The internal friction of torsional oscillations in ferromagnetic materials

The paper is a continuation of the first part [1] and deals with the internal friction of torsional oscillations of ferromagnetic materials in a static and an alternating magnetic field. The calculation differs considerably from the case of longitudinal oscillations, particularly in the following points. In the first place, the internal friction of torsional oscillations depends quite differently on the dimensions of the sample, and the continuous distribution of magnetic domains and Bloch walls cannot be so easily defined. Secondly, a magnetic field created as a result of eddy currents during torsional oscillations does not penetrate the surroundings, so that the internal friction in an electrically conducting medium is the same as in vacuum. Thirdly, the deformation here is an antisymmetrical function of the field, so that the coefficient is an even function while with longitudinal oscillations it was expressed by an odd function. Despite these different conditions the results are very similar to those with longitudinal oscillations and agree well with experiment. In an alternating magnetic field the internal friction of torsional oscillations has a sharp maximum atH=0.64H _S whereH _S is the saturated value of the field for which magnetoelastic effects disappear.

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[1] . , , . . -, , , , , , . , , , . , , , . , H=0,64H _S, H _S , .

     

Diffusion model of internal friction in nanocrystalline materials

The stress normal and tangenital components arise in grain-boundary segments differently oriented with respect to an external periodic load, causing the fluxes of vacancies and impurity atoms between neighboring segments. By solving the diffusion problem, one can find the velocity of mutual displacement of grains, the stress distribution in the segments with allowance for stress adjustment, and the amount of internal friction. The frequency dependence of the internal friction shows peaks associated with the redistribution of impurity atoms over the segments, grain sliding, and a high-temperature background.     

Measuring mechanical properties of ferromagnetic materials by internal induction

The voltage which is generated during the reversal of the magnetization inside the ferromagnetic specimen itself is measured. This internally produced voltage is sensitive to external forces and internal stresses. The method is compared with the application of strain gages.     

半金属磁性材料研究进展

半金属材料是一种新型的功能自旋电子学材料，是一种具有特殊能带结构的物质，近年来13益受到人们的关注。半金属材料从微观上具有导体和绝缘体双重性质：对一种自旋取向的电子其能带结构呈现金属性，而另一自旋取向的电子其能带结构呈现绝缘体性．文章着重对Half—Heusler结构半金属材料、CrO2铁磁半金属、Fe3O4亚铁磁材料半金属、反铁磁材料半金属和钙钛矿及双钙钛矿半金属的结构特性进行分析和综述，并对半金属材料的应用原理和应用前景作了阐述。     

The internal friction of iron whiskers

In this paper of the measurements internal friction of iron whiskers are described; a torsion pendulum was especially adapted for this purpose. It was found that the internal friction of whiskers having larger diameter than 10 m was usually amplitude dependent. This amplitude dependence approximatively fits the Granato-Lücke theory of dislocation damping.The effects of plastic deformation and its recovery in iron whiskers were also studied. The results and their discussion lead to the assumption that during the recovery an important role is played either by the interaction of dislocations with vacancies, or by the interaction of dislocation dipoles with the impurity atoms.     

Magnetic monopoles in ferromagnetic materials

The interactions of slow $\text{(10}{}^{\mathrm{?5}}\text{? v/c ? 10}{}^{\mathrm{?2}}\text{)}$ magnetic monopoles with ferromagnetic materials are studied. The spin-flip cross section σ and the energy loss dE/dx are calculated for magnetic monopoles impinging parallel to the magnetization direction. In iron, these reach a maximum at v/c ～ 3 × 10^?4, where they take the value of 100 Ų and 100 MeV/cm respectively. The electromagnetic signal of a monopole passing through a ferromagnet and the generation of spin waves are discussed.     

Critical current oscillations in strong ferromagnetic pi junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni, and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with a barrier thickness implying repeated 0-pi phase transitions in the superconducting order parameter. We show, in particular, that the Co barrier devices can be accurately modeled using existing clean limit theories and that, despite the high exchange energy (309 meV), the large IcR(N) value in the pi state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.     

Plasma oscillations and dynamical friction

The forceF of mutual interaction of two species of charged particles in relative motion with velocityU is, forU higher than the mean thermal velocities of particles, a decreasing function ofU. This property ofF(U) can give rise to the excitation of electrostatic oscillations.

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F, copta , U, U, , , U. F(U) .

     

Damping of longitudinal plasma oscillations

Attenuation of electron oscillations in a fully ionized plasma is investigated by solving linearized kinetic equation without external fields. The general dispersion relation for longitudinal plasma oscillations is obtained using the BGK model. Damping due to electron ion collisions is obtained with a correction term. It is also observed that damping rate decreases ask increases, which is in agreement with McBride.     

Possible nature of oscillations in the time dependences of the internal friction and effective shear modulus in Al-Cu alloys

The conditions of the appearance of in-phase oscillations in the time dependences of the low-frequency internal friction and effective shear modulus G _eff in Al-0.01 wt% Cu and Al-0.001 wt % Cu alloys are studied. The in-phase oscillations are shown to appear in the alloys only when their dislocation-impurity structure is disturbed from equilibrium and the impurity concentration near a dislocation is optimum. This fact suggests that the in-phase oscillations are due to a collective character of dislocation-impurity interactions, which leads to a transfer of energy of translational dislocation motion to transverse dislocation vibrations in the main slip plane.     

Thermal diffusion of hydrogen in ferromagnetic materials

The thermal diffusion of hydrogen dissolved in a ferromagnetic material is investigated. It is shown that the coefficient of thermal diffusion changes when the material changes into the ferromagnetic state. The possibility of a change in the sign of the thermal diffusion flow when passing through the Curie point is discussed.     

Mechanism of optical absorption in ferromagnetic materials

The optical spectrum is calculated for the nondiagonal component of the permittivity tensor of a ferromagnet.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Nonlinear longitudinal oscillations of relativistic plasma

The investigation has been made of nonlinear forced longitudinal oscillations of a relativistic plasma. It was demonstrated that: a) the nonlinear terms (v▽)p arising in the equations of motion do not restrict the electron oscillation amplitude increase in the region of plasma resonance; b) the nonlinear terms due to the relativistic nature of oscillations, firstly, limit the oscillation amplitude in the resonance region and, secondly, lead to the development of a parametric instability. The harmonic, sub- and ultraharmonic oscillations turned out to be unstable. The conditions for the instability as well as expressions for the growth rates of unstable oscillations were obtained.     

Relativisitic longitudinal non-Abelian oscillations in quark-antiquark plasma

We study the relativistic version of the non-Abelian, longitudinal wave in quark-antiquark plasma reported earlier by Bhat et al [Phys. Rev. D39, 649 (1989)]. We have also relaxed various approximations they made in their analysis. Both the quark and antiquark dynamics are taken in our analysis. The non-linearity arising from non-Abelian field as well as from plasma are included. Hence it is an exact longitudinal mode in relativistic quark-antiquark plasma, relevant to the study of quark gluon plasma. We find that earlier results are reproduced for non-relativistic and low amplitude oscillations, but are modified for relativistic or large amplitude waves. Further more, the above results are based on just four first-order equations for gauge invariant quantities derived from gauge covariant twelve first-order equations.     

Magneto-optical properties of metallic ferromagnetic materials

The authors have studied the magneto-optical Kerr rotation in more than 200 metallic systems comprising alloys as well as intermetallic compounds of 3d transition metals. For all these materials the crystal structure, the lattice constants, the room temperature magnetization and the room temperature Kerr rotation at two different wavelengths are specified. For several series of ternary compounds, comprising Heusler alloys Ni₂ In-type compounds and Cr₂₃C₆ type compounds, we determined the saturation moment at 4.2 K. For a number of representative alloys or compounds a study was also made of the wavelength dependence of the complex polar Kerr effect. The values of the Kerr rotation obtained at 633 nm were compared with the corresponding values of the measured magnetization. Systematic trends were observed and have been used to classify metallic systems into systems where the Kerr rotation will not reach values much in excess of 1° and systems where higher values are likely to be found.     

Magnetostriction of weak easy-plane ferromagnetic materials

A relationship determining the magnitude of magnetostriction in a given direction and its constant along the [100] easy direction are obtained with the use of the thermodynamic potential for a weak easy-plane ferromagnetic material based on calculations of the spontaneous components of the crystal strain tensor in terms of its elastic and magnetoelastic constants. An expression determining the specific compressibility of the crystal through its elastic constants is derived.