Magneto-thermo-mechanical characterization of giant magnetostrictive materials

The variations of magnetization and magnetostriction with temperature and stress were investigated through the analysis of the effective field,induced by temperature and stress.A nonlinear magnetostrictive model of giant magnetostrictive materials was proposed.The proposed model can be used to calculate the magnetostrictive characterization of giant magnetostrictive materials in different temperatures and under different stresses.The coupling effects of axial stress,magnetic field,and temperature on the magnetostriction of a Terfenol-D rod were numerically simulated as well as experimentally tested.Comparison between the calculating and experimental results shows that the proposed model can better describe the magneto-thermo-mechanical characteristics of Terfenol-D rod under different temperatures and compressive stress.Therefore,the proposed model possesses an important significance for the design of magnetostrictive devices.     

Effect of the shape anisotropy and configurational anisotropy on the magnetic structure of ferromagnetic nanodots

Processes of magnetization reversal of ordered square arrays of “nanodots” of different geometry coupled by dipole-dipole interaction that were formed by a focused beam of Ga⁺ ions from continuous polycrystalline Co films have been investigated. It has been established that the magnetic structure upon the magnetization reversal is mainly determined by the shape anisotropy of the nanodots. The round and square nanodots undergo magnetization reversal via a magnetic vortex, whereas the rectangular nanodots demonstrate a finite set of single-domain states depending on the orientations of the external magnetic field and easy axis of magnetization. It has been shown that the magnitude of the switching field of arrays of round and square nanodots and the distribution of switching fields in the system of rectangular nanodots upon magnetization reversal along the easy axis is substantially affected by the configurational anisotropy.     

超磁致伸缩复合材料的静态弹性模量及抗压强度

采用粘结技术制备出了性能良好的磁致伸缩复合材料，着重研究了合金含量、粉末粒度、磁场取向、粉末表面处理对样品的静态弹性模量及抗雎强度的影响规律。结果表明，粉末粒度、粉末表面处理对复合材料的抗压强度及静态弹性模量均有较大的影响；合金含量、磁场取向对复合材料的静态弹性模量影响不大，其中合金含量对复合材料的抗压强度有较大的影响。     

Thermodynamics of a helicoidal magnetic structure in a spin-wave approximation

Specific features of the heat capacity, magnetization, and magnetic susceptibility of a chiral magnet caused by the magnon contribution have been studied. It has been shown that the local heat capacity and the magnetization are functions of the coordinates and that their behavior depends on the magnitude of an applied magnetic field. The temperature dependences of these quantities have been investigated. The results of the approximate analytical calculations and numerical calculations have been compared between themselves.     

Dynamics of the evolution of a magnetization-reversal nucleus in orthorhombic antiferromagnets with a weak ferromagnetism

Dynamics of a two-dimensional nucleus of magnetization reversal in a magnetic field in an orthorhombic antiferromagnet with a weak ferromagnetism has been investigated analytically and numerically. It is shown that along the nucleus of the stable phase inside the metastable phase there can propagate both slow periodic and rapid periodic and solitary waves of magnetization. It is shown that the wave that describes the intergrowth of a domain of the reverse magnetization can have a velocity greater than the maximum velocity of the stationary motion of the domain wall, equal to the minimum phase velocity of spin waves.     

磁性材料在磁场中凝固取向的临界条件研究

抑制凝固过程中熔体内部的湍流是获得沿易轴取向晶体组织的关键。在凝固过程中，熔体内部的对流状态主要受冷却速率影响，修正的瑞利数被用来描述不同冷却速率下的熔体湍流程度。当反映熔体紊乱程度的瑞利数小于磁场作用下的临界瑞利数时，熔体的内部扰动被磁场抑制，生长中的晶体将克服热无序作用平行于磁场沿易磁化轴取向。修正的瑞利数和临界瑞利数接近时的凝固和磁场参数可作为磁性材料晶体生长取向控制的临界条件。     

Vibrating ferromagnets in a magnetic field

With the vibrating reed technique we have studied the magnetoelastic behavior and the dynamical response of amorphous and crystalline magnets in the ferromagnetic and re-entrant spin glass states. The measurements have been performed as a function of magnetic field, temperature and angle between the sample’s main area and the applied field. The resonance frequency of the reed is influenced by a static contribution (pole effect), a dynamic contribution with a large effect due to the pinning of magnetic domains and magnetostrictive effects. When the field is applied perpendicular to the main area of the sample, a large decrease of the resonance frequency is observed which is rather independent of the material characteristics and is not related to a change of the Young modulus. A field misalignment of a few degrees strongly affects the field dependence of the resonance frequency. We briefly discuss the influence of the re-entrant spin glass state on the magnetoelastic behavior and the characteristics of the internal friction of vibrating ferromagnets in a magnetic field.     

Investigation of the structure and properties of the material of various zones of the welded joint of the austenitic nitrogen-containing steel upon elastoplastic deformation

The structural, mechanical, and magnetic properties of metal cut out from the welded joint and from the near-weld zone of the welded joint of high-strength nitrogen-containing 04Kh20N6G11M2AFB austenitic steel have been investigated. The behavior of the magnetic parameters of materials under study subjected to various schemes of loading, such as tension, torsion, internal pressure, and combination of tension and torsion have been investigated. It has been established that the metal of the welded joint and near-weld zone of the welded joint, just as the base metal, has a stable phase composition and magnetic properties under various loading conditions. It has been concluded that 04Kh20N6G11M2AFB steel can be used in the fabrication of welded parts and elements of welded constructions that require low magnetization and high stability of magnetic characteristics under the force action.     

Ground state of the magnetization of nanowires

Analytical and numerical methods have been used to study the ground state of magnetization of a nanowire consisting of ferromagnetic crystallites that are coupled via exchange interactions. The random nature of the field of crystallographic anisotropy has been taken into account. It is shown that the magnetization of a nanowire is divided into weakly interacting regions of magnetization called blocks. Such characteristics, as the average size of a magnetic block, its effective anisotropy constant, and their dependence on the size of crystallites have been calculated analytically and simulated numerically in a two-angle approximation. The coordinate dependences of the magnetization dispersion at the edge and in the bulk of the magnet have been determined. It is shown that the functional dependence of the dispersion on the coordinate changes with moving away from the free edge of the nanowire.     

Resonance modes of two-layer exchange-coupled ferromagnetic film

Ferromagnetic and spin-wave resonances in two-layer exchange-coupled ferromagnetic films have been investigated numerically at different intensities of a magnetic field when it is directed in parallel or perpendicular to the film plane. Layers of the film have finite thicknesses and possess anisotropy of the easy-plane and easyaxis types. It has been shown that at a nonzero parameter of interlayer exchange coupling the dynamic component of magnetization upon ferromagnetic resonance is distributed nonuniformly across the film thickness. Its change has been described when the external magnetic field decreases from the saturating field to zero.     

On the frequency dependence of magnetic losses for rotational magnetization reversal of Fe-3% Si single crystals

The frequency dependence of magnetic losses for the rotational magnetization reversal has been studied on Fe-3% Si single crystals in the range of frequencies of 20–250 Hz and amplitudes of induction of 0.25–1.8 T. The separation of the total magnetic losses into components has been carried out. It has been shown that the dependence of the total and eddy-current losses in the cycle of magnetization reversal on the frequency of the rotation field has a nonlinear character at all inductions investigated. The specific features of the frequency dependence of the magnetic losses of single crystals observed in the work are qualitatively explained by the dynamics of their domain structure.     

Three-dimensional constitutive equations of ferromagnetic materials with magnetoelastic coupling: Determination of elastic coefficients under magnetic field

This paper determines the elastic coefficients in the constitutive equations of typical ferromagnetic materials. The constitutive equations are given by rate-type equations for the magnetization and the strain expressed in terms of the magnetic field and the stress. The authors have shown that the constitutive equations can describe the stress-dependent hysteretic magnetization curve, the stress magnetization effects, the stress-dependent magnetostriction curve, etc. They have determined the coefficients except the elastic coefficients as functions of the magnetization and the stress for typical ferromagnetic materials. In this paper, we attempt to obtain the elastic coefficients depending on the magnetization and the stress. To do it, applying the magnetoacoustic effect, we measure the stress–strain curves and the speeds of the longitudinal and the transverse ultrasonic waves under the magnetic field.     

Magnetic dichroism in X-ray photoemission

Dependences of the photoemission spectra of magnetic materials on the degree of circular and linear polarization of the exciting radiation, direction of magnetic moments, and escape angle of photoelectrons have been investigated. Using a simple model, analytical formulas have been obtained for the integrated intensity of the photoemission line as functions of the binding energy of inner levels, exchange splitting, and frequency of the X-ray radiation. The magnitude and the angular distribution of the dichroic signals obtained by the inversion of magnetization, by its rotation through 90°, and by the change in the helicity of the exciting radiation have been determined. It is shown that the dichroism of photoemission makes it possible to study not only ferromagnetic materials, but also antiferromagnetic structures.     

Magnetization reversal of a double-layer exchange-coupled ferromagnetic film

A theoretical investigation of magnetization reversal of a double-layer magnetic film with an easy-plane and easy-axis anisotropy has been performed. An analytical expression for the magnetization distribution in the layers in the presence of a magnetic field has been obtained. The calculations performed on the basis of the analytical relationships obtained show that when the thickness of the layers is close to the width of a magnetic inhomogeneity, it substantially affects the magnitude of the critical field for the magnetization reversal of the sample and the shape of the magnetization curve. It is also shown that a decrease in the exchange coupling between the layers leads to an increase in the magnitude of the critical field of magnetization reversal.     

Processes of the inhomogeneous magnetization reversal of cubic ferromagnets with defects

Within the framework of a phenomenological approach, processes of the magnetization of finite ferromagnets containing defects have been studied theoretically. The calculations are based on a model representation of magnetic inhomogeneities nucleating at defects as a magnetization distribution which corresponds to a 0° domain wall (DW). The region of stability of a 0° DW has been established in terms of the material parameters of a sample, characteristics of defects, and applied magnetic field. It is shown that the coercive force of the sample which is determined from an analysis of the critical fields of existence of a 0° DW agrees sufficiently well with the available models.     

Magnetic properties of Fe75Si10B15 amorphous metallic wires

Effects of the temperature of a preliminary treatment, specimen length, and elastic tensile stresses on magnetic properties of amorphous metallic wires of composition Fe₇₅Si₁₀B₁₅ have been investigated. It has been revealed that the temperature of the pretreatment and the length of the wires have important influence on their magnetic properties. As the wire length decreases, a segment appears in the functional dependence of the differential magnetic permeability of the wire on the external magnetic field where the permeability remains constant, which indicates a change in the fundamental mechanism of magnetization of the wire. It has been shown that the behavior of the experimental functional dependence of the coercive force and differential permeability on applied tensile stresses may be explained in the context of the model of propagation of domain walls that separate oppositely magnetized domains in the core of the wire with allowance for the dissipative term.     

Velocity of motion of domain walls with vortex structure in multilayer ferromagnetic films

A two-dimensional model of the magnetization distribution has been applied to investigating the influence of the layered structure of a ferromagnetic film in which the layers differ in saturation magnetization M _S on the velocity of the motion of domain walls with a vortex distribution of magnetization. Films with uniaxial planar magnetic anisotropy have been analyzed. It is established that, in three-layer films with values of M _S in the near-surface layers that exceed those of the internal layer in a wide range of the external field, the velocity of motion of the domain wall is higher than in a homogeneous film with the same average saturation magnetization.     

梯度强磁场下Al-18%Si合金中初晶硅的细化

实验研究了梯度强磁场下Al-18%Si(质量分数)合金中偏聚初晶硅的形貌及晶粒尺寸的变化.无磁场时初晶硅为粗大的板条状或五星状;施加梯度磁场时偏聚初晶硅呈弥散分布的等轴多边形,初晶硅显著细化.当磁化力维持不变时,偏聚初晶硅晶粒尺寸随磁场强度的增大而减小,晶粒数密度随磁场强度的增加而增大.实验结果预示,强磁场影响Si原子扩散.对磁场抑制扩散及初晶硅颗粒的受力进行了分析,较好地解释了实验结果.     

Effect of elastic and plastic deformations on the remanent magnetization of an ensemble of nanoparticles

A theoretical investigation of the effect of mechanical stresses on the remanent magnetization has been performed in terms of the model of single-domain noninteracting nanoparticles. Relationships have been obtained which define two main types of remanence in the entire range of stresses. In the low-field approximation, the magnetization of the first type, whose mechanism of formation is similar to that of the normal remanence, is quadratic in both the magnetic field and stresses and only slightly changes with increasing stresses. Depending on the relationship between the magnetostriction constants, this magnetization can both increase and decrease with increasing stresses. The magnetization of the second type, which arises as a result of a nonmonotonic behavior of the critical fields of nanoparticles depending on mechanical stresses, is proportional to the magnetic field and mechanical stresses. It has been shown that the longitudinal remanence arising in the field of stresses parallel to the magnetic field is always greater than the transverse remanence. The behavior of the remanence with increasing mechanical stresses depends substantially on whether this magnetization is formed in a loaded state or in a state unloaded after plastic deformation. In the range of deformations where the anisotropy of the applied stresses is less than the magnetocrystalline anisotropy, the plastic tension should lead to a decrease in the magnetization as compared to that arising in the plastically undeformed state. Plastic compression can lead to both an increase and a decrease in the remanence.     

Spontaneous volume magnetostriction of Dy2AlFe12Mn4 compound

The structure and magnetic properties of Dy2AlFe12Mn4 compound have been investigated by means of X-ray diffraction and magnetization measurements. The Dy2AlFe12Mn4 compound has a hexagonal Th2Ni17-type structure. Negative thermal expansion was found in Dy2AlFe12Mn4 compound from 229 to 280 K by X-ray dilatometry. The coefficient of the average thermal expansion is α= -3.8×10-5 K-1. The magnetostrictive deformations from 105 to 270 K have been calculated by means of the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation ωs; decreases from 6.2×10-3 to near zero with the temperature increasing from 105 to 270 K, the spontaneous linear magnetostrictive deformation λc. along the c axis is much larger than the spontaneous linear magnetostrictive deformation λa in basal-plane at the same temperature except close to 249 K.