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.     

High-frequency properties of two-layer exchange-coupled ferromagnetic structures

A numerical study of peculiarities of ferromagnetic resonance (FMR) in a two-layer exchange-coupled ferromagnetic structure has been performed at various magnitudes and directions of an external magnetic field. The layers have a finite thickness and are characterized by an anisotropy of the easy-plane or easy-axis type. The FMR frequencies have been found to decrease relative to the frequencies calculated in the model of an infinitely thin film. It has been demonstrated that with an increase in the layer thickness the direction of the displacement of the lower FMR mode depends on the magnitude and direction of the external magnetic field and that the frequency of the higher mode decreases at any direction of the external field. The amplitudes of the resonance curves decrease with increasing magnetic field, with the ratio of the half-width of the resonance curve to the resonance frequency decreasing up to the saturation point, after which this parameter varies only slightly. The FMR frequencies have been studied as a function of the constant of the interlayer exchange interaction for fields located in the saturation region.     

Ferromagnetic resonance in a two-layer exchange-coupled ferromagnetic film with a combined uniaxial and cubic anisotropy in the layers

Dynamic properties of ferromagnetic two-layer exchange-coupled (100) films with a combined cubic and uniaxial magnetic anisotropy of layers have been studied numerously upon the magnetization along the [100], [010], and [011] directions. The allowance for cubic anisotropy substantially affects the dependence of the frequencies of the ferromagnetic resonance on the field strength. Repeated changes in the localization of the ferromagnetic-resonance modes between the layers of the film have been found to occur with an increase in the strength of the magnetic field. At a certain relationship between the constants of the combined anisotropy for the directions [010] and [011], an increase in the field leads to a shift of the maximum of the dynamic-susceptibility distribution toward the interlayer boundary without a change in the localization of the modes.     

Dynamics of magnetic kinks in exchange-coupled ferromagnetic layers

A dynamic theory of magnetic kinks in a two-layered ferromagnet, which propagate in the direction of the normal to the interlayer boundary (planar “defect”), has been developed. Conditions for the passage of kinks through the boundary have been determined and the critical velocity for their absorption has been found. It has been shown that at kink velocities exceeding critical, the boundary, when absorbing a kink, emits an antikink. The dependence of the delay time of the kink in the defect on its velocity has been established.     

Low-frequency ferromagnetic resonance in a uniaxial magnetic film

The equations of the precession dynamics of the magnetic moment have been analyzed to study the specific features of the low-frequency ferromagnetic resonance corresponding to magnetic biasing along the hard magnetic axis of a film with an in-plane uniaxial anisotropy. It has been found that the precession trajectories exhibited a complicated hopping behavior and the resonance curve was asymmetric. This behavior and the asymmetry were due to an angular bistability arising from the presence of two symmetric angular positions of equilibrium near the anisotropy field.     

Magnetization reversal of Tb-Co/Fe19Ni81 films with a unidirectional anisotropy

Conditions have been found for the realization of induced in-plane uniaxial anisotropy of the amorphous Tb₃₁Co₆₉ films and of unidirectional anisotropy in the Permalloy layers in the composite two-layered Tb₃₁Co₆₉/Fe19Ni81 films. The mechanisms of magnetization reversal in such film structures have been studied in a temperature range of 5–300 K. It has been found that a decrease in temperature leads to a transformation of hysteresis loops, a significant increase in the field of unidirectional anisotropy, and a nonmonotonic change in the coercive force of the Permalloy layer. The regularities found are interpreted taking into account the variation in the properties of the amorphous layer Tb₃₁Co₆₉ with temperature under the assumption on a temperature-induced change in the localization of the interlayer magnetic boundary, which is formed upon a layer-by-layer magnetization reversal of the film structure.     

Autoresonance excitation of oscillations of domain walls in a ferromagnetic film

Nonlinear oscillations of domain walls in a ferromagnetic film with a plane-parallel periodic domain structure have been studied analytically and numerically in an external field with slowly changing amplitude and frequency. The conditions for the controlled autoresonance excitation of nonlinear oscillations of a domain wall in the presence of dissipation have been determined. The threshold values of the amplitude and rate of amplitude growth of the field of pumping have been found depending on the geometrical characteristics of the domain structure of the films whose exceeding generates an effect of autophasing in the system. It is shown that the magnetic saturation of a ferromagnetic film in the autoresonance regime can occur in fields that are substantially less than the fields of dynamic stationary magnetization.     

High coercivity characteristics of FePtB exchange-coupled nanocomposite thick film spring magnets produced by sputtering

Thick film magnets (∼3.2 μm) deposited on silicon substrate at processing temperatures lower than 400 °C, exhibiting high coercivity (H_c ∼ 604 kA m⁻¹), high maximum energy product [(BH)_max = 95 kJ m⁻³] and good surface properties, were obtained by sputtering technique. The effects of sputtering power, substrate temperature (T_s) and film thickness on the magnetic properties of the films were studied. Films exhibiting very good hard magnetic properties are mainly made up of nanosized hard (L1₀-FePt) and soft (Fe₂B and Fe₃B) magnetic phases, which are exchange coupled. The exchange spring behavior in these films was investigated and a high recoil rate of more than 90% was obtained. The ability to deposit permanent magnet thick films exhibiting good hard magnetic as well as surface properties on silicon, as demonstrated in the present work, is very promising for the fabrication of micromagnets for magnetic MEMS.     

On the magnetoelectric effect in a two-layered film with a ferromagnetic Spin-Spin interaction

Inhomogeneous magnetoelectric effect in a two-layered exchange-coupled film has been investigated theoretically. Smooth and jumplike changes (increases and decreases) in the polarization have been observed in accordance with changes in the inhomogeneities of the magnetization in the magnetic field. The specific features of polarization depending on the changes in the magnitude and direction of the magnetic field, magnitude of magnetic anisotropy, and thickness of the layers have been considered.     

Magnetization reversal of small two-phase chemically inhomogeneous particles with an extended interphase boundary

The hysteretic behavior of two-phase ferromagnetic particles with strong interphase exchange interaction has been considered theoretically. It has been shown that the mechanism of magnetization reversal is determined not only by the size but also by the degree of inhomogeneity of the particle and differs qualitatively from the case with a disturbed exchange interaction between the phases. A conclusion is made that no notice-able improvement of coercivity can be achieved by producing a highly anisotropic layer on the particle surface.     

Magnetization arrangement of hard magnetic phases and mechanism of magnetization and reversal magnetization of nano-composite magnets

During the process of directional solidification, laser remelting/solidification in the layer on sintered magnets, die-upsetting of cast magnets, or die-upsetting of nano-composites, the arrangements of the easy-magnetization-axes of the hard magnetic phases (Nd₂Fe₁₄B, SmCo₅ or Sm₂Co₁₇ type) in their designed directions have been studied. In Fe-Pt nano-composite magnets, attempts have been taken to promote phase transformation from disordered, soft magnetic A1 to ordered, hard magnetic L1₀ FePt phase at reduced temperatures. The dependence of the magnetization and reversal magnetization processes on the microstructures, involving the morphology and three critical sizes of particles of the FePt nano-composite magnets, are summarized. With the decrease of the nominal thickness of the anisotropic FePt film epitaxially grown on the single crystal MgO (001) substrate, the reversal magnetization process firstly changes from full domain wall displacement to partial magnetic wall pinning related to the morphology change, where the coercive force increases abruptly. The reversal magnetization process secondly changes from magnetic wall pinning to incoherent magnetization rotation associated with the particles being below the first critical size at which multi-domain particles turn into single domain ones, where the coercive force is still increased. And the reversal magnetization mode thirdly changes from incoherent to coherent rotation referred to the second critical size, where the increase of the coercive force keeps on. However, when the particle size decreases to approach the third critical size where the particles turn into the supperparamagnetic state, the coercive force begins to decrease due to the interplay of the size effect and the incomplete ordering induced by the size effect. Meanwhile, due to the size effect, Curie temperature of the ultra-small FePt particles reduces.     

超导环的磁化性质研究

计算了双结超导π环、零环及规则超导环阵列的自由能随外加磁场及温度的变化,给出了超导转变温度附近的系统相图。针对高温超导体及低温超导体中出现的顺磁迈斯纳效应,用π结、零结概念,给出了一种简单的理论解释。     

Magnetoelastic interactions in a cracked ferromagnetic body

The stress–strain state associated with a moving crack in a ferromagnetic material is investigated. The model considers a soft magnetic ferroelastic body and incorporates a realistic (nonlinear) susceptibility. The moving crack propagates in a direction perpendicular to the magnetic field. A closed-form solution yields the magnetoelastic stresses and a stress intensity factor. An applied magnetic field makes the stress intensity factor depend upon the crack velocity. The nonlinear susceptibility produces a completely different magnetoelastic stress state than a constant susceptibility, and the stress intensity is highly sensitive to material properties. The stresses around the crack are largely insensitive to the external magnetic field and crack speed except at critical combinations for which the stresses are singular. Some combinations of magnetic field and crack velocity cause the stress components ahead of the crack tip to change sign.     

Influence of dynamic crystallization on exchange-coupled NdFeB nanocrystalline permanent magnets

Dynamic crystallization was introduced to improve the magnetic properties of NdFeB nanocrystalline permanent magnets by optimizing microstructure. The microstructure was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It has been determined that, compared with the conventional heat treatment, dynamic crystallization can shorten the crystallization time. Moreover, dynamic crystallization can refine grains, enhance the exchange-coupled interaction among grains, and promote the magnetic properties. As a result, the optimal magnetic properties of Nd10.5(FeCoZr)83.4B6.1(Br＝0.685T, Hci ＝732 kA·m -1 , Hcb ＝429 kA·m-1 ,( BH )m=75 kJ·m -3 ) are obtained after dynamic crystallization heat treatment at 700 ℃ for 10 min.     

方钢纵向磁化的两个不同阶段

用磁偶板子链概念可以证明，随着磁化场的增强，方钢的纵向磁化将出现两个不同的阶段。在磁化场强较弱的第一阶段，端面上被激励出的磁荷将均匀分布在各条底棱上；而在磁化场强接近材料饱和磁化场强的第二阶段中，磁荷必均匀分布在所有的底棱和侧棱上。     

WC—Co硬质合金的饱和磁化强度

探讨了ＷＣ－Ｃｏ硬质合金饱和磁化强度的影响因素、磁化机制和比饱和磁化强度４πσ与抗弯强度σ（ＴＲＳ）的关系。结果表明，影响ＷＣ－Ｃｏ合金４πσ的主要因素是γ相含量和γ相成分，其次是γ相结构。其４πσ同合金标称钴含量或合金碳含量间均不存在单一性对应关系。γ相分布和热处理及添加稀土元素对合金４πσ的影响，实质上仍归结为上述基本因素的作用。ＷＣ－Ｃｏ合金的４πσ同抗弯强度间并不存在明确的对应关系，因而难以用饱和磁化强度（Ｍｓ或４πσ）取代σ（ＴＲＳ）的测定。     

Lanthanum ferrite ferromagnetic nanocrystallites by a polymeric precursor route

Reactive lanthanum orthoferrite nanoparticles were obtained by a polymeric precursor route. Nanoparticle growth and crystallization from amorphous precursor, as well as the formation of a grain boundary network in polycrystalline aggregates at different calcination temperatures were studied by conventional and high-resolution electron microscopy; electron and X-ray diffraction analysis; Raman; IR; and UV-vis spectroscopy. Microstructure measurements were compared to X-ray diffraction and chemical analysis results. Electron diffraction, combined with electron microscopy results were used to determine the content of amorphous phase. The coherent crystalline domain size and the particle size have been monitored by XRD and electron microscopy in order to determine the evolution of both crystal size and the onset temperature for crystallites formation. The results demonstrate that at 550 °C we obtain pure single-phase nanocrystalline LaFeO₃, sized ∼40 nm, without the presence of amorphous phase. The magnetization curves in the 5-350 K range indicate weak ferromagnetism of the LaFeO₃ powders.     

双层扁挤压组合筒光弹性分析

根据前期所做的扁挤压整体筒[1]和双层筒[2]光弹性实验(这两个实验模型具有相同的型腔尺寸(42×20mm)和外径(104mm),因此具有可比性),旨在对这两组实验数据作一综合分析,获得双层扁挤压组合筒的装配压力的分布规律和装配面挤压力的分布规律,并利用叠加原理得到组合筒挤压时的应力场。结果分析显示,扁挤压筒组合结构与整体结构相比,有效降低了型腔长轴方向的应力集中,改善了挤压筒的受力,为扁挤压筒的结构设计和强度计算提供了一定的实验依据。     

双层复合管材机头的优化设计

介绍了共挤双层复合塑料管材机头模具结构设计,为了提高复合管材的密实度和物料的加工性能,对机头的结构特点进行了优化设计,在芯棒处增设了双阻流装置。结果表明:物料经过双阻流芯棒时,密实度增加,塑化性能好,挤出稳定,管材的内、外表面光泽度好,强度高。