The effect of stripe domain structure on dynamic permeability of thin ferromagnetic films with out-of-plane uniaxial anisotropy

The permeability is calculated for a thin ferromagnetic film with the stripe domain structure and out-of-plane uniaxial magnetic anisotropy. Analytical expressions for the frequency dependence of components of permeability tensor are derived with the use of the Smit–Beljers method, with the thickness of domain walls and the domain wall motion being neglected. The effect of the domain width and the angle between the anisotropy axis and the film plane on the frequency dependence of the permeability is analyzed. General equations relating the static permeability components and the ferromagnetic resonance frequencies are found. The results of the approach are applied to the derivation of the constraint for the microwave permeability of thin ferromagnetic films. The analysis of the constraint as a function of the axis deviation angle, the domain aspect ratio and the damping parameter allows the conditions to be found for maximal microwave permeability. The results obtained may be useful in connection with the problem of developing high-permeable microwave magnetic materials.     

Diffusion of a pulsed field and electromagnetic forces in ferromagnets

Self-similar solutions of the magnetic field distribution in a conducting ferromagnet with a non-linear permeability are considered. The velocity of magnetic field penetration is calculated in various models of ferromagnetic properties of the conductor. Using the Maxwell tensor, equations are derived for bulk forces taking into account the possibility of saturation of the ferromagnet. A numerical solution for the distributions of the magnetic induction and of the density of the bulk force in a ferromagnetic conductor is obtained using the Preisach model taking hysteresis into account.     

Hysteretic vortex pinning in superconductor-ferromagnet nanocomposites

This Letter reports the observation of hysteresis in the vortex pinning in a superconductor-ferromagnetic epitaxial nanocomposite consisting of fcc Gd particles incorporated in a Nb matrix. We show that this hysteretic pinning is associated with magnetic reversal losses in the Gd particles and is fundamentally different in origin to pinning interactions previously observed for ferromagnetic particles or other microstructural features.     

非晶态合金薄带与膜的巨磁电阻抗效应理论及计算

通过建立具有平面近横向各向异性场的非晶态合金薄带及膜的磁畴结构模型，利用线性化Maxwell方程组及Landau-Lifshitz方程，推出了在高频交变磁场及外加面内轴向直流磁场H_ex作用下的铁磁材料的与取向相关的磁导率表达式，得到了对方位角平均的相对磁导率及阻抗的计算式，导出了磁导率与张量磁化率分量间的关系，对材料磁导率的实部及虚部随H_ex的变化进行了计算，并给出了对应的磁谱图.建立的磁导率与外磁场的理论关系可将Panina及Kraus给出的理论结果统一起来. 关键词： 非晶态合金薄带及膜 取向相关磁导率 GMI效应理论与计算 近横向各向异性场     

Estimation of the superhigh-frequency magnetic permeability of alsifer from the measured permeability of composites

The magnetic permeability of alsifer was restored from the frequency dependences of the dielectric and magnetic permeabilities of powder alsifer (AlSiFe alloy)–wax matrix composites. The permeabilities were measured using the coaxial line technique within a frequency range of 0.05–20 GHz. The effect of the concentration, shape, and size of powder particles on the microwave magnetic properties of composites was considered. A good agreement between the measurement results and the Maxwell–Garnett formula generalized with consideration for the particle shape, the percolation threshold, and the skin-effect was obtained. The found sizes of particles agreed with electron microscopy and granulometry data. Both the frequency and the ferromagnetic resonance line figure of merit (FOM) for lamellar particles proved to be higher than for spherical ones. Alsifer powders were shown to be promising fillers for radioabsorbing materials.     

Effective magnetic permeability of a turbulent fluid with macroferroparticles

A fully developed turbulent flow is capable to mix and homogenize a suspension of heavy macroscopic particles even at a high concentration of particles. If the particles are ferromagnetic, a kind of “turbulent ferrofluid" can be obtained. In the present work, we present a direct measurements of the effective magnetic permeability in a turbulent fluid with suspended ferromagnetic particles of typical size 0.01-0.1 mm and volume fraction c up to 25%. We show that the effective permeability can be fitted by the linear law = 1 + 5.3c for c? 10%. For higher volume fractions the permeability exceeds this linear relation. Received 11 January 2002     

Magnetic permeability spectrum of ferroxplans in the NFMR region. Like noninteracting particles

We calculate the magnetic permeability spectra of like noninteracting particles of ferroxplans in the region of natural ferromagnetic resonance (NFMR). The NFMR frequency is shown to depend essentially on the shape of the grains while the magnitude of the magnetic permeability at resonance depends on the degree of texture possessed by the material.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 43–46, January, 1989.     

Spiral magnetic structure in non-Heisenberg magnets with an easy-axis anisotropy

The investigations performed in this work have demonstrated that an easy-axis frustrated non-Heisenberg magnet can contain homogeneous phases with the vector (ferromagnetic) and tensor (nematic) orderings, as well as a spatially inhomogeneous phase of the magnetic spiral type. Depending on the relationships between the material parameters, either a ferromagnetic spiral or a spiral of the quadrupole–ferromagnetic type with different periods of the spiral structures can be formed in the system. The phase diagram of the system has been constructed.     

Effect of porosity on the permeability tensor of polycrystalline ferrites (independent grain approximation)

The effect of porosity on the form of the permeability tensor is calculated using the independent-grain approximation; this procedure is similar to the Schlömann method. The theoretical curve for the resonant-field distribution is approximated by the Lorentz curve using the method of least squares. It is shown that with this approximation, porosity increases the width of the ferromagnetic resonance line for a non-porous material by the width of the Lorentz distribution curve; thus the resonant field shifts toward lower values. Formulas are obtained for the resonant-field shift due to porosity and the broadening in the ferromagnetic resonance line; these formulas differ somewhat from the Schlömann formulas. In order to check the working formulas and the applicability of the independent-grain approximation, measurements were performed on the tensor for magnesium-chromium-copper ferrites with variable porosity and a magnetization on the order of 1200 gauss at a frequency of 4000 mHz. Specimens having the form of longitudinally magnetized circular cylinders were used so that there was no degeneration in uniform precession of magnetization with long spin waves. The observed effect of porosity on the width of the ferromagnetic resonance line (determined by measuring the tensor) was found to be in good quantitative agreement with calculation. The shift in the resonant field due to porosity was negligibly small, which also agreed with calculation. The experimental results show that when there is no degeneration in uniform precession with spin waves, the independent-grain approximation can be used in experiments even when the magnetization and resonant field are approximately equal.Here, we must allow for the static magnetic susceptibility in the formulas for the resonant-field shift and the broadening in the ferromagnetic resonance line.     

Effects of magnetic field on shape evolution of Fe–Co particles in a Cu matrix

The effects of magnetic field on the shape evolution of ferromagnetic fcc Fe–Co particles in Cu–0.83 at.% Fe–1.37 at.% Co alloy single crystals were examined using magnetic anisotropy measurements. The Cu–Fe–Co single crystals were aged at 993 K for 2 h to 24 h under a magnetic field of 10 T parallel to either the [001] or [011] direction. The magnetic anisotropy was examined by measuring magnetic torque around the (100) plane. It was found that the fcc Fe–Co particles are elongated in the direction parallel to the magnetic field. Furthermore, the elongation along [001] is more remarkable than that along [011]. The results are explained quantitatively by considering the minimization of the sum of the interface energy, elastic strain energy and magnetostatic energy of spheroidal particles.     

Negative Refractive Index Ferromagnetic Materials with Negative Permeability at Zero Applied Magnetic Field

Negative magnetic permeability of ferromagnetic materials is studied in the microwave frequency region. The magnetic permeability is analyzed by employing the dynamic equation of motion of magnetization for different directions of the applied magnetic field with respect to crystalline axes. In the calculations, a very thin ferromagnetic layer with the cubic magneto-crystalline anisotropy energy is considered. It is found that there can be multiple regions of the field value in which the real component of the dynamic permeability becomes negative. In addition, the low-field region for negative permeability can be extended even to the zero external static magnetic field.     

Synthesis and Structure of Magnetic Core-shell Ni–Ce Nanocomposite Particles

A new type of magnetic core-shell Ni–Ce nanocomposite particles (15–50 nm) is presented. SEM observations suggest the particles are strongly ferromagnetic, interacting with ordered chain-like features. Typical HR-TEM images demonstrate that many planar defects (nanotwins and stacking faults) exist in the surface shell and large Ni core zone (10–40 nm) of the particles; the inner shell layers (4–6 nm) consist of NiCe alloy and the outermost shell is NiO. Nano-diffraction patterns show an indication of well-defined spots characteristic of nanocomposite materials, of which certain crystal facet orientation relationships between orthorhombic [111] of NiCe and cubic [311] of Ni₂Ce, face-center crystal [222] of NiO, cubic [111] of nickel have been identified. This confirms the nature of this core-shell nanocomposite particle.     

Amphiphilic Core–Shell Nanocomposite Particles for Enhanced Magnetic Resonance Imaging

A scalable synthesis of magnetic core–shell nanocomposite particles, acting as a novel class of magnetic resonance (MR) contrast agents, has been developed. Each nanocomposite particle consists of a biocompatible chitosan shell and a poly(methyl methacrylate) (PMMA) core where multiple aggregated γ‐Fe₂O₃ nanoparticles are confined within the hydrophobic core. Properties of the nanocomposite particles including their chemical structure, particle size, size distribution, and morphology, as well as crystallinity of the magnetic nanoparticles and magnetic properties were systematically characterized. Their potential application as an MR contrast agent has been evaluated. Results show that the nanocomposite particles have good stability in biological media and very low cytotoxicity in both L929 mouse fibroblasts (normal cells) and HeLa cells (cervical cancer cells). They also exhibited excellent MR imaging performance with a T₂ relaxivity of up to 364 mM_Fe^?1 s^?1. An in vivo MR test performed on a naked mouse bearing breast tumor indicates that the nanocomposite particles can localize in both normal liver and tumor tissues. These results suggest that the magnetic core–shell nanocomposite particles are an efficient, inexpensive and safe T₂‐weighted MR contrast agent for both liver and tumor MR imaging in cancer therapy.     

Effect of domain walls on the propagation of microwaves

The propagation of microwaves in ferrimagnets is affected mainly by the gyrotropic terms of the magnetic permeability tensor, due in this frequency range to ferromagnetic and exchange resonances. It is shown that the change in the gyrotropic terms in the domain wall region leads to the existence of a localized mode propagating close to the domain wall. The domain wall constitutes an optical inhomogeneity for the microwave. The reflectivity and transmittivity of a domain wall are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 22–26, June, 1985.The authors are grateful to G. S. Krinchik for discussion of the paper.     

Measuring the 9-component magnetic permeability tensor of ferrites

The magnetic permeability tensor of ferrite monocrystals can, in the general case, be represented as consisting of nine components [1]. In [2] the 6-component magnetic permeability tensor was obtained for a triaxial ferrite monocrystal with low magnetic anisotropy energy magnetized by a constant field in the crystallographic plane (110). In the general case of magnetization in an arbitrary crystallographic direction the tensor becomes a 9-component tensor [3]. In the context of the increasing application of ferrite monocrystals in microwave engineering the question of measuring such a tensor is of current scientific and practical interest. In the present article we propose a method for measuring the 9-component tensor for ferrite materials at microwave frequencies.     

Polarization and transmission of microwaves in the magnetic suspension in the applied magnetic field

The production method of magnetic suspension consisting of ferromagnetic particles dispersed in cedarwood oil is presented at the beginning of this article. Next, the set-up for microwaves generation using a klystron is described. The main part of this paper concerning microwave transmission and polarization during its passage in samples of the produced magnetic suspension placed in a magnetic field is based on the following parameters: induction of this field, filling factor of magnetic suspension by ferromagnetic particles, dimensions of particles, viscosity of liquid carrier, and ratio of the magnetic field changes. Conducted investigations show that microwaves are damped and polarized in these magnetic suspensions. Obtained results are discussed and observed effects are explained by ordering of ferromagnetic particles in magnetic suspension by applied magnetic field.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.     

Magnetic nanostructured polymer composites

Magnetic polymer composites filled with microparticles of the nanocrystalline 5BDSR alloy have been studied. Measurements have been performed mainly by the ferromagnetic resonance method complemented by other spectroscopic methods. A quasi-stepwise structure of spectra near the direction of the magnetic field perpendicular to the nanocomposite plate has been found. It has been shown that incorporation of nanoparticles of technical carbon into the composite leads to a nonmonotonic concentrational dependence of the broadening of spectral lines, which is caused by spatial variations in the perpendicular magnetic anisotropy. The ferromagnetic resonance spectrum has been processed by taking into account the scatter of the magnetic anisotropy and demagnetizing factors.     

Nonlinear optical and magneto-optical effects in non-spherical magnetic granular composite

The magnetization-induced nonlinear optical and nonlinear magneto-optical properties in a magnetic metal-insulator composite are studied based on a tensor effective medium approximation with shape factor and Taylcr-expansion method. There is a weakly nonlinear relation between electric displacement D and elcctric field E in the composite. The results of our studies on the effective dielectric tensor and the nonlinear susceptibility tensor in a magnetic nanocomposite are surveyed. It is shown that such a metal-insulator composite exhibits the enhancements of optical and magneto-optical nonlinearity. The frequencies at which the enhancements occur, and the amplitude of the enhancement factors depend on the concentration and shape of the magnetic grains.     

FMR Investigation of the Magnetic Anisotropy in Films Synthesized by Co<Superscript>+</Superscript> Implantation into Si

Thin ferromagnetic films with the uniaxial magnetic anisotropy were synthesized by Co⁺ implantation into single-crystal silicon in the magnetic field. It was concluded that the formation of the induced magnetic anisotropy is due to the directional atomic pair ordering (Neel–Taniguchi model). The synthesized films were studied by the ferromagnetic resonance (FMR) method in the temperature range from 100 to 300 K. The FMR linewidth is almost independent of temperature, which is in agreement with the Raikher model describing the magnetic resonance of uniaxial magnetic particles. It is found that the temperature dependence of the anisotropy constant is linear. This dependence can be associated with the difference in the coefficients of thermal expansion of the Si (111) substrate and the ion-beam-synthesized cobalt silicide films.