walls is nonzero and is determined by the mean sublattice magnetization, the Dzyaloshinsky interaction constant, and the constant of the exchange interaction between the sublattices.     

Gyroscopic dynamics of antiferromagnetic vortices in the orthoferrite domain wall

The method of generation of antiferromagnetic vortices on the supersound domain wall in the orthoferrites was proposed. Moving antiferromagnetic vortices were accompanied by the solitary deflection waves. These waves were used for investigation of generation and nonlinear dynamics of the antiferromagnetic vortices on a moving domain wall with the help of two- and three-fold digital high-speed photography and Faraday rotation in the orthoferrites plates cut perpendicular to the optical axis. The full velocity of antiferromagnetic vortex nonlinearly increases and saturates on the spin velocity level c. The vortices with smallest topological charges saturate earlier than with big one. The vortices velocity along the domain wall u increases up to the maximum and goes to the dependence u²+v²=c². Vortex dynamics is quasirelativistic on quasirelativistic domain wall. The theory of gyroscopic force in the domain wall of orthoferrites was elaborated by Zvezdin et al. and was confirmed our earlier experimental results.     

Generation and gyroscopic quasi-relativistic dynamics of antiferromagnetic vortices in domain walls of yttrium orthoferrite

An unusual nonlinear relation between the velocity of an antiferromagnetic (AFM) vortex along a domain wall (DW) on the DW velocity is detected. This relation has a maximum whose position depends on the topological charge of the vortex. As the DW velocity increases from the value corresponding to the maximum to its limiting value, the AFM-vortex velocity decreases and tends to zero. The total AFM-vortex velocity increases nonlinearly with the DW velocity and levels off at 20 km/s, which is equal to the velocity of spin waves in the linear section of their dispersion law. The experimental data are approximated satisfactorily. The dynamics of AFM vortices in DWs of yttrium orthoferrite, just as the dynamics of the DWs, is quasi-relativistic and gyroscopic.     

Dynamics of domain walls under the action of pulse and gradient magnetic fields in rare-earth orthoferrites

Experimental and theoretical investigations of solitary domain wall dynamics in an yttrium orthoferrite plate under the action of a pulse magnetic field were carried out. The investigations are performed under conditions in which the change in the gradient magnetic field is comparable to the magnitude of the pulse magnetic field shifting the domain walls when the latter are displaced from their equilibrium position.     

Excitation of bending vibration by a moving domain wall in a plate of yttrium orthoferrite

Elastic vibrations have been experimentally found induced by a moving domain wall in a sample of yttrium orthoferrite 10⁻⁴ m thick. Calculations have been carried out to suggest their relevance to flexural Lamb waves.     

Dynamics of a Néel domain wall with a fine structure in rare-earth orthoferrites

The dynamics of an isolated domain wall (DW) with a fine structure moving at a supersonic velocity in a rare-earth orthoferrite is studied. A set of nonlinear equations of motion of the center of a DW structure line is derived. A steady-state solution to these equations adequately describes the experimental data for yttrium orthoferrite. The effect of an external magnetic field on the steady-state velocity of a DW with structural lines is investigated.     

Interaction of a shear wave with a moving domain wall in an iron garnet crystal

The boundary-value problem of the interaction of a plane monochromatic shear wave with a moving Bloch wall in an iron garnet crystal is solved in the framework of the nonexchange magnetostatic approximation on the basis of the method of phase invariants for wave problems with moving boundaries. For a shear wave incident on the domain wall, the possibility of the reflectionless birefringence is demonstrated. Numerical results illustrating the resonance properties of the magnetic subsystem are presented. It is established that, at the upper bound of the reflectionless birefringence range, the interaction of the shear wave with the domain wall manifests itself as a degenerate resonance with the solution in the form of two combined antiphase, collinearly propagating shear waves of infinitely large amplitudes, which form a zero resulting field.     

Ginzburg-Landau theory and solitary waves in shape-memory alloys

A one-dimensional dynamic Ginzburg-Landau theory of the martensitic phase transition in shape-memory alloys is established. The nonlinear equations of motion yield solitary wave solutions of kink and of soliton type. The kink solutions which cannot move without external force represent single domain walls either between austenite and martensite or between two martensite variants. The soliton solutions correspond to a matrix of austenite or of martensite containing a moving sheet of the other phase. The velocity of the solitons depends on their amplitude. In the static case they reduce to the critical nucleus. The energy of each type of solitary waves is calculated.     

Nonlinear propagation of ultra-low-frequency electromagnetic modes in a magnetized dusty plasma

A theoretical investigation has been made of nonlinear propagation of ultra-low-frequency electromagnetic waves in a magnetized two fluid (negatively charged dust and positively charged ion fluids) dusty plasma. These are modified Alfvén waves for small value of and are modified magnetosonic waves for large , where is the angle between the directions of the external magnetic field and the wave propagation. A nonlinear evolution equation for the wave magnetic field, which is known as Korteweg de Vries (K-dV) equation and which admits a stationary solitary wave solution, is derived by the reductive perturbation method. The effects of external magnetic field and dust characteristics on the amplitude and the width of these solitary structures are examined. The implications of these results to some space and astrophysical plasma systems, especially to planetary ring-systems, are briefly mentioned. Received 8 July 1999 and Received in final form 11 October 1999     

Excitation of Alfvén vortices in the ionosphere by the magnetospheric convection

We analyze conditions for excitation of ULF waves in the ionospheric Alfvén resonator (IAR), taking into account the altitude-inhomogeneous profile of the magnetospheric convection velocity. This profile is formed as a result of interaction of the convective flow with the neutral atmosphere at altitudes 90–150 km. ULF waves comprise oblique Alfvén waves, trapped into the IAR, and ionospheric drift waves, which are in resonance with them. These waves together form strongly anisotropic, closed current loops, whose scale along the magnetic field greatly exceeds their transverse scale, i.e., l_z ≫ l_⊥, and can be considered Alfvén vortices. Within the framework of the proposed model of the ionosphere, we study the instability threshold and the amplitude growth rate of the Alfvén vortices as functions of different parameters (wave vector k_22A5, angle between the wave vector and the convection velocity, ratio of the Alfvén-wave and Pedersen conductivities, etc.). Some estimates are given in application to the observed small-scale field-aligned currents in the auroral ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 5, pp. 376–390, May 2008.     

Generation of Elastic Waves by a Magnetic Field Associated with Reversible Rotations in Triaxial Magnets

In the first-order anharmonicity approximation of Hooke's law, the amplitudes of elastic waves, absorption coefficients, wave numbers of the fundamental wave and of the first harmonic generated by an alternating magnetic field with a preset orientation relative to the basis axes of a crystal having arbitrary dimensions are calculated for multidomain magnets with rigidly fixed domain boundaries in terms of concentrations of magnet phases and magnetic structure parameters with allowance for the wave equation and angular momenta.