Spin-wave spectroscopy and application of its methods to heterostructures of silicon dioxide with Co nanoparticles on a GaAs substrate

A method has been developed for determining magnetic and electrical characteristics of film nanostructures containing magnetic nanoparticles from dispersion curves of surface spin waves propagating in these nanostructures. The dispersion curves of spin waves are determined by the dynamics of the spin component described by the generalized Landau-Lifshitz equations and an alternating electromagnetic field induced by a spin wave. Since spin waves are very sensitive to inhomogeneity of magnetic parameters, spin disorder, and conductivity of an object near or inside which these waves propagate, they can be used for determining magnetic and electrical characteristics of the objects under investigation. The developed calculation method, which can be employed both in spin-wave spectroscopy and in analysis of dispersion curves obtained by other methods, has been used for determining parameters of heterostructures consisting of a SiO₂ film with Co nanoparticles on a GaAs substrate. It has been found from the shape of dispersion curves of the surface spin waves that, in the film near the interface, spins of the nanoparticles are close to a ferromagnetic ordering, whereas near the free surface, the spin orientation of nanoparticles is more chaotic. It has been revealed that a conducting layer is formed in GaAs, and the SiO₂(Co) film near the interface has an increased conductivity.     

Two-magnon scattering and the collective excitation of spin waves

The excitation of strongly coupled spin waves is studied on a model of coupled oscillators. In particular, the question of a uniform mixing of degenerate spin waves is discussed. Attention is also paid to the _k -dependence of the frequency intervals in which the spin waves are appreciably excited.     

Magnetization dynamics in thin films and multilayers

The behavior of spin waves is influenced by essentially all the parameters which characterize a magnetic material – exchange interactions, anisotropy, surface effects, dipolar interactions, phase transitions and imperfections. Thus measurements of spin wave frequencies can give important information on characterizing different magnetic materials and structures. In this paper we outline the major results and calculational methods for long-wavelength spin waves in thin films and multilayers. While the primary attention is on ferromagnet-based structures, long-wavelength spin waves in antiferromagnets are also discussed. We indicate how particular measurements of spin wave frequencies can be used to extract the fundamental parameters of the different structures.     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

The CESR and spin wave studies of A-15 films in normal and superconducting states

The electron spin resonance studies have been reported for A-15 superconductors, namely Nb₃Ge, Nb₃Si and V₃Si possessing different T_c values and CESR, Platzmann-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that T_c of Nb₃Ge depends upon the presence and separation of spin wave absorptions from the CESR, and T_c is found to increase when the separation is reduced. It is concluded that the exchange interactions in the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible for superconductivity in A-15 materials studied.     

Spin-wave diffraction in a homogeneous ferromagnetic layer

The authors report theoretical and experimental research on diffraction effects associated with the excitation and propagation of surface spin waves. Expressions are derived for the Green's matrix of a point source of surface spin waves in the far-field region, and a simple scalar form of notation for the Green's function of a point source in an anisotropic medium is proposed. This form can be used to calculate the far diffraction field of an arbitrary planar antenna. The Green's functions obtained for the point source are used to calculate the radiation patterns of linear microstrip transducers, as well as the fraction of energy picked up by an analogous transducer positioned opposite the radiating transducer in the far-field region. The computational method is used to reduce the error in local monitoring of the quality of yttrium-iron garnet (YIG) films by the double-measurement method. The authors present the results of an experimental study of the dispersion characteristics of surface spin waves for different angles between andH _i, as well as a theoretical and experimental study of transducers that focus surface spin waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 32–48, April, 1989.The authors would like to thank S. G. Suchkov and Yu. A. Zyuryukin for valuable comments during a discussion of the results.     

立方铁磁体中的自旋波局域模

本文计算了简单立方铁磁体中由于磁性杂质的存在而形成的局域自旋波。其空间对称有s型模、p型模和d_r型模三种。求出了各类局域模的波函数和自旋偏离的相角分布,其相应的能级与参量J′S′/JS的关系均由曲线表出。其次分析了在体心立方和面心立方铁磁体中可能出现的局域模的对称类型。我们指出,s型局域自旋波是最有可能实际观测到的。作为观测局域自旋波的样品的铁磁体,其居里点应随渗杂浓度的增加而上升。按照这一判据,我们举出一些可以试用的实验样品。最后,通过总自旋量子数的分析,一般地断定了辐射场不可能激 关键词：     

SPECIFIC HEAT OF A HEISENBERG SYSTEM WITH FINITE SIZE

Based upon the spin wave theory, the influence of the size of a three-dimensional Heisenberg system on its thermodynamic properties was studied. It is found that the specific heat increases due to the finite size and free surface of the system. For a magnetic film with finite thickness, the interaction of spin waves was also discussed. There exist three additional scattering processes, namely, the scattering between spin waves with wave-vectors parallel to the surface of the film (two-dimensional spin wave), the scattering between two and three-dimensional waves, and the scattering between those waves with the same component in the direction along the thickness of the film. As a result, the T⁴ term, arising from the coupling of spin waves, in the expression of the specific heat of the system, splite into three parts proportional to T^5/2,T^7/2 and T⁴, respectively. Here T is the temperature.     

Phonon and magnon widths in metamagnets

Microscopic expressions for the couplings of the phonons corresponding to the rigid motions of the waters of hydration with both transverse and longitudinal spin fluctuations are given for the linear chain metamagnetic systems FeCl₂·2H₂O, CoCl₂·2H₂O, and CoBr₂·2H₂O. The processes which can give large, temperatures dependent widths of spin waves and of some phonons for temperatures approaching the Neél temperature are discussed.     

Raman scattering studies of spin‐waves in hexagonal BaFe12O19

We present the results of polarized Raman spectroscopy of hexagonal BaFe₁₂O₁₉ single crystal. The spectra, recorded from 200 to 800 cm^–1 and 1100 to 1700 cm^–1 in the 20–250 K temperature range, are analyzed on the basis of both crystal vibrations and spin‐waves. In the low wavenumber range, the Γ‐point phonons are observed. In the high wavenumber range, phonon mixings are observed; more interestingly, four modes of spin‐waves are identified in hexagonal BaFe₁₂O₁₉. Both have not been studied previously. Our analyses of the spin‐waves provide an optical method for quantitatively estimating the spin exchange interactions in hexagonal BaFe₁₂O₁₉. The four strong exchange integrals are found to have the values of J_ce = 1.31 meV, J_ae = 1.36 meV, J_cd = 1.46 meV, and J_bd = 1.71 meV. Our results also indicate that at ~200 and ~80 K, there would be additional spin‐ordering transitions in hexagonal BaFe₁₂O₁₉. Copyright © 2012 John Wiley & Sons, Ltd.     

Reflection and scattering of spin waves by surface roughness of a ferromagnet

Abstract

A detailed theory of volume spin wave reflection from the randomly rough surface of a ferromagnet is presented. The contribution to damping of the reflected wave is calculated. This contribution is due to the scattering of the initial volume wave into secondary surface and volume spin waves. The value of damping is proportional to the correlation length and the square of the roughness amplitude. Numerical calculations of the attenuation rate as a function of the angle of incidence and the ratio between the surface anisotropy and the wavenumber are provided. They yield the angle of incidence where the attenuation has a maximum. In analogy to optics, this angle is similar to the Brewster angle. Numerical estimations of damping and a comparison of its value with the ferromagnetic resonance linewidth are also made. Finally, the results of the calculation of the scattering of surface exchange spin waves by surface roughness are presented and discussed.     

Brillouin-zone mapping of the spin waves in a ferromagnetic bilayer system

The eigenproblems of spin waves in a heterogeneous ferromagnetic bilayer system with periodic boundary conditions are solved using the interface-rescaling approach. Brillouin zone mapping and the eigenmodes of the system are investigated. We find three types of spin waves may exist in the system: the bulk mode, the interface mode, and the perfect confined mode. The fine structure of the energy band in the heterogeneous bilayer system is first given for the whole two-dimensional Brillouin zone. Conditions for the existence of the interface mode are discussed. Finally, we analyze the resonant-confined spin waves in bulk modes and their oscillating behavior.     

The effect of parametrically excited spin waves on the dispersion and damping of magnetostatic surface waves in ferrite films

This paper describes an experimental study of variations of the dispersion and damping of magnetostatic surface waves in ferrite films, caused by three-and four-magnon interactions with parametric spin waves excited by an auxiliary surface magnetostatic pump wave with frequency f _p. The variations in the dispersion and damping were identified, respectively, with variations Δk″ in the real part and Δk′ in the imaginary part of the wave number of the surface magnetostatic wave. The Δk″ and Δk′ values were determined from the ratio of the changes of the phase increment Δφ and the amplitude increment ΔA of the surface magnetostatic wave to the length L of the nonequilibrium section of the film, where the parametric spin waves exist. It is found that, when three-magnon decay processes are allowed for the pump wave and the surface magnetostatic probe wave, the probe wave can substantially alter the distribution of the parametric spin waves in the film. Zh. éksp. Teor. Fiz. 115, 318–332 (January 1999)     

铁磁/反铁磁双层薄膜中自旋波特性研究

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了非周期性边界条件下,铁磁/反铁磁双层薄膜中自旋波低温激发问题.重点讨论了表面各项异性对薄膜中自旋波谱、色散关系的影响.结果表明:表面各向异性对铁磁层中自旋波谱影响较大,对色散关系影响甚微。     

Josephson and quasiparticle currents in tunneling junctions between partially dielectrized (partially gapped) superconductors with spin density waves

The current-voltage characteristics (CVC) are calculated for the Josephson, interference, and quasiparticle components of the current through a tunneling junction formed by two superconductors with spin density waves (SDW). The treatment is based on the model of partial dielectrization (gapping) of the Fermi surface and the assumption of pinning of the spin density waves. The following particular cases are studied in detail: asymmetric SDW superconductor-ordinary superconductor junctions and symmetric junctions between two identical SDW superconductors. The positions and nature of the singularities in the CVC are determined. For a symmetric contact the possibility of the existence of asymmetric CVC’s is predicted. The calculations are in qualitative agreement with the experimentally observed behavior of the CVC’s of tunneling junctions and microcontacts containing the SDW superconductor with heavy fermions URu₂Si₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1743–1749 (October 1999)     

Surface Spin Waves in Anisotropic Heisenberg Magnet with a Single-Ion Uniaxial Anisotropy

The technique of Green's function is used to investigate the surface spin waves in semiinfinite anisotropic Heisenberg magnet with a single-ion uniaxial anisotropy. The dispersion relations of surface excitations are obtained for the ferromagnetic, antiferromagnetic and ferrimagnetic systems, respectively. Results in some particular cases are discussed.     

Quantum antiferromagnetism in 2 dimensions

We study the spin dynamics of the 2-dimensional spin 1/2 Heisenberg model at finite temperatureT, in particular the frequency region <TJ (the coupling constant). The main conclusion reached is that the spin fluctuations have relaxational character, no propagating spin waves exist for <T. Comparison with experimental results on La₂CuO₄ give good agreement. Similarities and differences to previous theoretical treatments are discussed.     

A light scattering study of the ferromagnet CrBr3

Measurements of light scattering from acoustic spin waves in CrBr₃ in both the ferromagnetic and paramagnetic states are reported. The dispersion is shown to be adequately described in terms of a single anisotropy constant. Lifetime measurements are compared to those obtained by ferromagnetic resonance techniques. Anomalous effects arising from the very high magnetooptic coupling are discussed.     

非均匀交换各向异性铁磁介质的非线性表面自旋波

利用Landau-Lifshitz 方程，研究了具有非均匀交换各向异性的半无限大铁磁体的非线性表 面自旋波理论。导出了部分钉扎纯交换铁磁介质的磁化强度所满足的边界条件和非线性表面 自旋波的色散关系，并获得了自旋波振幅沿z方向驻波的一维非线性Schrdinger方程和包 络振幅沿平面传播的二维非线性Schrdinger方程，结果表明铁磁体磁化强度的包络振幅随时空变化的性质是由二维非线性Schrdinger方程决定的。因此预言铁磁介质的表面非线性激发应是二维孤波的形式。对于弱非线性表面自旋波，对非线性Schrdinger方程存在孤子形式解的可能性作了讨论. 关键词： 表面自旋波 Landau-Lifshitz方程 非线性Schrdinger方程 孤子