Modeling of magnetoelastic shear waves due to point source in a viscoelastic crustal layer over an inhomogeneous viscoelastic half space

In the present study, propagation of magnetoelastic shear wave due to a momentary point source in a viscoelastic crustal layer over inhomogeneous viscoelastic half space has been discussed. Green’s function technique and Fourier transform along with method of successive approximation are used to find the closed-form solutions for displacement and generalized shear wave period equation. Attenuation of the resultant shear wave is computed and effects of magnetic field, width of the layer, complex wave number, viscosity, and inhomogeneity parameters are distinctly marked on dissipation curves using two-dimensional and surface plots. It is found that effect of layer’s magnetoelastic coupling parameter on attenuation pattern of shear wave is just the reverse of half space magnetoelastic coupling parameter. Similarly, internal friction of layer has somewhat different effect on shear wave angular frequency than lower half space viscosity. Certain published results are also derived as special cases to the present study.     

Shear Surface Wave on a Moving Bloch Wall

Using Galileo's transformation for moving to the rest frame of the Bloch wall in the exchange-free magnetostatic approximation, we obtain the dispersion relation for a shear surface wave guided by a moving 180-degree domain boundary of a ferromagnetic crystal. It is found that the motion of the domain boundary has the orienting action on the wave normal of the shear surface wave and significantly changes the spectrum of forward-propagating waves in the frequency band below the scattered-field ferromagnetic resonance.     

Magnetoelastic waves in an in-plane magnetized ferromagnetic plate

The spectrum of magnetoelastic waves propagating along the magnetic field in an in-plane magnetized ferromagnetic plate is numerically investigated in the exchangeless approximation. No restrictions are imposed either on the field pattern of backward volume magnetostatic waves (BVMSWs) or elastic waves supported by a plate of a given geometry across the plate or on the relationship between the sound velocity v _S and the phase velocity of the magnetoelastic waves v=ω/q (ω is the frequency, q is the wave number). The resonance interaction of the BVMSWs and elastic waves is accompanied, as a rule, by the formation of “stop” bands δω that are proportional to the magnetoelastic coupling constant b. When the BVMSWs are in resonance with Lamb and shear elastic modes the values of the magnetoelastic gaps δω at v≈v _S turn out to be of the same order. For v≫v _S , the efficiency of the interaction between the BVMSWs and transverse Lamb modes is almost one order of magnitude higher. If the frequency spacing Δω between the elastic modes is smaller than the mag-netoelastic gap in the spectrum (Δω≤δω), which takes place, particularly, in the region of crowding the elastic mode spectrum (v≈v _S), the resonant interaction results in mixing the dispersion laws for the elastic modes. Namely, a surface mode may transform into a volume one and a shear mode, into the Lamb mode or into a shear mode with another number. The resonance interaction of the shear and Lamb elastic modes not only forms the magnetoelastic gaps δω∼b ² but also changes the efficiency of elastic wave coupling with the magnetic subsystem. This may show up as the coexistence of the effects of “repulsing” both the dispersion laws and the damping decrements of the elastic waves at the resonance frequency. It is shown that magnetostriction splits the cutoff frequencies of both transverse Lamb modes and shear modes, as well as the long-wave (q → 0) frequency limits f ₀ of the BVMSW modes. This may cause the resonance interaction between BVMSW modes of equal evenness in a narrow frequency band Δ∼b near f ₀.     

Fast compressional wave attenuation and dispersion due to conversion scattering into slow shear waves in randomly heterogeneous porous media

Within the viscosity-extended Biot framework of wave propagation in porous media, the existence of a slow shear wave mode with non-vanishing velocity is predicted. It is a highly diffusive shear mode wherein the two constituent phases essentially undergo out-of-phase shear motions (slow shear wave). In order to elucidate the interaction of this wave mode with propagating wave fields in an inhomogeneous medium the process of conversion scattering from fast compressional waves into slow shear waves is analyzed using the method of statistical smoothing in randomly heterogeneous poroelastic media. The result is a complex wave number of a coherent plane compressional wave propagating in a dynamic-equivalent homogeneous medium. Analysis of the results shows that the conversion scattering process draws energy from the propagating wave and therefore leads to attenuation and phase velocity dispersion. Attenuation and dispersion characteristics are typical for a relaxation process, in this case shear stress relaxation. The mechanism of conversion scattering into the slow shear wave is associated with the development of viscous boundary layers in the transition from the viscosity-dominated to inertial regime in a macroscopically homogeneous poroelastic solid.     

Interaction of a shear wave with a moving domain wall in a ferromagnetic crystal with allowance for the external magnetic field

The boundary-value problem of the magnetoelastic wave interaction with a moving domain wall in a ferromagnetic crystal is solved in the nonexchange magnetostatic approximation with allowance for the external magnetic field. It is shown that the difference introduced by magnetic field between the ferromagnetic resonance frequencies of the domains does not cause any noticeably departure of the refraction characteristics of reflected and transmitted waves from those observed at zero frequency mismatch. By contrast, the magnitudes of the transmission and reflection coefficients strongly depend on the external magnetic field and on the mobility of the domain wall. The dependence of the magnitude of the reflection coefficient on the external magnetic field at a fixed angle of shear wave incidence is found to possess two ferromagnetic resonance peaks. The positions and heights of the peaks may vary depending on the mobility of the domain wall.     

晶格量子涨落对spin-Peierls系统低能量激发态的影响

在非绝热近似下，研究了一维spin-Peierls系统中晶格量子涨落 对系统性质的影响，讨论了系统的二聚化相变、单粒子激发和双粒子束缚态. 结果表明，量子晶格涨落会抑制晶格的二聚化，破坏系统的spin-Peierls基态稳定性.在临界点，系统发生从二聚化spin-Peierls态向无能隙态的相变. 自旋声子耦合强度对束缚态能隙的影响比单粒子激发谱能隙显著. 关键词： sin-Peierls系统 非绝热近似 玻色化 相图     

曲线坐标系下孔隙介质圆柱纵向表面波的传播特性

为了研究孔隙介质圆柱纵向表面波的传播规律,分析其频散和衰减特性,在正交曲线坐标系下建立了表面波的频散方程,通过数值计算得到频散曲线,将纵向导波最低模态与表面波进行对比,并分析了曲率半径及孔隙参数对表面波频散和衰减的影响。结果表明,当频率足够大时,导波最低模态的频散曲线与表面波近似;在同一频率下,表面波的相速度随曲率半径的增大而增大,随孔隙度的增大而减小;表面波的衰减随孔隙度的增大而增大。研究结果为开展孔隙介质圆柱结构的超声无损评价提供了一定的理论参考。     

Coupled Magnetoelastic Waves and the Effective Spin-Phonon Coupling Parameter in High-Temperature Superconductors

The present paper is devoted to finding the spectrum of spin-phonon interaction and elucidating the mechanism of critical temperature increase in high-temperature superconductors. The spin-wave dynamic equations and the spectrum of coupled spin-phonon oscillations are obtained. The dependence of the spin wave spectrum on the spin-phonon coupling parameter is investigated. It is demonstrated that the superconducting transition temperature is directly proportional to the spin-phonon interaction force.     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

Dispersion of interface waves in sediments with power-law shear speed profiles. I. Exact and approximate analytical results.

In the upper tens of meters of ocean bottom, unconsolidated marine sediments consisting of clay, silt, or fine sand with high porosity are "almost incompressible" in the sense that the shear wave velocity is much smaller than the compressional wave velocity. The shear velocity has very large gradients close to the ocean floor leading to strong coupling of compressional and shear waves in such "soft" sediments. The weak compressibility opens an avenue for developing a theory of elastic wave propagation in continuously stratified soft sediments that fully accounts for the coupling. Elastic waves in soft sediments consist of "fast" waves propagating with velocities close to the compressional velocity and "slow" waves propagating with velocities on the order of the shear velocity. For the slow waves, the theory predicts the existence of surface waves at the ocean-sediment boundary. In the important special case of the power-law depth-dependence of shear rigidity, phase and group velocities of the interface waves are shown to scale as a certain power of frequency. An explicit, exact solution was obtained for the surface waves in sediments characterized by constant density and a linear increase of shear rigidity with depth, that is, for the case of shear speed proportional to the square root of the depth below the sediment-water interface. Asymptotic and perturbation techniques were used to extend the result to more general environments. Theoretical dispersion relations agreed well with numerical simulations and available experimental data and, as demonstrated in a companion paper [D. M. F. Chapman and O. A. Godin, J. Acoust. Soc. Am 110, 1908 (2001)] led to a simple and robust inversion of interface wave travel times for shear velocity profiles in the sediment.     

Exchange mechanism for localization of phonons near the surface of a magnetically ordered crystal

It is shown that first-principles inclusion of nonuniform exchange interaction leads to the creation of a new type of generalized shear acoustic wave propagating near the mechanically free surface of a magnet. Criteria are formulated that can be used to specify conditions under which an “exchange” type of surface acoustic wave can exist at the boundary between a magnetic and a nonmagnetic medium once the spectrum of magnetoelastic oscillations of the unbounded magnetic crystal is known. Fiz. Tverd. Tela (St. Petersburg) 40, 299–304 (February 1998)     

Spin-phonon coupling induced frustration in the exactly solved spin-1/2 Ising model on a decorated planar lattice

Spin-1/2 Ising model with a spin-phonon coupling on decorated planar lattices partially amenable to lattice vibrations is examined using the decoration-iteration transformation and harmonic approximation. It is shown that the magnetoelastic coupling gives rise to an effective antiferromagnetic next-nearest-neighbour interaction, which competes with the nearest-neighbour interaction and is responsible for a frustration of decorating spins. A strong enough spin-phonon coupling consequently leads to an appearance of striking partially ordered and partially disordered phase, where a perfect antiferromagnetic alignment of nodal spins is accompanied with a complete disorder of decorating spins. Thermal dependences of the specific heat are explored in detail.     

Specific Features in the Spectrum of Nonlinear Magnetoelastic Waves Propagating in a (111) Plate with Combined Anisotropy

Nonlinear magnetoelastic waves with stationary profiles propagating in a cubic ferromagnetic with [111] uniaxial induced anisotropy are investigated theoretically. It is demonstrated that a new type of nonlinear waves engendered by the resonant motion of the 60-degree domain wall propagates along the [111] axis without magnetization escape from the plane of spin rotation. Some other cases of nonlinear magnetoelastic wave propagation in the examined magnetic are also studied.     

Observation of indirect parallel pumping of magneto-elastic modes in layered YIG/GGG structures

In this work we report new experimental results on nonlinear excitation of magnetoelastic (ME) modes in layered YIG/GGG waveguide structures at GHz frequencies, obtained by a guided-wave light scattering technique. It is shown that the fundamental spin-dipole wave (SDW) mode induces a secondary microwave field at double frequency that can efficiently excite shear elastic modes of the structure. The distinctive feature of a mechanism for ME coupling proposed is that it is free from the selection rules and provides continuous excitation of elastic modes within a wide frequency range by means of the standard microstripe line excitation system.     

Investigation of acoustic waves of higher order propagating in plates of lithium niobate

This paper presents theoretical investigation of higher order acoustic plate waves propagating in single crystals of lithium niobate. The dependencies of wave velocity and electromechanical coupling coefficient of antisymmetric, symmetric, and shear horizontal modes on the parameter hf (h=plate thickness, f=operating frequency) are calculated as a function of propagation direction on X-, Y-, and Z-cut lithium niobate plates. It is found that several modes can provide values of K2 that are much greater than can be obtained with surface acoustic waves (SAWs). For example, K2 as high as 0.26 and 0.38 can be obtained from SH1 and A2 modes, respectively. This compares with a maximum value of K2=0.055 for SAWs. It is further shown that there are several crystal cut and propagation directions that can allow efficient excitation and detection of a single mode with minimal interference due to other modes.     

Spin-driven phonon splitting in bond-frustrated ZnCr2S4

Magnetic susceptibility, specific heat, thermal expansion, and IR spectroscopy provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr2S4 at TN1 = 15 K and TN2 = 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at TN2 reveals a temperature hysteresis typical for a first-order transformation. Because of strong spin-phonon coupling, both magnetic transitions at TN1 and TN2 induce a splitting of phonon modes. The anomalies and phonon splitting observed at TN2 are suppressed by strong magnetic field. Regarding the small positive Curie-Weiss temperature Theta approximately 8 K, we argue that this scenario of two different magnetic phases with different magnetoelastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange.     

Plasmonic wave propagation along a magnetized plasmon–dielectric boundary

The emphasis of this paper is on investigation of the TM/TE depolarization along a magnetized surface. In this regard, initially, the wave equation with appropriate boundary conditions is solved for the propagation constant of the wave along the interface. Next, in order to investigate the electromagnetic field TM/TE depolarization along a linear magnetoplasmon–dielectric interface, coupled partial differential equations are derived for the amplitudes of the TE and TM polarized fields along the surface. These equations are then solved using the Fourier transform and 4th order Runge–Kutta methods and the envelopes of the electric field are obtained. The dependency of the propagation constants of the slow and fast modes on the frequency and magnitude of DC magnetic bias field is investigated. A discussion of TE mode deflection is also provided.     

Efficiency of shear surface wave transformation by the motion of the confining domain wall

The transformation of a shear surface magnetoelastic wave by the motion of the 180° confining domain wall in a ferromagnet is considered. Changes in the wave spectrum due to the motion of the wall are correlated with the variations of the energies of the elastic and magnetic subsystems. The efficiency of surface wave transformation by the domain wall motion is estimated in terms of energy. The frequency dependences of the mean energy density of the wave are found. It is shown that the energy density grows with wall velocity.     

金属包层对称平面单轴晶体波导的模式场(Ⅱ)

对于晶体光轴平行于波导界面的结构,讨论了光在金属包层对称平面单轴晶体波导(波导层是单轴晶体,两个波导界面均为金属)内的传输特性.解析地得到了这种结构下波导模式场的精确解.模式场的性质因单轴晶体的性质不同而异.对于正单轴晶体,波导的主模是横电波,任何频率的光波均可激励该模式;当频率满足一定条件时,波导内传输单模,否则,将激励起高阶模式.高阶模既非TE波,也非TM波,而是两者耦合而成的混合模.对于负单轴晶体,波导的主模是一种混合模,该模式同样可被任何频率的光波所激励;当频率满足一定条件时,波导内传输单模,否则 关键词： 平面金属波导 单轴晶体 模式场 混合模     

光子晶体光纤中布里渊散射声波模式特性的分析

推导了光子晶体光纤中声波微小位移波动方程; 研究了泵浦波长以及纤芯折射率对声波模式的影响; 应用石英圆柱模型研究了小芯径光子晶体光纤中纤芯直径对布里渊声波模式色散的影响. 结果表明在光子晶体光纤中, 纵向声波和横向声波共同作用产生质点声场, 两者相互耦合将产生混合声波模式; 可以通过改变泵浦波长或光子晶体光纤纤芯折射率来改变参与布里渊散射(BS) 过程的声波模式的传播常数; 随着光子晶体光纤(PCF) 纤芯直径的增大, 声波模式耦合程度得到加强, 相速度呈减小趋势, 且同一传播常数下, 声波模式数呈增多趋势; 随着泵浦波频率的增大, 声波相速度减小.