Magnetoelasticity,rotational invariance and magnetoacoustic birefringence in CeAl2

A thorough experimental investigation of elastic and magnetoelastic properties for CeAl₂ is given. The temperature dependence of the various elastic modes is accounted for with a magnetoelastic coupling of the strain components to crystal field split magnetic ions. Large magnetic field effects for thec ₄₄-mode in various geometries are observed. The rotational invariant magnetoelastic interaction is deduced from the experiment and quantitatively accounted for. Magnetoacoustic analoga of the Faraday and Cotton-Mouton-Voigt effect are observed for the first time in the paramagnetic phase.     

Effect of electric current on the magnetic and magnetoelastic properties of amorphous ferromagnetic alloys

The magnetic and magnetoelastic parameters of Fe₆₄Co₂₁B₁₅ and Fe_81.5B_13.5Si₃C₂ amorphous ferromagnetic alloys treated by direct electric current in air are studied as functions of the applied magnetic field and current density. The samples of the alloy have the form of narrow strips with different lengths. It is found that the magnetoelastic parameters of the dc-treated alloys depend on the magnetic field in a qualitatively different way. From the behavior of the magnetic and magnetoelastic parameters of the alloys in the magnetic field a model of magnetization nonuniform distribution in amorphous ferromagnetic alloys subjected to dc treatment is proposed.     

Micromagnetic Description of the ΔE-Effect in Amorphous Ferromagnets

Dependences of the magnitude of the E-effect in Fe_81.5B_13.5Si₃C₂ amorphous metal alloys on the external magnetic field and the temperature of thermomagnetic treatment are investigated. Four characteristic regions are distinguished in the dependence of the E-effect on the magnetic field strength. The experimental results are explained in the context of micromagnetism theory.     

Investigation of the parametric excitation and relaxation of phonons in antiferromagnetic α-Fe2O3

Parametric excitation of magnetoelastic waves was investigated in the easy-plane antiferromagnet α-Fe₂O₃ by parallel and perpendicular microwave pumping over a wide range of frequencies, magnetic fields, and temperatures, and the parametric resonance thresholds were measured. The frequencies of the natural magnetoelastic vibrations of the sample were investigated as a function of the magnetic field and temperature. The results of the measurements were used to calculate the parameters of the magnetoelastic wave spectrum and the rate of relaxation of the excited quasi-phonons. Possible mechanisms for quasi-phonon damping were analyzed.     

Peculiarities in the magnetic,magnetoelectric, and magnetoelastic properties of SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

Results of a complex investigation of the magnetic, magnetoelectric, and magnetoelastic properties of a SmFe₃(BO₃)₄ single crystal are presented. Samarium iron borate is similar to another easy-plane iron borate, NdFe₃(BO₃)₄, in that it possesses a large value of the magnetic-field-induced polarization (about 500 μC/m²), the sign of which changes when the field direction is changed between axes a and b of the crystal. However, the temperature dependence of the magnetic susceptibility and the field dependence of polarization and magnetostriction of the two compounds are significantly different, which is explained by the weak effect of external magnetic field on the ground-state multiplet of samarium ion, which is characterized by an extremely small value of its g-factor.     

Crystal field effect in the thermal expansion of hexagonal rare earth compounds

The low temperature thermal expansion in hexagonal rare earth materials exhibits crystal field effects. These are quantitatively explained with a magnetoelastic coupling of Γ₁-symmetry strains to a 1=2 quadrupolar operator of the magnetic ion. For PrNi₅ the temperature dependence of both c- and a-axis thermal expansion is correctly accounted for. For dhcp Pr both the cubic and the hexagonal sites of the Pr-ions contribute to the thermal expansion. The magnetoelastic coupling constants are an order of magnitude larger than for cubic compounds.     

Effect of heat treatment on the magnetic and magnetoelastic properties of cobalt ferrite

The influence of different heat treatments on the magnetic and magnetoelastic properties of highly magnetostrictive CoFe₂O₄ has been investigated. The first order cubic anisotropy coefficient, coercive field, magnetostriction and high strain sensitivity were observed to decrease as the heat treatment temperature increased. The saturation magnetization of the samples on the other hand increased with increase in heat treatment temperature. These changes were not accompanied by any observable changes in crystal structure or composition and are indicative of migration of Co²⁺ from the octahedral sites (B-sites) to the tetrahedral sites (A-sites) and Fe³⁺ from the A-sites to the B-sites of the spinel structure. Different distributions of the cations at the two distinct lattice sites can strongly affect the magnetic and magnetoelastic properties of these materials.     

4f-electron-phonon coupling and lattice dynamics in the ferromagnet LiTbF4

The polarized Raman spectra of LiTbF₄ have been investigated at several temperaturesT150 K, and the temperature dependence of the selfenergies of optical phonons due to the magnetoelastic interaction have been determined. The sizes of these temperature shifts, of the magnetic phonon splittings and of the level repellings between resonant optical phonons and crystal field states have been calculated using the theory of magnetoelastic interaction by Fulde and Thalmeier without any adjustable parameters, considering the modulation of the crystalline electric field by the optical phonons. The phonon modes of LiTbF₄ have been determined in a rigid-ion approximation. Reasonable to good agreement with the experimental results has been obtained for most of the phonon modes.     

Mössbauer and magnetic study of mechanical alloying of Fe3Si

Using Mössbauer spectroscopy and measurement of hysteresis loops and thermomagnetic curves, phase composition and magnetic parameters of Fe₃Si mechanically alloyed powders were studied in dependence on milling time and subsequent heat treatment at a thermomagnetic experiment. Samples of as-prepared powders show high value of coercivity, the saturation magnetization and the content of amorphous Fe₃Si phase raise with increasing time of milling, the content of α-Fe diminishes. Heat treatment of samples with long enough milling time can produce almost perfect Fe₃Si alloy.     

Optical phonons of Rare-Earth halides in a magnetic field

A theory is developed which explains the observed magnetic field splitting of doubly degenerateE-phonons in Rare Earth Chlorides. It is based on first and second order magnetoelastic interactions. Quantitative calculations are done for CeCl₃ for which the field dependence of the splitting is derived and estimates of the coupling constants are given.Furthermore we propose a mechanism for the observed reduction of the phononlinewidth in a magnetic field.     

Magnetic investigations of Dy(Fe0.9M0.1)2 (M=B,Al, Ga) cubic laves phases

The magnetic properties of the pseudo-binary compounds Dy(Fe_0.9M_0.1)₂ (M=B, Al, Ga) have been investigated. Substitution of M atoms for Fe atoms decreases the Curie temperature and varies the magnetic moment of the Fe atom. The coercivity of the samples increases very rapidly with decreasing temperature. This increase is considerably enhanced by the substitution elements. Pronounced thermomagnetic history effects are observed in the temperature dependence of the magnetization. These effects may be explained as an exponential increase in magnetic hardness on lowering the temperature.     

Magnetization studies in niobium and YBa2Cu3O7 samples

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H _max to +H _max) and reverse (+H _max to −H _max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH _peak,H _I,H _II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M _rem(H)−M _FC(H)+M _ZFC(H)) being equal to, less than or greater than zero, whereM _rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.     

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMnsGe2（R=Sm,Gd）

The temperature dependence of lattice constants a and c of intermetallic compounds RMn₂Ge₂ (R=Sm, Gd) is measured in the temperature range 10－800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism－ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10^-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn－Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn－Mn exchange interaction.     

赝二元化合物Dy(Fe0.9M0.1)2(M=B,Al,Ga)的磁性研究

研究了赝二元化合物Dy(Fe_0.9M_0.1)₂(M=B,Al,Ga)的磁性。M对Fe的置换,使居里温度下降,Fe原子的磁矩改变。矫顽力随温度的降低,增加很快,增加的幅度与M有关。磁化强度的温度关系,在磁场小于临界值时,出现反常和热磁效应。用Fe原子局域环境的变迁和低温下内禀磁硬度的急剧增加,解释了上述现象。 关键词：     

Thermal-expansion and magnetostriction anomalies in phase transitions in La1−x SrxMnO3

A study is reported on phase transitions in the La_1?x Sr_xMnO₃ system, both spontaneous and induced by a pulsed magnetic field of up to 250 kOe, accompanied by anomalies in magnetoelastic properties. The temperatures of the polaron (charge) and magnetic ordering, as well as those of structural transitions, are observed to be in good agreement with the results obtained by other methods. Jumps in the field dependence of longitudinal and transverse magnetostriction associated with field-induced orbital ordering have been found. A strong temperature dependence of the corresponding threshold fields is observed.     

Effects of a magnetic field on a modulated phase

The effects of a magnetic field on a modulated phase are studied. A modulated phase is found to have two critical fields H₁ and H₂. For a large enough magnetic field, H₁ and H₂ can be approximated by a linear law. As a result, the minimum magnetic field needed to destroy a modulated phase is a constant. The minimum magnetic field also greatly depends on the order of a commensurate phase. A very high order commensurate phase and an incommensurate phase cannot survive a magnetic field. The behaviour of a magnetoelastic chain in a magnetic field can be described by a harmless devil's staircase. The inverse temperature is found to play a role similar to that of a special magnetic field. The deeper physics underlying these new phenomena is the breaking of the left-right symmetry of a phase diagram. As a result a controllable path to a modulated phase is found.     

Electric and magnetic properties of the synthetic metal-copper subrodanide Cu(SCN)13

The temperature dependence of the resistance and magnetic susceptibility of the copper subrodanide Cu(SCN)_{$\text{1}\text{3}$} samples were measured at the magnetic field up to 7.3 kG and the temperature range 4.2–400 K. The resistance of the pressed pellet samples linearly increases with the temperature increasing and at 100–150 K the ?(T) line slope smoothly changes. The magnetic susceptibility at zero field linearly decreases with decreasing of the temperature, showing unusual exponential field dependence, which is rather strong at the room temperature and disappears at T → 0. It was concluded that a copper subrodanide is a synthetic metal.     

Parametric excitation of magnetoelastic vibrations in hematite single crystals in weak magnetic fields

This paper reports on the results of investigations into the magnetoelastic vibrations excited by radio pulses in thin single-crystal hematite plates according to the scheme of parallel parametric pumping. It is demonstrated that this technique can be used to examine the multimode spectrum of natural vibrations of crystals in weak external magnetic fields H₀ < 200 Oe and to measure some acoustical quantities in this range. By using the parametric amplification of sound, it is established that an increase in the magnetic field is accompanied by the appearance of weakly damped low-intensity modes in the spectrum of natural vibrations.     

Temperature anomalies in the nonlinear magnetoacoustic properties of manganese-zinc spinel single crystals

The absorption of a magnetoelastic wave and the nonlinear resonance interaction of counterpropagating magnetoelastic waves in a manganese-zinc spinel single crystal have been investigated near the conjectured spin-reorientational transition. The convolution signal vanishes near T _tr=291 K; this is explained by the relaxational dynamics of the magnetic mode near the transition. The increase in the strength of the field corresponding to the maximum of the convolution with decreasing temperature is explained by a change in the demagnetizing field. Fiz. Tverd. Tela (St. Petersburg) 39, 652–655 (April 1997)