Dipol-octopol interaction spin model for temperature-induced metamagnets

A two-sublattice Ising model is proposed with special dipol-octopol interaction. This special interaction term is not the even function of the spin sublattice operators. Thermodynamical properties of this model are investigated in the molecular field approximation. Our results agree satisfactory with experimental data for Mn₃GaC.     

Anomalies in Ising metamagnets

Ising metamagnets in a field with weak ferromagnetic intralayer interactions and highly coordinated antiferromagnetic interlayer couplings are studied using mean field theory and Monte Carlo simulations. In the antiferromagnetic phase anomalies in the magnetization and the specific heat are observed, reflecting the competing ordering tendencies of the external field and the interlayer couplings. Results are compared to recent experimental findings on FeBr₂.     

Molecular fields in chainlike metamagnets

A two-sublattice metamagnet is formed of ferromagnetic Ising chains which interact ferromagnetically and antiferromagnetically. The intrachain coupling is strong compared to the interchain ones, as is observed in CoTAC. We treat the chains exactly and describe interchain interactions with molecular fields. The Neel temperature and tricritical point differ qualitatively from conventional molecular field results.     

Magnetic domain formation in itinerant metamagnets

We examine the effects of long-range dipolar forces on metamagnetic transitions and generalize the theory of Condon domains to the case of an itinerant electron system undergoing a first-order metamagnetic transition. We demonstrate that, within a finite range of the applied field, dipolar interactions induce a spatial modulation of the magnetization in the form of stripes or bubbles. Our findings are consistent with recent observations in the bilayer ruthenate Sr(3)Ru(2)O(7).     

Phonon and magnon widths in metamagnets

Microscopic expressions for the couplings of the phonons corresponding to the rigid motions of the waters of hydration with both transverse and longitudinal spin fluctuations are given for the linear chain metamagnetic systems FeCl₂·2H₂O, CoCl₂·2H₂O, and CoBr₂·2H₂O. The processes which can give large, temperatures dependent widths of spin waves and of some phonons for temperatures approaching the Neél temperature are discussed.     

Variation of the critical behavior in metamagnets

From a study of a simple mean field model for metamagnets we predict that new types of critical behavior may be observed in FeBr₂ and other similar systems.     

Cylindrical domain structure in metamagnets and its stability conditions

The possibility of the existence of a cylindrical magnetic domain structure near the first-order phase transition line is investigated theoretically in metamagnets, compounds with anisotropy energy substantially exceeding the exchange interaction energy. The domain structure of such compounds is related to the kinetics of the transition from the paramagnetic to the antiferromagnetic state. The static properties of isolated cylindrical magnetic domains and their equilibrium lattices are studied using an energetic approach. It is shown that for a low-temperature metamagnetic phase transition domains of the existence of a cylindrical domain structure, lattices and isolated cylindrical magnetic domains, abut, on the domain of plane-parallel domain structure existence from both sides. The features of their behavior are determined as a function of the magnitude of the external magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 84–88, March, 1988.The author is grateful to Yu. I. Gorobets for supporting the research and for useful remarks, and also to D. A. Yablonskii and I. M. Vitebskii for fruitful discussions.     

Low-temperature quasi-adiabatic magnetization reversal of rare-earth Ising metamagnets

The magnetization and magnetically induced elastic strains of rare-earth Ising antiferromagnets DyAlO₃ and TbAlO₃ are shown to exhibit an unusual behavior associated with low-temperature metamagnetic phase transitions. As an external magnetic field is applied and then removed slowly, the state of the magnetic system in these compounds follows quite different paths on the H-T diagram. Small alternating-sign variations in the field magnitude cause the magnetic system to switch reversibly from one path to another, which is accompanied by sharp changes in the magnetization and elastic strains. The observed anomalies are shown to be due to the magnetization process being quasi-adiabatic in character in the compounds under study. This fact should be taken into account in interpreting the data on the magnetization and magnetostriction in Ising antiferromagnets undergoing metamagnetic transitions at low temperatures. Experimentally, quasi-adiabatic magnetization makes it possible to determine the critical fields for metamagnetic transitions very exactly and to investigate the H-T phase diagram at temperatures that are far below the minimum temperature of a helium bath and are unattainable under strictly isothermal conditions.     

A note on the field-induced phase transitions of dilute metamagnets

The magnetization processes and the magnetic-field-induced phase transitions of dilute two-sublattice and totally disordered metamagnets with mixed ferro- and antiferro-magnetic exchange interactions are discussed. It appears, in particular, that the minimum value of the tricritical concentration, at which a change of the phase transition from first to second order occurs, is identical to the critical concentration of magnetic atoms.     

Optimal orientation field to manufacture magnetostrictive composites with high magnetostrictive performance

Magnetostrictive properties have relationship with the applied orientation field during the preparation of giant magnetostrictive composites. To understand the dependence of the optimal orientation field on particle volume fraction, composites with 20%, 30% and 50% particles by volume were fabricated by distributing Terfenol-D particles in an unsaturated polyester resin under various orientation fields. Their magnetostrictive properties were tested without pre-stress at room temperature. The results indicate that as the particle volume fraction increases, the optimal orientation field increases. The main reason for this phenomenon is the packing density for the composites with higher particle volume fraction is larger than that for those with lower particle content.     

用于电流传感的声表面波磁致伸缩效应

利用声表面波(SAW)磁致伸缩效应可以实现一种快速、高灵敏度的电流检测方法,但磁致伸缩薄膜内部矫顽力导致了明显的磁滞误差。磁致伸缩薄膜的栅阵化设计可以减小磁致伸缩时薄膜内部矫顽力,抑制磁滞现象,从而实现高灵敏和低迟滞误差的SAW电流检测。结合有限元和耦合模理论对沉积铁钴(FeCo)薄膜栅阵的声表面波电流传感器中的磁致伸缩效应进行分析,对传感响应进行仿真,确定优化的传感结构参数。为验证理论分析,实验研制了频率为150 MHz的声表面波电流传感器件,并结合差分振荡电路及亥姆霍兹线圈,建立传感器测试系统.实验结果表明,磁致伸缩薄膜的栅阵设计大幅降低了迟滞误差,并显著提升了传感器灵敏度。     

A one-dimension coupled hysteresis model for giant magnetostrictive materials

This paper addresses the development of a one-dimension model for quantifying magnetic-elastic-thermal coupling and hysteresis inherent to giant magnetostrictive materials. Firstly, the anhysteretic law is modeled by considering the Gibbs free energy function G(σ, M, T), and thermodynamic relations are used to obtain the constitutive expressions. These expressions character the effects of coupling between stress, magnetization, and temperature in the giant magnetostrictive material but hysteresis, i.e. strain and magnetic intensity described by above the constitutive expressions are single-valued function of the magnetization. And then pinning is incorporated to describe hysteresis based on Jiles–Atherton model. The model considered in the paper is demonstrated valid by comparing the predicted results with experimental data. Moreover, the model proposed in the paper is convenient to be used in engineering applications since the parameters referred to the model have definite physical mean and can all be easily determined by experiments.     

磁致伸缩超声换能器阻抗匹配网络的设计

根据磁致伸缩超声换能器的等效电路模型和阻抗匹配理论,该文设计了一种优化的π型匹配网络.该网络具有调谐、变阻和滤波的功能,适用于工作在中低频超声频率范围内的磁致伸缩超声换能器系统的阻抗匹配.仿真和实验结果表明:接入该π型阻抗匹配网络后,驱动电源与换能器之间实现了最大功率传输,换能器两端电信号的波形质量得到优化,换能器激励...     

Anisotropy of constrained magnetostrictive materials

The magnetic anisotropy of a ferromagnetic material that is free to deform is defined as the effective anisotropy, which is the sum of intrinsic anisotropy and magnetostriction-induced anisotropy. Prior works [1] and [2] (Baltzer, 1957; Kittel, 1949) indicate that if the material is undeformed then the measured anisotropy is same as its intrinsic anisotropy. When magnetostrictive materials are used as actuators or sensors they are often mechanically loaded, resulting in a restriction on the deformation. To capture their behavior in such scenarios, a modelling approach is required. Therefore, in this work, the thermodynamic accuracy of the common expressions for magnetostriction-induced and stress-induced anisotropies is first investigated. A 3D magnetoelastic model is then developed using Armstrong's implementation of an energy model. This 3D magnetoelastic model is capable of predicting the stresses induced when the magnetostriction of these materials is constrained. Using this model, it is shown that when the bulk magnetostriction of the material is clamped, the measured anisotropy will not in general be the same as the intrinsic anisotropy. It is also shown that when the magnetostriction is clamped at the microscopic level, i.e. if the material is locally constrained at the exchange length scales, then the measured anisotropy is the intrinsic anisotropy.     

A nonlinear magneto-thermo-elastic coupled hysteretic constitutive model for magnetostrictive alloys

This paper presents a general hysteretic constitutive law of nonlinear magneto-thermo-elastic coupling for magnetostrictive alloys. The model considered here is thermodynamically motivated and based on the Gibbs free energy function. A nonlinear part of the elastic strain arising from magnetic domain rotation induced by the pre-stress is taken into account. Furthermore, the movement of the domain walls is incorporated to describe hysteresis based on Jiles–Atherton's model. Then a set of closed and analytical expressions of the constitutive law for the magnetostrictive rods and films are obtained, and the parameters appearing in the model can be determined by those measurable experiments in mechanics and physics. Comparing this model with other existing models in this field, the quantitative results show that the relationships obtained here are more effective to describe the effects of the pre-stress or in-plane residual stress and ambient temperature on the magnetization or the magnetostriction hysteresis loops.     

稀土超磁致伸缩材料的应用

本文重点介绍稀土超磁致伸缩材料在声学方面，特别是在水声换能器方面的应用。     

The phase diagram of simple metamagnets as determined by the cluster variation method

In order to explore the global properties of a simple Ising metamagnet we computed the values of the coupling parameters for which the tricritical behavior is replaced by bicritical endpoints (Lifshitz point). The transition points are determined by means of the cluster variation method. The metamagnets studied have an antiferromagnetic coupling between the spins on two chosen sublattices and a ferromagnetic coupling between spins on the same sublattice. The following lattices and sublatices were considered: two-dimensional square, simple cubic and two different subdivisions of the fcc and bcc lattices each. The method used is based on the coincidence of two roots for the bicritical endpoints and of three roots for the tricritical point. In contrast to the molecular field and the pair approximation results, the presence or absence of the Lifshitz point depends on the lattice structure considered. We discuss the comparison of our results with the results from the renormalization theories.     

Cantilevered actuator using magnetostrictive thin film

This paper describes a cantilevered magnetic actuator driven by magnetostriction in a low magnetic field. The dimensions of the two layers actuator were 1×5 mm and amorphous FeSiB was used as the magnetostrictive material. Since the FeSiB has excellent soft magnetic characteristics, the actuator with FeSiB was able to work in magnetic field strength of less than 10 kA/m. The theoretical formulas for the amount of the displacement and the force of the actuator were obtained. The theoretical results agreed with the experimental one. According to the theoretical formula, the displacement was calculated with the parameter of the mechanical properties of the substrate. To obtain the large displacement, the actuator with Co substrate was designed based on the theoretical formula. The displacement of 153 μm was obtained using Cu substrate of 1.1 μm thickness in the magnetic field of 10 kA/m. © 2008 Elsevier B.V. All rights reserved