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.     

Magnetic characterization of cold rolled and aged AISI 304 stainless steel

Magnetic easy axis predicted by the orientation distribution of the maximum amplitude of magnetic Barkhausen emission (MBE), which is obtained by magnetization in radial directions from the center of the specimens has been applied to determine the magnetic anisotropy on cold rolled and aged 304 SS in two sets of specimen. The maximum of the MBE has been found to orient along the rolling direction (RD) compared to the transverse direction (TD), indicating the presence of magnetic easy axis along the rolling direction for both sets. The strain induced martensite phase transformation has been determined using X-ray diffraction technique. The orientation distribution function (ODF) analysis has been carried out to obtain the crystallographic texture with cold rolling. ODF analysis revealed the 110 texture as the major. The magnetic anisotropy factor has also been determined with cold deformation and noticed that the strength of magnetic anisotropy decreases above 50% deformation for both the sets. Results have been explained considering two competitive effects, formation of crystallographic texture in the martensite phase and presence of compressive residual stresses along RD during cold rolling.     

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.     

Nuclear magnetic resonance in magnets with a spiral magnetic structure

Specific features of nuclear magnetic resonance (NMR) in a ferromagnet with a crystallographic magnetic anisotropy of the easy-plane type placed in a constant magnetic field have been studied. The symmetry of the magnet admits the existence of the Dzyaloshinskii-Moriya interaction, which leads to the formation of a new ground state, namely, a soliton lattice (spiral structure). Within the spin-wave approximation, the following basic local characteristics of the NMR of this structure have been calculated: resonance frequency, enhancement factor, and line broadening and their field dependences have been investigated. The magnetic resonance susceptibility of the electron-nucleus spin system has been calculated; the shape of the NMR absorption line has been analyzed. The problem of the evolution of the NMR absorption line upon the change in the magnitude of an external magnetic field has been solved. The possibility of the experimental detection and investigation of the structural and dynamic features of the spiral magnetic structure by the NMR method is discussed.     

Study of exchange and anisotropy parameters versus insertion of H and C in the RFe11.35Nb0.65 compounds (R=Ho,Lu)

Magnetisation measurements were performed on oriented powder samples of the RFe_11.35Nb_0.65 compounds (R=Ho, Lu) and their hydrides and carbides. The study of the angular dependence of the parallel and perpendicular components of the magnetisation vector, reference to the applied field, allows to determine accurately the magnetocrystalline anisotropy parameters. Powder neutron diffraction experiments led to a determination of the crystallographic and magnetic structures of these materials, in particular the values of the iron and holmium moments on the different sites. Analysis of the results allowed estimation of the exchange interactions and of the magnetocrystalline anisotropy originating from both sublattices and an explanation of the spin reorientation process occurring in the Ho-based hydride.     

Angular dependence of the FMR linewidth and the anisotropy of the relaxation time in iron garnets

This work is devoted to the problem of extracting the contribution of the anisotropy of relaxation to the angular dependence of the FMR linewidth and to the opportunity of determining the values of the parameters of relaxation. The results of the FMR study of films based on the yttrium iron garnet prepared by the method of liquid-phase epitaxy are given. The orientational dependence of the linewidth has been calculated using the traditional method of measuring an FMR spectrum and a method based on scanning at an angle to the resonance field for obtaining the minimum linewidth. A model for calculating the linewidth has been proposed that takes into account the anisotropy of the relaxation term in the equation of motion of the magnetic moment. The model leads to a dependence that agrees well with the experimental data, which makes it possible to state that the anisotropy of relaxation most likely takes place in the samples under consideration at the temperatures employed.     

Characterization of texture and residual stress in a section of 610 mm pipeline steel

Gas pipelines are inspected for defects such as corrosion. The most commonly used nondestructive inspection tool uses the magnetic flux leakage (MFL) technique. The MFL signals depend on the magnetic behaviour of the pipe, which is sensitive to its microstructure and crystallographic texture as well as both residual and applied stresses. Here a section of commercial X70 pipeline is characterized using microstructural examination, X-ray diffraction (to determine crystallographic texture) and neutron diffraction (for residual stress measurement). The results correlate well with the manufacturing steps used for this type of pipe. Magnetic characterization is also performed using magnetic Barkhausen noise measurements, which reflect the magnetic anisotropy in the pipe and thus the MFL signal. These results do not correlate simply with crystallographic texture and residual strain results, but this is not unexpected given the complex nature of the material and its stress state.     

Bulk and surface spin-wave modes in a longitudinally magnetized thin film

Based on the solution to the equation of magnetization motion, an expression for the tensor of the magnetic susceptibility has been obtained, whose imaginary part determines the spectrum of spin-wave resonance in a thin film magnetized in its plane along the (coinciding-in-direction) axes of the uniaxial bulk and surface anisotropy, for the basic types of the surface pinning of spins. The presence of damping and the finiteness and asymmetry of surface pinning lead to an essential rearrangement of the spectrum of spin-wave resonance.     

Influence of Oblique Sputtering on Stripe Magnetic Domain Structure and Magnetic Anisotropy of CoFeB Thin FilmsEI北大核心CSCD

Magnetic anisotropy is one of the most important fundamental properties of magnetic thin film. The strength of magnetic anisotropy determines the ferromagnetic resonance frequency of magnetic films in the high-frequency applications. Because of the directionality of conventional static magnetic anisotropy in magnetic film, the high-frequency device usually shows an obvious directionality. When the microwave magnetic fi eld deviates from the perpendicular direction of magnetic anisotropy, the devices cannot reveal their best performance. The magnetic film with a stripe magnetic domain structure displays an in-plane rotatable magnetic anisotropy, which can be an important strategy to solve the problem of magnetic fi eld orientation dependent performance in high-frequency device. Therefore, the magnetic domain, the magnetic anisotropy, and the high-frequency behaviors for magnetic fi lms with a stripe magnetic domain structure have received extensive attention. Previously, most of the studies focused on the stripe magnetic domain structure of polycrystalline thin films. However, less attention was paid on amorphous magnetic thin films. Since the amorphous magnetic films have no long-range ordered crystal structure, no magnetocrystalline anisotropy, no grain boundary defects resistance hindering the domain wall displacement, they usually show excellent soft magnetic properties and have been widely applied in high-frequency devices. CoFeB alloy is one of the most important amorphous magnetic materials and has been extensively applied in various spintronic devices. In this work, amorphous CoFeB magnetic thin films were prepared by using a method of oblique sputtering technique at room temperature. The influences of oblique sputtering on the stripe magnetic domain structure, the in-plane static magnetic anisotropy, the in-plane rotational magnetic anisotropy, and the perpendicular magnetic anisotropy of the amorphous CoFeB films were studied by scanning probe microscope, vibrating sample magnetometer, ferromagnetic resonance. It is found that the method of oblique sputtering could effectively reduce the critical thickness for the appearance of stripe magnetic domain in amorphous CoFeB films. For a non-oblique sputtered CoFeB film, the critical thickness for the appearance of the stripe magnetic domain is above 240 nm. In contrast, after been subjected to the oblique sputtering, the critical thickness becomes below 240 nm. The different magnetic characterizations indicate that for the growth of CoFeB films with stripe magnetic domain structure, the oblique sputtering could not only enhance the strength of in-plane static magnetic anisotropy, but also improve the in-plane rotational magnetic anisotropy and the perpendicular magnetic anisotropy. All of the magnetic anisotropies are increased with the angle of oblique sputtering. The observation results of XRD and TEM prove that the prepared CoFeB thin films tend to amorphous structure. The characterization of SEM observation indicates that although the amorphous CoFeB films do not possess long-range ordered crystalline structure, they still could form a kind of columnar structure. The slanted columnar structure of CoFeB films could significantly increase the perpendicular magnetic anisotropy, thus lead to the appearance of stripe magnetic domain structure.     

On the properties of a stochastic magnetic structure of low-dimensional ultradisperse ferromagnets

Analysis is given of the effect of the dimensionality of structural inhomogeneities on the properties of the magnetic structure of ultradisperse ferromagnetic materials. The coordinate dependence of the magnetization dispersion of magnets with one-dimensional and two-dimensional inhomogeneities of magnetic anisotropy has been calculated. The linear dimension of a magnetic block for one-and two-dimensional inhomogeneities of the anisotropy field has been estimated. A method for exact calculation of the distribution function of magnetization rotations in the block is suggested.     

Application of ultrasonic methods to determine elastic anisotropy of polycrystalline copper processed by equal-channel angular pressing

Anisotropy of elastic properties of ultrafine-grained polycrystalline copper after one, two and four passes of equal-channel angular pressing (ECAP) is investigated by means of ultrasonic methods. For each material, Young’s and shear moduli in the principal processing directions are evaluated and the symmetry and orientation of the anisotropy are identified. The relation between the determined symmetry and the processing mechanisms is discussed. It is shown that the material after one and two passes of ECAP exhibits a measurable anisotropy, while the material after the fourth pass behaves isotropically. Within the discussion, it is shown that the origin of the observed anisotropy may be attributed to the spatial arrangement of grain boundaries rather than to the crystallographic texture. In the light of this conclusion, the obtained results correspond well with optical and transmission electron microscopy observations of the microstructures of ECAPed materials documented in the literature.     

Magnetization of superparamagnets in the state of mechanical anisotropy

The internal energy of magnetic anisotropy in some particles dominates over the thermal energy, even at room temperature. The existence of strong magnetic anisotropy of nanoparticles can significantly affect the process of magnetization of superparamagnets. However, if the axes of magnetic anisotropy of nanoparticles are randomly oriented, then their presence does not affect the process of magnetization, which occurs according to the classical Langevin theory. However, if the axes of nanoparticles are polarized (mechanical anisotropy), then the magnetization curve of a superparamagnet under the conditions of mechanical anisotropy lies between the Langevin curve and the curve of hyperbolic tangent and with increasing anisotropy moves progressively farther from the Langevin curve and approaches the curve of hyperbolic tangent. It has also been shown that, in the case of powder superparamagnets, the presence of mechanical anisotropy leads to significant changes in the Curie constant.     

Dynamics of a lattice of magnetic nanoparticles under action of a pulse of a magnetic field

Processes of quasistatic and dynamic magnetization reversal have been studied for the case of planar 6 × 6 lattices of magnetic nanodipoles that possess a cubic crystallographic anisotropy. The response of the total magnetic moment to a magnetic-field pulse of various duration and polarization have been determined for different equilibrium configurations of the lattices. Along with the in-plane configurations of magnetic moments, configurations with one and two dipoles oriented perpendicular to the plane of the lattices have been considered.     

Magnetic Barkhausen measurements for evaluating the formation of Lüders bands in carbon steel

Localized inhomogeneous plastic deformation phenomenon was experimentally investigated in a structural steel by using the magnetic Barkhausen noise measurements. ANSI 1050 steel plates, throughout oriented in the rolling transversal direction, were selected additionally inducing different strains. Scanning of the magnetic Barkhausen energy (MBN_energy) shows the formation of Lüders bands as well as the displacements in the material deformation zone. The MBN_energy angular dependency on the applied stress and on the anisotropy coefficient were determined as well as the presence of Lüders bands in samples oriented according to the rolling direction.     

广义矢量法预估多晶材料的磁晶各向异性

本文根据三维取向分析的广义矢量法,分别确定W20冷轧无取向硅钢片的表面层、1/4层及中心层的晶粒取向分布函数(ODF);扩展运用晶粒取向分布无样品对称多晶材料之磁各向异性理论,定量算出板片上不同方向的磁转矩。理论预算值与实测磁转矩曲线符合较好。     

Tension–compression asymmetry in severely deformed pure copper

In this work, we investigate the tension–compression asymmetry in the flow response of pure copper, severely deformed by a single pass of equal channel angular extrusion (ECAE), and tested afterwards in tension and compression along three orthogonal directions. The tension and compression responses differ in flow stress, hardening rate and transient behavior. The asymmetry in tension and compression responses depends on the direction of straining. Predictions from a microstructurally based hardening law implemented in a viscoplastic self-consistent polycrystal model demonstrate that the tension–compression asymmetry and its anisotropy arise not only from crystallographic texture but also from the directional substructure induced by the severe pre-straining. Slip activity differs in each grain and in each axial test, depending on its crystallographic orientation, orientation relationship with the previously generated substructure and pre-strain history. Asymmetry occurs because tension and compression represent two different types of strain path changes. Consequently, macroscopic deformation is a reflection of the type of strain path change represented by the post-ECAE axial test.     

Magnetic Ni-Fe-Co alloys with a stepwise hysteresis loop

Conclusions It is possible to create a cubic recrystallization texture in Ni-Fe-Co alloys with a close-packed hexagonal lattice, which contain 50% Ni and up to 25% Co. Magnetic properties were investigated on spiral cores of these alloys in the textured state after various heat-treatment regimes in a transverse field. In this case, the previously unknown stepwise hysteresis loop, whose appearance is explained by the interaction of the energies of crystallographic anisotropy and induced uniaxial anisotropy and the external magnetic field, were observed for alloys with a high positive constant of crystallographic anisotropy K₁.Dresden Technical University, (German Democratic Republic). Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 56–59, March, 1981.     

Study of the structure and magnetic properties of Co nanoparticles in the matrix of highly porous amorphous carbon

This paper presents the results of investigations into the structure and the magnetic properties of powders consisting of Co nanoparticles in the matrix of a high-porous amorphous carbon material. An analysis of X-ray diffraction and nuclear magnetic resonance spectra revealed the presence of hcp and fcc phases of cobalt in the particles of the powder and demonstrated the existence of a highly defective state, which could be related to a high concentration of defects of the “displaced-plane” type. The magnetometric measurements showed that the powder particles were in the ferromagnetic state and were characterized by a high field of the local magnetic anisotropy.     

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.     

Effects of initial orientation,sample geometry and friction on anisotropy and crystallographic orientation changes in single crystal microcompression deformation: A crystal plasticity finite element study

The study presents crystal plasticity finite element simulations of cylindrical Cu single crystal micropillar compression tests. The aim is to study the influence of the stability of the initial crystal orientation, sample geometry (diameter-to-length ratio) and friction on the anisotropy and crystallographic orientation changes during such tests. Initial anisotropy (initial orientation) has a strong influence on the evolution of crystallographic orientation changes and also, to a minor extent, on the sample shape during compression. Pronounced orientation changes occur at an early stage of compression (at engineering strains of 0.2), entailing as a rule a large orientation spread within the initially uniformly oriented sample. A non-zero friction has a stabilizing effect on the course of the compression test even in cases where strong orientation changes occur. The evolution of orientation changes during compression is in part due to rigid body rotations (shape inclination due to buckling) rather than exclusively to crystallographic reorientation. Orientations that are crystallographically unstable and non-symmetric during compression tend to entail shape instability of the pillars at an earlier stage than observed for more stable cases.