Evolution With Temperature of the Magnetic Anisotropy of Ba Ferrite Particles

The magnetic anisotropy of pure and Co/Ti-doped Ba ferrite particles is analyzed through the evaluation of the dependence on temperature of the constants of magnetocrystalline and shape anisotropy, which both are present in the platelet-like Ba ferrite particles with hexagonal structure. In undoped Ba ferrite, the magnetocrystalline anisotropy constant is predominant on the conflicting shape anisotropy constant at all temperatures, which indicates that the magnetic anisotropy is uniaxial, with preferred direction for the magnetization along the c axis of the hexagonal particles. In doped particles, where the magnetocrystalline anisotropy is weakened by the ionic substitutions, while at high temperatures the magnetic anisotropy is substantially uniaxial with c as axis of easy magnetization, when the temperature decreases, the shape anisotropy constant becomes larger than the magnetocrystalline anisotropy constant, and consequently, the magnetic anisotropy is not uniaxial, but it presents multiple preferred directions for the magnetization.     

Small-Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets Near Saturation

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function S_H can be extracted from experimental SANS data. A sum rule for S_H suggests measurement of the volumetric mean square anisotropy field. When magnetocrystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of S_H.     

Microwave properties of polymer composites containing combinations of micro- and nano-sized magnetic fillers

We investigated the microwave absorbing properties of composite bulk samples with nanostructured and micron-sized fillers. As magnetic fillers we used magnetite powder (Fe3O4 with low magnetocrystalline anisotropy) and strontium hexaferrite (SrFe12O9 with high magnetocrystalline anisotropy). The dielectric matrix consisted of silicone rubber. The average particle size was 30 nm for the magnetite powder and 6 micro/m for the strontium hexaferrite powder. The micron-sized SrFe12O19 powder was prepared using a solid-state reaction. We investigated the influence of the filler concentration and the filler ratio (Fe3O4/SrFe12O19) in the polymer matrix on the microwave absorption in a large frequency range (1 / 18 GHz). The results obtained showed that the highly anisotropic particles become centers of clusterification and the small magnetite particles form magnetic balls with different diameter depending on the concentration. The effect of adding micron-sized SrFe12O19 to the nanosized Fe3O4 filler in composites absorbing structures has to do with the ferromagnetic resonance (FMR) shifting to the higher frequencies due to the changes in the ferrite filler's properties induced by the presence of a magnetic material with high magnetocrystalline anisotropy. The two-component filler possesses new values of the saturation magnetization and of the anisotropy constant, differing from those of both SrFe12O1919 and Fe3O4, which leads to a rise in the effective anisotropy field. The results demonstrate the possibility to vary the composite's absorption characteristics in a controlled manner by way of introducing a second magnetic material.     

Effect of annealing on magnetization process of Ni3Fealloy

The magnetocrystalline anisotropy, the magnetostriction, and the structure-sensitive properties in the magnetization process were studied in the ordered-disordered states of Ni₃Fe alloy. The single crystals were cooled very slowly at the rate of 1°C/day to obtain the fully ordered state. Magnetization curves and hysteresis loops were measured using picture frame specimens. The magnetocrystalline anisotropy and the magnetostriction were also measured in disk specimens. The magnetic domain was also observed. The experimental results obtained in the ordered state are considerably different from those obtained by previous investigators. The difference is attributed to the degree of order-the magnetic properties obtained by previous investigators are those of a partially ordered state     

Effect of annealing on magnetization process of Ni3-xFe1+x (-0.2<x<0.2) alloys

The magnetocrystalline anisotropy, the magnetostriction and the structure-sensitive properties in the magnetization process were studied in the ordered and disordered states of specimens between 70.1 wt% Ni and 79.7 wt% Ni. The single crystals were cooled at the rate of 1 K/day to obtain the fully ordered state. Magnetization curves and hysteresis loops were measured using picture frame specimens. The magnetocrystalline anisotropy and the magnetostriction were also measured in disc specimens. New experimental results obtained in the order state are considerably different from those obtained by previous investigators. The difference is attributed to the degree of order and the magnetic properties of the previous investigators are those of a partially ordered state     

Structure,synthetic methods,magnetic properties and biomedical applications of ferrofluids

This paper is aimed at conducting a survey of the synthetic methods and magnetic properties of nanoparticles as ferrofluids used in biomedicine. As compared with other works in the field, the distinctive feature of the current work is the systematic study of recent advances in ferrofluids utilized in hyperthermia and magnetic resonance imaging (MRI). The most important feature for application of ferrofluids is super-paramagnetic behavior of magnetic cores with relatively high saturation magnetization. Although Fe₃O₄ nanoparticles have traditionally been used in medicine; the modified Mn-ferrite has recently received special attention due to its higher saturation magnetization and r₂-relaxivity as a contrast agent in MRI. Co-ferrite nanoparticles are also good candidates for hyperthermia treatment because of their high coercivity and magnetocrystalline anisotropy. The thermal decomposition and hydrothermal methods are good candidates for obtaining appropriate super-paramagnetic particles.     

Ferromagnetic Resonance Studies of Exchange Biased CoO/Fe Bilayer Grown on MgO Substrate

Dynamic and static magnetizations of an exchange biased bilayer system which is constructed as a proximity of a CoO layer on an Fe-layer grown on the (100) oriented MgO substrate by ion beam sputtering technique have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The room-temperature FMR measurements reveal that the Fe layer is epitaxially grown on MgO substrate with four-fold magnetocrystalline anisotropy and the hard magnetization axis of the sample is the [100] crystallographic directions of MgO substrate. We have determined the g-value, effective magnetization, magnetocrystalline anisotropy constants and contributions to FMR linewidth due to the intrinsic Gilbert damping and inhomogeneity of magnetization by using Landau–Lifshitz–Gilbert (LLG) equation. We observed an unusual FMR line shape attributed to impedance switching of resonance cavity and complex component of conductivity of sample system. The low-temperature FMR measurement shows asymmetric hysteretic behavior of resonance field related to magnetic coupling of ferromagnetic and antiferromagnetic layers. From both FMR and VSM measurements between 10–300 K, the magnetocrystalline anisotropy is observed to dominate above blocking temperature, while unidirectional anisotropy is observed to dominate below blocking temperature over internal magnetic anisotropy. FMR spectra have a comparatively small linewidth between 40–100 Oe, which indicates to a high crystallinity of the Fe film. Gilbert constant was calculated as 0.007 from the linewidth fitting of FMR spectra. This small value is a suitable for reducing the critical switching current used in magnetic tunneling junction. Detailed exchange bias studies were carried out for hard and easy axis of the sample in the temperature range of 10–300 K. From both low-temperature FMR and VSM measurements, the blocking temperature of the system was determined as ～60 K.     

Crystallographic and magnetic investigation on W-type-hexaferrite single crystals in the solid solution series SrZn2-xCoxFe16O27

The saturation magnetization and the magnetocrystalline anisotropy were measured on single crystals in the solid solution series SrZn2-xCoxFe16O27(SrZn2-xCox-W) at 298 K and 6 K. Lattice constants and Curie-temperatures are also given as a function of the composition. The magnetocrystalline anisotropy changes from uniaxial to planar dependent on temperature and cobalt substitution atx approx 0.5for 6 K andx approx 0.85for 298 K. The six-fold anisotropy in the basal plane of the planar W-hexaferrites increases strongly with increasing cobalt content. Crystals with a low Co substitution (x = 0.33 and 0.67) have an anomalous discontinuity in their hard direction magnetization curves; indicating a first-order magnetization process.     

Low Temperature and High Magnetic Field Dependence and Magnetic Properties of Nanocrystalline Cobalt Ferrite

The DC magnetic hysteresis loop measurements were carried out for temperatures varying from 5 to 300 K over a field range of ±10 T on nanocrystalline (～35 nm) cobalt ferrite samples (crystallized to \(Fd\bar {3} m\) space group with cubic symmetry) to validate the law of approach at low temperature for the nanocrystalline cobalt ferrite. A magnetocrystalline anisotropy constant and saturation magnetization have been obtained by analyzing the magnetization curve in saturation using the “law of approach (LA) to saturation.” The magnetocrystalline anisotropy constant is found to be almost constant in the temperature range of 5 to 150 K due to the freezing of spin at low temperature. Also, spin freezing leads to a decrease of coercivity with the increase in the temperature.     

On the stability of certain magnetic modes in fine ferromagnetic particles

Conditions of stability of the magnetization curling mode in fine spherical and cylindrical particles and of the magnetization helicoid structure in fine cylindrical particles are derived, and magnetization reversal processes in such particles are considered. The application of Ritz's method for solving the variational problem of finding the local magnetization vectors in the volume of a particle provides a convenient way of finding the conditions mentioned above. It has been shown that the magnetization curling mode in a spherical fine particle with radius larger than a critical one is stable in a well-determined interval of the external magnetic field, provided the magnetocrystalline anisotropy constant is less than a certain value K0. In such a case the hysteresis loop of a spherical particle measured in an easy direction is no longer rectangular. The magnetization curling and helicoid modes are both unstable in fine cylindrical particles with positive or zero values of the magnetocrystalline anisotropy constant. The hysteresis loop of such particles magnetized along the axis of rotation is rectangular.     

Co-Fe metal/native-oxide multilayers: a new direction in soft magnetic thin film design I. Quasi-static properties and dynamic response

The progression toward gigahertz data rates in magnetic recording has introduced considerable challenges to soft magnetic materials design. The difficulties lie in satisfying two sets of conflicting demands: 1) simultaneously achieving soft magnetic properties, high saturation magnetization, and a high resistivity, with the latter required to limit eddy-current losses and 2) balancing the inherent tradeoff between bandwidth and permeability imposed by the direct and inverse dependences, respectively, of these two parameters on the anisotropy field. This paper describes a new soft magnetic composite system that meets these requirements: a metal/native-oxide multilayer (MNOM) film consisting of nanogranular high-moment Co/sub x/Fe/sub 100-x/ layers separated by ultrathin magnetic native oxide layers. The high-resistivity magnetic oxide layers isolate the metallic layers electrically, while coupling them magnetically and minimizing the decrease in volume-averaged saturation magnetization that exists in traditional metal/nonmagnetic oxide composites. In addition, the "exchange-averaged" soft magnetic properties of the MNOM composite include an ideal low-dispersion in-plane uniaxial anisotropy whose magnitude varies linearly with the fraction x of Co in the alloy. The resulting anisotropy control, together with the large saturation magnetization, permits the permeability and resonance frequency to be tuned over a wide range to meet specific application requirements.     

Directional solidification and magnetic properties of MnSb–Sb eutectic composite

The microstructures, solidification behaviour, crystal orientation and magnetic properties of MnSb–Sb eutectic with a high entropy of solution have been studied systematically. The present approach reveals the inherent correlations between them. MnSb–Sb eutectic is characterized by a non-faceted–non-faceted type of growth and well-distributed aligned MnSb rods embedded in a Sb matrix on a micron scale, an ideal composite structure, during directional solidification. The scaling laws of the variation in interrod spacing (or rod diameter d) with growth velocity are quantitatively developed. A parallel orientation relation between [001] of MnSb and rod axis is identified through electron diffraction patterns, indicating that the highly anisotropic composite has been fabricated successfully. The hysteresis loops of directionally solidified MnSb–Sb composites are determined along the MnSb rod axis. The saturation magnetization and constant permeability have been evaluated by considering the alignment, size and magnetocrystalline anisotropy.     

The effect of texture and internal stresses on the perpendicular anisotropy of vacuum evaporated ferromagnetic films

The effect of crystallographic texture and macrostresses on perpendicular anisotropy was studied. The effect of texture is shown to be considerable for materials with high magnetocrystalline anisotropy (Co, Fe). Tensile stresses can both increase and reduce perpendicular anisotropy (in materials with positive and negative magnetostriction respectively). In the films of Fe in the presence of texture [100] contribution of stresses to perpendicular anisotropy may be positive, zero or negative depending on the degree of the texture perfection.     

Anisotropic and high magnetization rare earth transition metal compounds containing metalloids

The structural and magnetic behavior is presented for selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series. Focus is on materials with high Fe content and resulting high magnetizations. The unusual axial ratios and features of the sheet type structures of these materials have pronounced consequences on such properties as magnetic anisotropy and magnetic hardness. Individual site anisotropy contributions are studied by temperature dependence of magnetization along easy and hard magnetic axes. As an example it is found that tetragonal Nd₂Fe₁₄B has axial anisotropy with H_A = 76 kOe at 300 K but shows tendencies for a spin reorientation around 150 K. Y₂Fe₁₄B has axial anisotropy with H_A = 25 kOe but does not exhibit a similar spin reorientation. This indicates that the two crystallographic Nd sites (4f and 4g) have axis and plane preference respectively, with different temperature dependencies. Axial Nd anisotropy is a consequence of the lack of Nd coordination along the z axis due to intervening thick Fe layers. Both extrinsic (fine particle) and intrinsic magnetic hardness is observed. Crystallographically disordered materials show intrinsic hardness based on domain wall pinning by local fluctuations of magnetic parameters. Strong nucleation phenomena are characteristic for ordered materials in bulk and powder form. The unusually high achievable ratios of extrinsic coercivities to anisotropy fields in the metalloid stabilized materials are related to their chemically relatively inert layer structure. This appears to lead to less corrugated surface structures and is so responsible for the characteristic domain wall nucleation processes.     

Structure and properties of novel ternary Fe-rich rareearth-carbides

Alloying of intermetallic rare-earth compounds of the type R₂ Fe₁₇ leads to ternary carbides of the form R₂Fe₁₇C_x. In this work, the authors discuss C-induced changes in Curie temperatures, unit cell dimensions, magnetization, and magnetocrystalline anisotropy. They report on HREM (high-resolution electron microscopy) observations of stacking faults in these materials and discuss their possible influence on the attainment of coercive forces     

Simultaneous substitution of Co2+and Zn2+ions in submicronic acicular γ - Fe2O3particles

In iron sesquioxide of acicular shaded γ-Fe2O3simultaneous substitution of Co²⁺and Zn²⁺ions leads to the formation of mixed-defect ferrites and modulation of magnetic properties is of interest for their application to high density magnetic recording. It is shown that the coercive force, remanent magnetization and saturation-magnetization are controlled by a judicious choice of the contents of Co²⁺and Zn²⁺, while it is indispensable to optimize the morphological features like the average size of the crystallites, the shape, the size and texture of the particles. A high value of coercive force (650-700 Oe) and of the remanent magnetization (35-45 emu/g) had been obtained with a minimal content of cobalt ions (Co²⁺= 2.5 to 3% by wt.) permitting limits to the magnetocrystalline anisotropy of these compounds and their thermal variation near the ambient temperature. The influence of the zinc content had been systematically studied notably in relation to its effect on the structural, morphological and magnetic properties of the ferrites.     

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spin valves mechanically coupled to a ferroelectric substrate. The switching between low- and high-resistance states results from the strain-driven magnetization reorientations by about 90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weak cubic magnetocrystalline anisotropy. Such reorientations, not requiring external magnetic fields, can be realized experimentally by applying moderate electric field to a thick substrate (bulk or membrane type) made of a relaxor ferroelectric having ultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giant magnetoresistance may be employed as electric-write nonvolatile magnetic memory cells with nondestructive readout.     

静电纺丝法制备磁性纳米纤维材料的研究进展

磁性纳米纤维材料不但具有普通纳米粒子的特殊效应,而且具有独特的形状各向异性和磁晶各向异性效应,在高密度磁记录、电磁波吸收、催化剂、医学和生物功能材料等领域具有重要应用。静电纺丝技术已被证明是一种制备纳米纤维最简单有效的方法。结合最新文献,重点阐述了以静电纺丝技术为主的磁性纳米纤维制备工艺以及不同工艺对磁性纳米纤维的形貌和性能的影响。简要介绍了磁性纳米纤维的应用,指出了发展新型结构可控磁性纳米纤维材料、研究其定向排布及组装技术、开发其在各领域的实际应用是未来主要的研究方向。     

Static magnetic properties of Co and Ru substituted Ba-Sr ferrite

M-type hexagonal ferrite powders, Ba_0.5Sr_0.5Co_xRu_xFe_(12−2x)O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) have been synthesized by conventional ceramic method. Magnetic properties have been investigated as a function of substitution of Co and Ru ions at applied external field of 10 kOe. XRD and SEM revealed hexagonal structure for these ferrites. The Co and Ru ions substitution cause increase in saturation magnetization and rapid decrease in magnetocrystalline anisotropy at lower substitution. The magnetic parameters variation has been explained by taking into account preferential site occupancy of sublattice sites by substituted ions. Curie temperature decreases with substitution due to weakening of superexchange interaction. The obtained hysteresis parameters suggest that the proposed materials cannot be used for recording applications.     

The magnetic anisotropy of thin epitaxial CrO<Subscript>2</Subscript> films studied by ferromagnetic resonance

The magnetic anisotropy of thin epitaxial films of chromium dioxide (CrO₂) has been studied as a function of the film thickness by the ferromagnetic resonance (FMR) technique. CrO₂ films with various thicknesses in the range from 27 to 535 nm have been grown on (100)-oriented TiO₂ substrates by chemical vapor deposition using CrO₃ as a solid precursor. In a series of CrO₂ films grown on the substrates cleaned by etching in a hydrofluoric acid solution, the FMR signal exhibits anisotropy and is strongly dependent on the film thickness. The magnetic properties of CrO₂ films are determined by a competition between the magnetocrystalline and magnetoelastic anisotropy energies, the latter being related to elastic tensile stresses caused by the lattice mismatch between the film and the substrate. In the films of minimum thickness (27 nm), this strain-induced anisotropy is predominant and the easy magnetization axis switches from the [ 001] crystallographic direction (characteristic of the bulk magnet) to the [ 010] direction.