Mechanico-electromagnetic waves in elastic dielectric crystals in a magnetic field

The mechanico-electromagnetic coupled waves under an external magnetic field are theoretically investigated, where elastic dielectric crystals with higher symmetry are taken into consideration. There are three types of the coupled waves: the predominantly electromagnetic waves, the predominantly mechanical transverse and longitudinal waves. Their propagation velocities, and the amplitude ratios of the polarization, the mechanical vibration, and the electromagnetic fields are obtained, which depend not only on the material constants but also on the suppressed magnetic intensity and its direction relative to the propagation direction.     

The scattering of oblique flexural waves of a cracked conducting plate in a uniform magnetic field

Summary Following a classical plate bending theory for magneto-elastic interactions under quasistatic electromagnetic field, we consider the scattering of time harmonic flexural waves by a through crack in a conducting plate under a uniform magnetic field normal to the crack surface. It is assumed that the plate has the finite electric conductivity, and the electric and magnetic permeabilities of the free space. An incident wave giving rise to moments symmetric about the crack plane is applied in an arbitrary direction. Fourier transform method is used to solve the mixed boundary value problem which reduces to a pair of dual integral equations. These dual integral equations are further reduced to a Fredholm integral equation of the second kind. The dynamic moment intensity factor versus frequency for several values of incident angle is computed and the influence of the magnetic field on the normalized values is displayed graphically.     

Anisotropic effects in the magnetoelastic damping of electrolytic nickel in a direct magnetic field

In the present work a quantitative study of the dependence of the e.m.f. U_W induced in a search coil which surrounds a ferromagnetic sample performing torsional oscillations at a constant amplitude on the amplitude and the frequency of the oscillations is made. The dependence of U_W on the intensity of the longitudinal magnetic field H is also studied. On the basis of these studies experimental arguments are presented for the existence of magnetoelastic hysteresis and macro-currents. A supplementary mechanism through which the energy of the vibrating system can be dissipated is also presented.     

Propagation of waves in magneto-thermo-viscoelastic solids subjected to a uniform magnetic field

The propagation of magneto-thermo-mechanical (MTM) plane waves in electrically and thermally conductive magneto-thermo-viscoelastic (MTVE) unbounded solids is investigated with account for the mutual effects of the magnetic, thermal and strain fields. Concerning the mutual and thermo-electric effects in isotropic solids the governing equations are first linearized. In the linearization, the material is assumed to be subjected to a uniform and primary magnetic field in any direction while the material undergoes infinitesimal deformations. It is shown that the governing equations at the intermediate state are fulfilled by the presumed MTM-fields. Furthermore, the dispersion relation which allows us to consider the entire frequency range, the effect of the magnetic field and some nondimensional material parameters is obtained. Therefore, several modes of MTM-waves arise depending upon the direction of the magnetic field such as the uncoupled magnetic and mechanical S-mwaves, the coupled S-wave, the modified mechanical $\text{P}$-and thermal waves, and the modified and coupled MTM-waves. It is seen that all modes of the wave are dispersive and dissipative due to the conductivity and the viscosity of the material. Then the phase velocities and the attenuation constants for the coupling modes are obtained, and some limiting values are discussed. From the expressions follow, in particular, the results for the elastic case, the propagation of mechanical waves in nonconductive materials.     

Theory of paramagnetic spin waves in simple metals in a direct-current magnetic field

The transverse dynamic spin susceptibility of a normal, charged Fermi liquid is found. The formula depends on the first three antisymmetric Landau parametersB ₀,B ₁,B ₂ and is exact to orderk ² in the expressions both for the energy of spin waves and their oscillator strengths. It contains spin-wave poles with1=0 and 1 and easily reproduces all the results obtained previously by perturbation theory.On leave of absence from: Institute of Physics, University of Lattakia, Syria.     

Influence of magnetic breakdown on low-frequency conductivity and weak damping electromagnetic waves in metals

Low-frequency electroconductivity under magnetic breakdown conditions is investigated in the two qualitatively different situations: (a) coherent and (b) stochastic. In the former situation (a) the consideration is fulfilled in terms of the incidental magnetic breakdown spectrum of electrons. In the latter situation (b) it is fulfilled in terms of discrete random walking of classical electron. In these cases the general expressions of the conductivity tensor under conditions of both strong and weak spatial dispersion are derived. It is shown that magnetic breakdown does not violate the conditions of compensation of the volumes of electrons and holes in the case of closed trajectories. In the coherent situation the magnetic breakdown has the most essential influence upon Landau adsorption which is determined by a macroscopic layer of valuesP _z (projection of the momentum of the electrons on the direction of the magnetic field). In the stochastic case it is shown that magnetic breakdown results in the appearance of peculiar ergodic states in which an electron is spread on all bands (which take part in the breakdown) with equal probability. Together with it the probability of breakdown disappears from the low-frequency conductivity in a wide interval of magnetic fields and the conductivity is expressed in terms of the classical motion along an united compound orbit. The results of both coherent and stochastic calculations are used to investigate the weak damping electromagnetic waves. It is shown that the magnetic breakdown essentially changes only the dependence of the helicon damping on the magnetic fieldH. In many situations the dependence is represented by nonmonotonic functions ofH. In the coherent and stochastic cases these functions are qualitatively different.     

Evanescent waves in the magnetic field of an electric dipole

The magnetic field of radiation emitted by an electric dipole contains travelling and evanescent waves when represented as an angular spectrum. The evanescent waves decay exponentially away from the xy-plane, and will therefore not contribute to the detectable radiation in the far field, in general. It is well known, however, that in a small region around the z-axis the evanescent waves of the electric field do end up in the far zone. We have studied the corresponding magnetic evanescent waves, and we have found that the evanescent waves of the magnetic field do not contribute to the far zone in the neighbourhood of the z-axis. When considering the neighbourhood of the xy-plane, it appears that both the electric and magnetic evanescent waves end up in the far field, and the travelling and evanescent waves contribute equally to the radiation in the far zone. Close to the dipole the radiation field diverges, and we have shown that this is entirely due to the evanescent waves.     

The viscous damping of capillary — Gravity waves

Summary Complete solutions are obtained for linear capillary — gravity oscillations at the surface of a viscous liquid of finite depth. The limits to propagation of both high and low wavenumbers are discussed as well the nature of the purely damped motions found beyond them. Numerical results are presented for four liquids covering a wide range of viscosities.With 6 Figures     

Effects of an external magnetic field on thermo-acoustical waves in a linear isotropic thermo-elastic dielectric material

Summary Thermo-mechanico-electromagnetic coupled waves propagating in a linear isotropic thermo-elastic dielectric material are theoretically investigated, in case an external magnetic field is applied to the material. Here the constitutive equations derived from the Clausius-Duhem inequality and Vernotte's heat conduction law are adopted. There are three types of coupled waves: the predominantly electromagnetic wave, the predominantly mechanical transverse wave and the predominantly thermo-mechanical longitudinal wave. The first and second waves have no thermal coupling. The third wave has thermal coupling and its propagation velocity and attenuation constant are perturbed by the external magnetic field.     

The effect of a strong magnetic field on the damping of second sound in dilute3He-4He mixtures below 100 mK

We have measured the increase in damping of second sound in 0.1% and 0.05%³He-⁴He solutions upon application of a magnetic field of 10 T. The increase is caused by changes in the thermal conductivity and viscosity of the mixture as the system becomes spin polarized. We show how to fit the data for the viscosity in zero-field to obtain an estimate of the interaction between the³He quasiparticles. It is then possible to predict the viscosity in a field of 10 T using the calculation of Hampson et al. The data agree with the theory to within 2%.     

Plane waves in electrically conducting and magnetizable viscoelastic isotropic solids subjected to a uniform magnetic field

The propagation of plane electromagneto-mechanical (EM M) waves in homogeneous, initially unstressed and isotropic, electrically conductive magnetoviscoelastic solids in a uniform primary magnetic field is investigated. Depending upon the direction of the applied magnetic field, several modes of the waves arise such as coupled mechanical (M) and electromagnetic (EM) waves. The phase velocities and the attenuations of the waves are obtained both analytically and numerically. Some interesting behavior of the phase velocities and the attenuations are, in particular, detected for certain frequencies, intensity and direction of the primary magnetic field, viscoelastic parameters and conductivity. For example, there are anomalous dispersions of the coupled modes of EM- and M-waves depending upon the intensity of the primary magnetic field, viscoelastic parameters and the conductivity of the material.     

Diffraction of normal compression waves by a penny-shaped crack in the presence of an axial magnetic field

The problem of diffraction of normally incident compressional waves by a penny-shaped crack located in a perfectly conducting, infinite, isotropic, elastic solid permeated by an uniform magnetostatic field is considered. Using an integral transform technique, the problem is reduced to that of solving a Fredholm integral equation of the second kind having a finite integral kernel. The dynamic singular stress distributions near the crack tip are obtained in closed form and the effects on the dynamic stress-intensity factors due to the presence of the magnetic field are shown graphically. For low frequencies, the dynamic stress-intensity factors are expressed in series of ascending powers of the normalized frequency. The approximate solutions are compared with exact solutions.     

Influence of the concentration of a dispersed phase and of the magnetic field on the attenuation of ultrasonic waves in magnetic fluids

The regularities of the propagation of elastic waves of ultrasonic range in magnetic fluids have been experimentally established. The influence of relaxation processes — nonlocal heat exchange and viscous dissipation — on the coefficient of attenuation of sound has been revealed and a comparison to the existing theoretical data has been made. The dependences (anisotropic in form) of the attenuation of the wave’s amplitude on the value and direction of the magnetic field have been determined and an interpretation of the results obtained has been proposed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 133–140, September–October, 2007.     

The ultrasonic wave damping in the region of a magnetic phase transition in manganese-zinc spinels

Data on the damping of ultrasonic waves in the vicinity of the point of inversion of the first anisotropy constant are reported for manganese-zinc spinel crystals grown by various methods. A low-frequency maximum in the ultrasonic wave damping caused by the magnetoelastic interaction in the region of a magnetic phase transition in the spinel is observed for the first time. A difference in the acoustic properties of crystals of various types observed in the region of the phase transition is explained by the difference of internal fields related to differences in the degree of macroscopic disorder and in the arrangement of Fe²⁺ and Fe³⁺ ions in the spinel crystal lattice.     

The hall cross in a perpendicular inhomogeneous magnetic field

Equations are derived which give the voltage that is generated in a Hall conductor when an inhomogeneous magnetic field is applied normal to the plane of the Hall conductor. These equations are used to solve a specific example of a Hall cross in such a field, using finite difference techniques. The results are shown to be in excellent agreement with measured data.     

A dynamic determination of the anisotropy field and the damping constant of thick magnetic films

A new dynamic method for the measurement of Hkand the damping constantTof thick magnetic films is presented from both the theoretical and practical standpoint. The method is based on a property of the differential equation (1), which has been found to be adequate if applied to the read cycle. It is found that, if the easy-axis sense signals obtained with linearly rising hard-axis fields of different rise times are plotted vs. the respective hard-axis fields, the maxima of the sense signals lie almost precisely on a straight line. This straight line cuts the field axis at Hk. From the slope of this line it is possible to deduceT, which is primarily the eddy-current time constant for thick films. The resistivity of the magnetic material can, in this case, be evaluated directly if the physical dimensions of the film are known. The influence of dispersion and skew is discussed and found to be reasonably small. The further advantages are that no instrumental integration is needed and that the reading is quite precise. Small variations of Hkcan, therefore, be easily recorded. Practical results obtained with thick magnetic films illustrate this method.     

Propagation features of plane waves of defect field across the interface boundary between viscoplastic media with arbitrary damping

Dynamic equations of the field theory of translational defects have been applied to the analysis of propagation of plane waves of defect field across the interface between viscoplastic media with arbitrary damping. Results have been obtained, which determine the reflection and refraction laws and the reflection and refraction coefficients in case of arbitrary viscosity value of the contacting media. Dependencies of the reflection and refraction coefficients on the incidence angle of primary waves have been calculated and analyzed.