Magnetoelectric and magnetoelastic interactions in NdFe3(BO3)4 multiferroics

Complex experimental and theoretical investigations of the magnetic, magnetoelectric, and magnetoelastic properties of neodymium iron borate NdFe₃(BO₃)₄ along various crystallographic directions have been carried out in strong pulsed magnetic fields up to 230 kOe in a temperature range of 4.2–50 K. It has been found that neodymium iron borate, as well as gadolinium iron borate, is a multiferroic. It has a much larger (above 300 μC/m²) electric polarization controlled by the magnetic field and giant quadratic magnetoelectric effect. The exchange field between the rare-earth and iron subsystems (～50 kOe) has been determined for the first time from experimental data. The theoretical analysis based on the magnetic symmetry and quantum properties of the Nd ion in the crystal provides an explanation of the unusual behavior of the magnetoelectric and magnetoelastic properties of neodymium iron borate in strong magnetic fields and correlation observed between them.     

Magnetoelastic effects in multiferroic YMnO3

We have investigated magnetoelastic effects in multiferroic YMnO(3) below the antiferromagnetic phase transition, T(N) ≈ 70 K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO(3) decreases normally above T(N), but decreases anomalously below T(N), whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T(N). The unit cell volume also undergoes an anomalous contraction below T(N). By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO(3) couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.     

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.     

Elastic properties of magnetic materials

This review discusses the theoretical aspects of magnetoelastic coupling with emphasis on the magnetic perturbation of elastic properties. The basic theory of magnetostriction is set out with application to ferromagnets, ferrimagnets and antiferromagnets, and is followed by a discussion of the physical origin of the magnetoelastic coupling coefficients in both localized and itinerant magnetic systems. Magnetic contributions to elastic compliance are then discussed and sound velocity anomalies near magnetic phase transitions investigated, including the cooperative Jahn-Teller limit for which the acoustic mode itself drives a structural transition even when magnetic ordering does not occur. The review concludes with discussion of magnetoelastic (or mixed magnon-phonon) waves in low temperature magnetically ordered phases and with a study of local striction phenomena in magnetically dilute materials. The latter leads to a recognition of internal rearrangement modes which may also be present in concentrated magnetic systems, and which may or may not couple significantly to bulk homogenous strain.     

Stabilization of the long-range magnetic ordering by dipolar and magnetoelastic interactions in two-dimensional ferromagnets

The possibility of the long-range magnetic order stabilization in two-dimensional ferromagnets with the account of dipolar and magnetoelastic interactions is investigated. The mechanisms of the magnetic order stabilization by both types of interactions are studied. The Curie temperature is estimated. The comparisons with experimental data are made. Received 22 June 2001 and Received in final form 17 January 2002     

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.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Single crystal elastic constants and magnetoelasticity of holmium from 4·2 to 300 K

The five independent elastic coefficients of holmium single crystals have been determined by means of an ultrasonic pulse technique at a frequency of 10 MHz, between 4·2 and 300 K. From the elastic constants the temperature variation of the directional adiabatic compressibilities, the limiting Debye temperature and the elastic anisotropy ratio were calculated. The elastic coefficients exhibit anomalies at the magnetic ordering transitions known to occur in holmium. Anomalous behavior in the elastic constants was also observed at about 80 K. The limiting value of the Debye temperature was found to be 191·5 K. The present measurements of the elastic constants, and the reported magnetostriction and thermal expansion data, enabled the calculation of the magnetoelastic contribution to the total Hamiltonian of holmium in the magnetically ordered states. A very small discontinuity in the temperature dependence of the magnetoelastic energy was observed at the Curie point of holmium. Below the Neel temperature, the magnetoelastic energy varies smoothly with decreasing temperature, attaining a value of ?2·13 J cm^?3 at liquid helium temperature. The temperature dependence of the magnetoelastic energy in the vicinity of the Curie point in holmium suggests that the magnetic transition from the antiferromagnetic arrangement into the ferromagnetic state is of second order.     

Magnetization process in holmium: easy axis spin reorientation induced by the magnetostrictive basal plane distortion

We report on the change of the easy axis direction in holmium, from the a to the b axis, under the application of a magnetic field in the basal plane. This spin reorientation is observed by measuring the magnetic torque in Ho(n)/Lu(15) superlattices (n and 15 are the number of atomic planes in the Ho and Lu blocks). We also observe that, at the field H0 and temperature at which the reorientation occurs, both axes are easy directions. Based on the fact that the field H0 depends on n in the same way as the field-induced magnetoelastic distortion does, we propose that this spin reorientation originates from the strong field-induced magnetoelastic deformation within the basal plane. The modulation of the alpha strains with sixfold symmetry originates a 12-fold term in the magnetic anisotropy energy.     

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.     

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.     

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)     

Transient magnetostriction in ferro- and ferri-magnetic materials

An acoustic impulse excited by a laser pulse generates some transient magnetostriction effects in various ferromagnetic and ferrimagnetic materials. A crystal of small magnetic anisotropy energy gives a steady magnetoelastic oscillation which is resonant with the sample dimension. The other materials show the surface effect only. The temperature dependence is studied up to the Curie temperature. Around the phase transition point the long-wavelength fluctuation of magnetization is excited by the elastic impulse, and some critical phenomena are observed. These effects are discussed by the usual theory of magnetoelastic interaction.     

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.     

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.     

Crossover of polaron conductivity and the inhomogeneous state of lanthanum manganites in the magnetic phase transition region

The transport properties of lanthanum manganites over a wide range of temperatures below the magnetic phase transition point are discussed within the model of a two-phase composite whose phases differ in the magnetic order and charge carrier concentration. The volume ratio of the phases depends on the temperature and the magnetic field. The magnetoelastic polarons are charge carriers in both phases, and the metal-dielectric transition occurs as a percolation transition accompanied by the crossover of the polaron conductivity. The results obtained by numerical simulation of the resistivity, magnetoresistance, and thermopower are compared with the experimental data for La_0.7Mn_1.3O_3?δ thin films. The theoretical and experimental data are in good agreement.     

Magnetic exchange driven magnetoelastic properties in GdCu2

The magnetoelastic properties of GdCu₂ have been investigated by thermal expansion and magnetostriction measurements. GdCu₂ orders antiferromagnetically with a noncollinear magnetic structure. The anisotropic magnetostriction is of similar magnitude as in other RCu₂ compounds and can be explained by a contribution of the bilinear exchange interaction to the magnetoelastic energy. For several compounds this contribution is as important as the single ion magnetoelastic exchange. The pressure dependence of the Néel temperature of GdCu₂ is found to be in agreement with the data of thermal expansion.     

磁致伸缩材料磁弹性内耗的场依赖特性及其用于磁场传感研究

分析和测试了磁致伸缩材料磁弹性内耗的偏置磁场依赖特性,发现Terfenol-D的品质因数(与内耗的量值成反比)强依赖于偏置磁场.利用磁致伸缩材料磁弹性内耗强依赖于偏置磁场的特性,提出了一种静态和准静态磁场的磁传感器方法,即将磁致伸缩材料与压电变压器单元层叠构建一种复合变压器.分析表明:在谐振状态下,复合变压器的输出电压正比于其品质因数,于是复合变压器的输出电压强依赖于偏置磁场;磁致伸缩材料的ΔE效应对复合变压器输出电压的影响很小.制备Terfenol-D/PZT8复合变压器进行了实验,结果表明,在近谐振状态下,当激励输入电压振幅为0.5V时,复合变压器工作的输出电压对静态磁场的灵敏度达到～5.12mV.Oe-1.