Parametric excitation of spin waves in uniaxial ferrites

Parametric excitation of spin waves in uniaxial ferrites with large anisotropy is investigated theoretically. First-order processes in a sphere with arbitrary orientation of an external static magnetic field, in which case the pumping is oblique, are studied. The threshold field and the parameters of the excited spin waves are calculated numerically for two ferrites — easy-axis and easy-plane. Zh. Tekh. Fiz. 68, 65–71 (May 1998)     

Parametric excitation of spin waves in strongly anisotropic uniaxial ferrites

The thresholds of parametric spin-wave excitation are measured in a Ba₂Zn₂Fe₁₂O₂₂ easy-plane ferrite at a pumping frequency of 36 GHz in the temperature range from 5 to 350 K at different angles between the external dc magnetic field and the anisotropy axis, where the pumping is, in a general case, oblique. The dependences of the spin-wave damping parameter on wave number, temperature, and the angle between the spin-wave propagation direction and the anisotropy axis have been found. The results of measuring the ferromagnetic resonance parameters in this ferrite and an easy-axis Ba(Fe_0.95Sc_0.05)₁₂O₁₉ ferrite are presented. Conclusions on the relaxation processes in strongly anisotropic hexagonal ferrites are made. Fiz. Tverd. Tela (St. Petersburg) 41, 1652–1659 (September 1999)     

Effect of a longitudinal magnetic field on the excitation of plasma wake waves

The effect of a longitudinal magnetic field on the linear wake fields excited by a relativistic electron bunch in a cold homogeneous plasma is considered. The obtained results prove that the presence of an external magnetic field leads to a dependence of the wake wavelength on the transverse coordinate, to a change in the wave amplitude with increasing distance from the bunch, and to the emergence of anharmonicity. It is found that a strong magnetic field reduces the wave amplitude significantly for narrow bunches and changes the amplitude insignificantly for broad bunches.     

Parametric excitation of spin waves in ferromagnets by longitudinal pumping localized in space

The equations of motion for the slowly varying complex amplitudes of spin waves parametrically excited by a localized pumping magnetic field have been derived. A solution of these equations satisfying given boundary and initial conditions has been obtained. The energy dissipated by spin waves decreases with the pumping intensity beyond a certain pumping power, which can be termed the regeneration threshold. The losses vanish and change sign at the instability threshold. Both thresholds depend heavily on the linear dimension L of the pumping zone, increasing with decreasing L. Owing to the regeneration process, the dissipation length of spin waves increases without bound as the pumping power approaches the instability threshold. Consequently, perturbations of a uniform state due to the boundary penetrate throughout the pumping zone, regardless of the dimension L. As a result, the full pattern of parametric instability is strongly affected by the zone boundary: 1) the spatial distribution of wave amplitudes becomes nonuniform everywhere inside the zone; 2) the amplitude growth rate in the unstable regime decreases at all points when perturbations due to the boundary reach these points; 3) the instability threshold is independent of the spin-wave frequency offset from the parametric resonance frequency. The calculated minimum instability threshold as a function of the bias magnetic field (the “butterfly” curve) changes shape with L, in agreement with the available experimental data. Zh. éksp. Teor. Fiz. 111, 199–219 (January 1997)     

Toward the theory of spin waves on the surface of a nanotube with a superlattice in a magnetic field

Electron spin waves on the surface of a semiconductor nanotube with a superlattice in a magnetic field have been considered. The spin-wave spectra and regions of collisionless wave damping have been found. It has been shown that the spin waves do not exhibit damping on small-radius tubes with a degenerate electron gas.     

Anomalous spin excitation spectrum of the Heisenberg model in a magnetic field

Making the assumption that high-energy fermions exist in the two dimensional spin- 1/2 Heisenberg antiferromagnet, we present predictions based on the pi-flux ansatz for the dynamic structure factor when the antiferromagnet is subject to a uniform magnetic field. The main result is the presence of gapped excitations in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This is qualitatively different from spin-wave theory predictions and may be tested by experiments or by quantum Monte Carlo.     

Nonlinear dynamics of semiclassical spin in a time-dependent magnetic field

The dynamics of magnetic nanoclusters (or molecules) with a large spin in a magnetic field whose strength varies in proportion to time is analyzed. Such a field breaks the symmetry relative to rotations through 2π, as well as clockwise and counterclockwise rotations, and induces a number of new coherent quantum effects in the spin dynamics, such as the formation of a band energy spectrum with continuous spin states or the emergence of “Bloch” oscillations in spin precession and interband Zener tunneling. Bloch oscillations are manifested in experiment as equidistant identical jumps on the magnetization curve. The interband Zener tunneling gives rise to additional jumps and peaks on the susceptibility of the system.     

Microscopic theory of dipole-exchange spin waves in magnetic multilayers

A microscopic model is employed to calculate the spectrum of dipole-exchange spin waves in multilayers in which thin ferromagnetic films are separated by non-magnetic spacers. Two configurations are considered: in one the films have magnetizations parallel to each other, in the other the magnetizations are antiparallel. The calculations extend a previous microscopic formalism that allows the calculation of the dipole-exchange spin wave spectrum in thin films. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness.Received: 26 May 2004, Published online: 12 August 2004PACS: 75.30.Ds Spin waves - 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces     

Pulsed magnetic resonance as a probe of longitudinal spin waves and magnetic second sound

Under appropriate conditions, longitudinal spin waves and spin wave second sound, a magnetic temperature wave, can be observed in quantum liquids and solid using pulsed magnetic resonance. A “hole-burning” experiment yields directly the spectrum ω(k) of the propagating magnetic wave.     

微波激发下非线性磁表面波传播特性

研究了微波激发下非线性反铁磁晶体与线性铁磁晶体交界面上transverseelectric(缩写为TE)表面波的传播特性.用反函数形式给出了反铁磁晶体中TE表面波磁场的精确解.分析了小功率和大功率传输时的场分布特征.求得了TE表面波的色散方程,进而讨论了系统的微波频率特性.分析表明,非线性TE表面波存在频率通带和禁带,其带宽是功率的函数,通过调节导波功率可以有效地实现通带和禁带之间的转换 关键词： 反铁磁晶体 铁磁晶体 非线性表面波     

Induced and parametric excitation of spin waves by a light field with a discrete spectrum and optically induced spin echo

Induced and parametric excitation of spin waves by a light field and light-induced spin echo are considered. The static and steady-state dynamic magnetizations in the light field are determined, and the phase diagrams of the system in magnetic and light fields are constructed. The threshold of parametric excitation, the amplitude of stationary oscillations, and the trajectories described by the end point of the magnetic moment vector are calculated for the case of light-induced nonlinear ferromagnetic resonance. The trajectories can be topologically different. The transition between states with different trajectories occurs as a phase transition. The parameters of the light-induced spin echo are determined.     

Quantization of spin direction for solitary waves in a uniform magnetic field

It is known that there are nonlinear wave equations with localized solitary wave solutions. Some of these solitary waves are stable (with respect to a small perturbation of initial data) and have nonzero spin (nonzero intrinsic angular momentum in the center of momentum frame). In this paper we consider vector-valued solitary wave solutions to a nonlinear Klein-Gordon equation and investigate the behavior of these spinning solitary waves under the influence of an externally imposed uniform magnetic field. We find that the only stationary spinning solitary wave solutions have spin parallel or anti-parallel to the magnetic field direction.     

Refraction of spin waves by bifocal surface ferromagnetic lens in external magnetic field

Behavior of spin-wave propagation in ferromagnetic medium with non-uniform distribution of magnetic parameters is studied. In particular, the influence of external magnetic field, spin-wave frequency and exchange parameter on behavior of surface spin-wave propagating through inhomogeneity made in form of lens (lens is biaxial ferromagnet placed into uniaxial ferromagnetic medium) is studied.     

Tunneling of dipolar spin waves through a region of inhomogeneous magnetic field

We show experimentally and by numerical simulations that spin waves propagating in a magnetic film can pass through a region of a magnetic field inhomogeneity or they can be reflected by the region depending on the sign of the inhomogeneity. If the reflecting region is narrow enough, spin-wave tunneling takes place. We investigate the tunneling mechanism and demonstrate that it has a magnetic dipole origin.     

Self-excited ionization waves at a turbulence threshold in homogeneous longitudinal magnetic field

Experimental observations of the effect of homogeneous longitudinal magnetic fleld on the propagation of spontaneously existing s variety of ionization waves and subharmonic wave in neon d.c. discharge are reported. The measurements were realized in a part of positive column where the motion of the waves is coherent and wave interactions can be neglected. Changes in wave amplitudes are explained in terms of parameters of Landau amplitude equation and their dependence of the magnetic field.     

Interaction of space-charge waves in an electron beam with electromagnetic waves in a longitudinal magnetic field

The effect of space charge on the interaction between an electron beam and an electromagnetic wave in a longitudinal magnetic field has been considered. The equations describing the interaction processes have been formulated within the two-dimensional linear theory and solved using the dispersion-equation approach and the successive approximation method.     

Nonlinear excitation of the second harmonic in a semiconductor superlattice placed into a magnetic field

The nonlinear interaction of waves in a periodic structure placed into a magnetic field is considered. The structure is comprised of alternate semiconducting and insulating layers. The three-wave interaction technique is applied to studying the nonlinear processes. It is shown that nonlinear phenomena in media with translational symmetry and in homogeneous media differ. Conditions for the resonant interaction between the first and second harmonics are analyzed. It is found that the second harmonic is generated when the interacting first spatial harmonics both co-and counterpropagate, which is possible only in periodic structures. Resonance events improving the generation efficiency are discussed.