Effect of optical phonon‐magnon interaction on the magnon softening at finite temperature

A magnon‐phonon interaction model is set up in a two‐dimensional ferromagnetic compound square‐lattice system. Using the Matsubara Green function theory we calculated the magnon dispersion curves on the main symmetric line in Brillouin zone, compared the influences of the magnetic ion optical phonon with non‐magnetic ion optical phonon on the magnetic excitation of the system and discussed the influences of various parameters on the magnon softening. The lower Debye temperature of ferromagnetic materials is, the more likely the magnon softening occurs. It turned out that the optical phonon‐magnon coupling plays an important role on the magnon softening, the longitudinal optical phonon contributes the most to the magnon softening and magnon damping. It is also found that the contribution of the non‐magnetic ion to the magnon softening and magnon damping is more significant than that of magnetic ion when the mass of the magnetic ion is less than that of the non‐magnetic ion, or the mass of magnetic ion and the non‐magnetic ion are equal.     

Laser stimulated magnon fluorescence of EuO in quantizing magnetic fields

The effect of a strong quantizing magnetic field on the magnon damping in the electron-magnon system of EuO in a phonon field is discussed. It is found that as the laser frequency approaches the cyclotron frequency the magnon population may become unstable (magnon fluorescence).     

光频支声子对二维Heisenberg铁磁系统磁激发的影响

在二维复式正方Heisenberg铁磁系统的基础上建立了磁振子-声子相互作用模型. 利用松原格林函数理论研究了系统的磁激发，计算了布里渊区的主要对称点线上的磁振子色散曲线，比较了光频支声子与声频支声子对系统磁激发的影响以及各项参数的变化对磁振子软化的影响. 发现光频支声子-磁振子耦合对磁振子软化起主要作用，尤其是纵向光频支声子对磁振子软化起更大的作用，并且非磁性离子的光频支声子对磁振子软化的作用比磁性离子的光频支声子对磁振子软化的作用更显著.     

Magnon instability in the free-carrier absorption in magnetic semiconductors under a strong d.c. magnetic field

The magnon damping in an electron-magnon system in the presence of a laser beam and a d.c. magnetic field is discussed. It is shown that near the laser-cyclotron resonance, and for the laser beam propagating perpendicularly to the magnetic field the magnon population in a relatively narrow range of k may grow with time.     

Magnon instability by a radiation field in magnetic semiconductors and strong d.c. magnetic fields

The magnon damping in an electron-magnon system in the presence of a laser beam and a d.c. magnetic field is discussed. It is shown that near the laser-cyclotron resonance, and for the laser beam propagating perpendicularly to the magnetic field the magnon population in a relatively narrow range of k may grow with time.     

有限温度下光频支声子-磁振子相互作用对磁振子寿命的影响

在二维复式正方Heisenberg铁磁系统的基础上建立了磁振子-声子相互作用模型.利用松原格林函数理论研究了系统的磁振子寿命,计算了布里渊区的主要对称点线上的磁振子衰减的变化曲线,比较了磁性离子的与非磁性离子的光频支声子对磁振子衰减的影响以及各项参数的变化和温度对磁振子衰减的影响.发现光频支声子-磁振子耦合对磁振子衰减起主要作用,尤其是纵向光频支声子对磁振子衰减起更大的作用,并且非磁性离子的光频支声子对磁振子衰减的作用比磁性离子的光频支声子对磁振子衰减的作用更显著.根据关系式-Im∑^*(1)(k)=/(2τ)可以对磁振子寿命进行判断. 关键词： 光频支声子-磁振子相互作用 磁振子衰减 磁振子寿命 绝缘复式正方铁磁体系统     

Green’s function approach to the low temperature properties of Cs<Subscript>2</Subscript>CuCl<Subscript>4</Subscript>: anisotropy effects

We have studied the effect of both axial and transverse anisotropy on the critical field and thermodynamic properties of the field induced three dimensional antiferromagnetic Heisenberg model on the frustrated hexagonal lattice for Cs₂CuCl₄ compound. The spin model is mapped to a bosonic one with the hard core repulsion constraint and the Green’s function approach has been implemented to get the low energy spectrum and the corresponding thermodynamic properties. To find the critical field (B _c ) we have looked for the Bose-Einstein condensation of quasi-particles (magnons) which takes place when the magnon spectrum vanishes at the ordering spiral wave vector. We have also obtained the dispersion of magnon spectrum in the critical magnetic field for each anisotropy parameter to find the spiral wave vector where the spectrum gets its minimum. The magnon energies show a linear dispersion relation close to the quantum critical point. The effect of hard core boson interaction on the single particle excitation energies leads to a temperature dependence of the magnon spectrum versus magnetic field. We have also studied the behavior of specific heat and static structure factor versus temperature and magnetic field.     

Light scattering from thermal acoustic magnons in yttrium iron garnet

Brillouin scattering measurements on yttrium iron garnet (YIG) have revealed the scattering due to thermal acoustic magnons. The intensity and frequency shift of the magnon peaks as a function of applied magnetic field and incident light wavelength have been investigated and are discussed here within the framework of a simple light scattering theory.     

Magnons localised on surface steps: a theoretical model

We present the solution of the full magnetic problem arising from the absence of magnetic translation symmetry in two dimensions due to an extended magnetic surface step on the surface boundary of an insulating magnetic substrate. The calculation concerns in particular the spin fluctuation dynamics of a magnetic atomic step in the surface of a ferromagnetic simple cubic lattice, the spin order being in the direction normal to surface boundary. Only exchange interactions are considered between the spins in the model. The theoretical approach determines the evanescent spin fluctuation field in the two dimensional plane normal to the direction of the step edge. This field arises owing to the absence of magnetic translation symmetry in this plane, and is completely independent of the form of the surface defect, underlying the general character of the calculation. We show the existence of optical localised magnon modes propagating along the step, their fields being evanescent in the plane normal to the step direction. Received 17 February 1999     

Magnon squeezing states in a ferromagnet

In this Letter we discuss squeezing state of magnon in ferromagnet, which permits a reduction in the quantum fluctuation of the spin component to below the zero-point quantum noise level of coherent magnon states. We investigate the generation of squeezed magnon state through calculating the expectation values of spin component fluctuation. The mean field theory is introduced in dealing with the nonlinear interaction terms of Hamiltonian of magnon system.     

Decay kinetics of metastable spin-aligned crystals

The decay kinetics of metastable spin-aligned atomic crystals is considered. It is shown that the most rapid decay channels due to the creation of spin waves may be suppressed by means of the external magnetic field applied against the direction of magnetic momenta polarization. The primary channels of decay which cannot be eliminated by magnetic field are connected with the decay of the polarized state into one magnon and two phonons due to dipole-dipole interaction and into one magnon and one phonon due to a second order process that is associated simultaneously with the hyperfine interaction and the magnon-phonon interaction.     

Brillouin scattering study of multilayer cobalt-niobium films

Spin waves which are characteristic of periodic structures of thin ferromagnetic Co films alternating with nonmagnetic Nb films have been investigated by means of Brillouin light scattering. The dependence of the magnon frequencies on the magnetic inplane field and on the wave-vector was measured for several samples with different numbers and thicknesses of the layers. The experimental data are in good agreement with a theory of magnetostatic surface spin waves in such media elaborated by Grünberg and Mika. The amplitudes of the transverse magnetization in the different layers of the stack have been calculated. The highest frequency branch resembles the Damon-Eshbach surface magnon. With decreasing frequency the branches acquire volume mode character. In addition, a number of phonon branches has been observed which are interpreted as plate modes of the combined Co-Nb layer on the Si substrate.     

Dispersion properties of a nonreciprocal magnon crystal

A magnon crystal is created on the basis of a metal–dielectric–ferrite structure inserted into a spatially periodic magnetic field. The dispersion characteristics of the crystal are measured using a movable probe. The nonreciprocity of the magnon crystal is determined. This property is reflected in the difference betwen the dispersion characteristics of waves propagating in opposite directions. It is shown that the Brillouin zone is located asymmetrically with respect to zero in the space of wave numbers.     

High frequency modes in vortex-state nanomagnets

The magnon mode excitation spectrum is obtained from a linearized set of Landau-Lifshitz equations for vortex ground state cylindrical nanomagnets in an external magnetic field. It is shown that there is a rich spectrum of doublet states, and the splitting can be amplified in an external magnetic field.     

Solitons and magnons in Sine-Gordon like magnetic chains

We present theoretical results on the dynamic structure factors of both the classical Sine-Gordon chain and thexy-like ferromagnetic chain in a symmetry breaking magnetic field. We investigate the lowest order corrections to the noninteracting soliton/magnon picture and show that interference effects between solitons and magnons considerably reduce the intensity of the soliton induced central peak. We discuss the additional contribution of two magnon processes to the central peak and find that the combined strength is in agreement with numerical results. We calculate magnon intensities including quantum effects and find that the intensity depends strongly on temperature and wavevector. Quantitative results are given for the one-dimensional magnet CsNiF₃ and compared to neutron scattering data. The soliton induced line-width of the long wavelength magnon is also given.     

Magnon propagation in a nanometric magnetic cluster chain: Effects of additional clusters near the chain

A fundamental understanding of nanoscaled materials has become an important challenge for any technical applications. For magnetic nanoparticles, the investigations are in particular stimulated by the magnetic storage devices. In this paper we present a theory of the magnon propagation in a nanometric chain and of the effects of neighbor magnetic clusters. We show that with an appropriate choice of the additional clusters, it is possible to control the transmission spectrum of the magnons. The additional clusters can also create a resonator with zero of transmission. This model could be useful to construct device to transmit or stop the magnon signal.     

Magnon bands in twisted bilayer honeycomb quantum magnets

We study the magnon bands of twisted bilayer honeycomb quantum magnets using linear spin wave theory. Although the interlayer coupling can be ferromagnetic or antiferromagnetic, we keep the intralayer one ferromagnetic to avoid possible frustration. For the interlayer ferromagnetic case, we find the magnon bands have similar features with the corresponding electronic energy spectrums. Although the linear dispersions near the Dirac points are preserved in the magnon bands of twisted bilayer magnets, their slopes are reduced with the decrease of the twist angles. On the other hand, the interlayer antiferromagnetic couplings generate quite different magnon spectra. The two single-layered magnon spectra are usually decoupled due to the opposite orientations of the spins in the two layers. We also develop a low-energy continuous theory for very small twist angles, which has been verified to fit well with the exact tight-binding calculations. Our results may be experimentally observed due to the rapid progress in two-dimensional magnetic materials.     

Field-induced paramagnons at the metamagnetic transition of Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4, therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.     

Exact soliton solution of spin chain with an external magnetic field in linear wave background

Employing a simple, straightforward Darboux transformation we construct exact N-soliton solution for anisotropic spin chain driven by an external magnetic field in linear wave background. As a special case the explicit one- and two-soliton solution dressed by the linear wave corresponding to magnon in quantum theory is obtained analytically and its property is discussed in detail. The dispersion law, effective soliton mass, and the energy of each soliton are investigated as well. Our result show that the stability criterion of soliton is related with anisotropic parameter and the amplitude of the linear wave.     

New properties of magnon density in uniaxial anisotropic ferromagnet on the background of spin wave

The N-soliton solutions of magnetization in uniaxial anisotropic ferromagnet on the background of spin wave are presented by using the effective Darboux transformation method. With the analytical solutions new properties of magnon density is studied in detail. On the ground state background the magnon density is constant for the spin wave solution and the magnetic soliton, respectively. However, on the spin wave background the magnon density possesses of temporal or spatial periodic oscillation. Moreover, the soliton solution possess the breather character in its propagation along the ferromagnet. These results show that during soliton propagation a periodic magnon exchange occurs between the magnetic soliton and the spin wave background.