Effect of frustration on the magnetization in nickel ferrite-chromites

A study is reported on the behavior of the isotherms of the magnetization σ(H) and of the longitudinal λ_‖(H), transverse λ_⊥(H), volume ω(H), and anisotropic λ_t(H) magnetostrictions measured at T=80 K in the Cu_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ and Zn_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ ferrite-chromites having a frustrated magnetic structure. It has been established that these ferrite-chromites do not undergo technical magnetization and that the growth of the magnetization with the field is accounted for by two paraprocesses of different natures.     

Time-dependent magnetization in co-precipitated cobalt ferrite

A study of the magnetic aftereffect in co-precipitated cobalt ferrite is presented. Measurements of the magnetic viscosity S were performed at room temperature along the demagnetization curve for different applied fields H_ap over a wide range of fields (0 kOe<H_ap<−7 kOe). The interrelation function η=(∂M_rev/∂M_irr)_Hi between the DCD reversible M_rev and irreversible M_irr magnetization components was determined as well. The experimental results for S_η(H_i), where H_i is the internal field, showed a broad distribution with a maximum at H_i=2.7 kOe. However, the irreversible susceptibility χ_irr displays a maximum at H_c=0.75 kOe, the coercivity of the material. The experimental behavior of η and the non-proportionality between S_η and χⁱ_irr suggest that the magnetic viscosity in this material is principally supplied by events of nucleation of inverse domains and the depinning of domain walls. When the main mechanism of reversal magnetization changes to rotation of magnetic moments for all the grains, the magnetic viscosity decreases.     

Some comments on magnetization reversal in uniaxial ferrite magnets

The interpretation of Hadjipanayis et al. of their experimental results on magnetization reversal in ferrite magnets is critically analyzed on the basis of the present knowledge of the mechanisms governing magnetization, magnetization reversal and coercivity, both in single and polydomain particles.     

Influence of induced anisotropy on the processes of magnetization reversal of cobalt circular nanodots

The magnetic structure and the processes of magnetization reversal of individual cobalt nanodots and arrays of cobalt nanodots have been studied using the magneto-optical Kerr effect and magnetic force microscopy. Arrays of nanodots have been prepared by ion etching from a continuous cobalt film. Magnetic anisotropy is induced during deposition of the cobalt films. The nanodots have the diameter d = 600 nm and the period varying from 1.5d to 3.0d. Magnetic force microscopy images have shown that the induced magnetic anisotropy affects the orientation of magnetization of noninteracting nanodots and the direction of displacement of the magnetic vortex center in the nanodots coupled by the dipole-dipole interaction.     

The origin of frustration of magnetic coupling in the NiFeCrO<Subscript>4</Subscript> ferrite

The magnetostriction of the NiFeCrO₄ ferrite is investigated for the first time. It is found that the frustration of magnetic coupling occurs only in the B sublattice of the NiFeCrO₄ ferrite, whereas the A sublattice has a usual magnetic structure. The inference is made that the frustration of magnetic coupling in the B sublattice is caused not only by the negative direct BB exchange interaction Cr _B ³⁺ -Cr _B ³⁺ but also by the positive indirect AB exchange interaction Fe _A ³⁺ -O^2?-Cr _B ³⁺ . Reasoning from the experimental data and an analysis of the exchange interactions in the NiFeCrO₄ ferrite sample, it is demonstrated for the first time that the magnetic moments of Fe _A ³⁺ ions in this ferrite deviate from collinearity. It is established that, at low temperatures, the B sublattice of the NiFeCrO₄ ferrite is responsible for the total magnetic moment n_0exp.     

Stress effect on the magnetization process in Ni-Cu-Zn ferrite in a weak field

The effect of stress on the magnetization process for Ni-Cu-Zn ferrite was studied by the measurement of the nonsaturated hysteresis loop under external applied stress. A weak field was applied to estimate the effect of stress during the reversal magnetization process. The experimental results show that the nonreversible reversal magnetization process is affected considerably by the compressive stress in the applied field from 0.20 to 0.84 Oe. The compressive stress acts as positive field to impede the nonreversible process which takes place during the reversal magnetization process. The tensile stress had a counter effect but the effect was too small. Stress perpendicular to detecting direction seems to exhibit an effect opposite to that of parallel stress. The sample was toroidal to estimate the differencein the effect of stress between detecting direction and radial direction perpendicular to the detecting direction. The effect of stress in the detecting direction is larger than in the radial direction for the Ni-Cu-Zn ferrite with negative magnetostriction.     

Influence of rate of change of magnetization processes on sensitivity of magnetic adaptive testing

The rate of change of magnetizing field (field-slope), applied for the nondestructive method of magnetic adaptive testing, influences both signal-to-noise ratio and sensitivity of the chosen magnetic parameters with respect to the investigated degradation of the ferromagnetic material (degradation functions). Dependence of the degradation functions sensitivity on the field-slope is analyzed in this paper. It is shown that whereas sensitivity of the top-responsive degradation functions from around the top permeability of the nondegraded (reference) material drops down with increasing field-slope, sensitivity of the mild-responsive degradation functions from regions with lower permeability of the reference material is frequently field-slope-independent. The most favorable choice of the best degradation functions and of the proper magnetizing field-slope remains to be a question of optimum adaptation of the tests both to the investigated material and to the applied measuring technique.     

Zn-doping effect on the energy barrier to magnetization reversal in superparamagnetic nickel ferrite nanoparticles

Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni_1-xZn_xFe₂O₄ nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature T̄_B were obtained from the characteristic temperature behavior of the imaginary susceptibility χ_imag. An Arrhenius-type law, which accurately describes the relationship between the observation time τ_obs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal U_eff. A Zn-content dependence of the energy barrier is observed, where U_eff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of U_eff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins in the A and B sub-lattices of the ferrimagnetic inverse spinel. PACS 75.50.Bb; 75.50.Gg; 75.30.Et     

Temperature dependent coercivity and magnetization of nickel ferrite nanoparticles

Magnetic nanoparticles of nickel ferrite (size: 24±4 nm) have been synthesized by chemical coprecipitation method using stable ferric and nickel salts. Coercivity of nanoparticles has been found to increase with decrease in temperature of the samples. It has been observed that the coercivity follows a simple model of thermal activation of particle’s moment over the anisotropy barrier in the temperature range (10-300 K), in accordance with Kneller’s law for ferromagnetic materials. Saturation magnetization follows the modified Bloch’s law in the temperature range from 300 to 50 K. However, below 50 K, an abrupt increase in magnetization of nanoparticles was observed. This increase in magnetization at lower temperatures was explained with reference to the presence of freezed surface-spins and some paramagnetic impurities at the shell of nanoparticles that are activated at lower temperatures in core-shell nickel ferrite nanoparticles.     

Influence of magnetic anisotropy constant and particle domain magnetization on magneto-dielectric response of substituted manganese ferrite particles dispersed in kerosene

This paper presents a dielectric and magneto-dielectric study of magnetic fluids made of three different, partially substituted, manganese ferrite particles dispersed in kerosene. Measurements were performed using a wholly automated spectrometer in the frequency range 1 mHz–10 MHz and a temperature range from −200°C up to 100°C. We can distinguish the two phases (solid and liquid) in pure kerosene and three phases for all the magnetic fluid samples. We observed the effect of anisotropy constant and domain magnetization of the particles on magneto-dielectric measurements. We also observed the temperature sensitivity of these fluids.     

Free magnetization oscillations in garnet ferrite films with quasi-planar anisotropy

Free magnetization oscillations in garnet ferrite films with quasi-planar anisotropy was studied. The oscillations were excited by a pulse of an in-plane magnetic field. An analytic expression relating the oscillation frequency to the film parameters and the external magnetic field was derived; the expression is in good agreement with the experimental data. The planar anisotropy is shown to increase the free-oscillation damping.     

Fractal model of magnetization reversal in a strained garnet ferrite film

The domain structure in strained garnet ferrite films and its behavior in an external magnetic field are studied using the Faraday effect. Based on the experimental results, a model of magnetization reversal in thin polycrystalline layers is proposed that describes the process of remagnetization as the development of fractal clusters. The model proposed is verified using a computer simulation of magnetization reversal.     

Influence of relaxation phenomena in dynamic magnetization processes on magnetic power losses of non-oriented electrical steel sheets

We have reported that the magnetic power losses per magnetizing cycle change with the magnetization pausing time in a constant rate of magnetization change and some relaxation phenomena exist in the magnetization pausing period. We discuss the influence of the relaxation phenomena on the dynamic magnetic power losses and suggest that the relaxation phenomena seems to be causes of the frequency dependence of the magnetic losses and of the variation of losses that cannot be expressed only by magnetization speed dependence of losses.     

Nonlinear precession dynamics of magnetization in (111) garnet ferrite films

The nonlinear dynamics of magnetization precession in perpendicular-magnetized (111) garnet ferrite films is studied by numerically solving the equations of motion of magnetization. Bifurcational changes in the magnetization precession and dynamic-bistability states are detected. The conditions are found under which both regular and stochastic large-amplitude dynamic regimes arise.     

Effect of the number of nanodisks in two-dimensional arrays on magnetization reversal processes

The magnetic properties of nanodisks packed into square arrays with various numbers of elements in the face have been studied. It has been shown that the vortex nucleation field oscillates with increasing number of nanodisks; the oscillation behavior depends on the nanodisk thickness. The synchronism of the vortex state formation varies with increasing number of nanodisks. The effect of the magnetostatic interaction of nanodisks on the critical fields of magnetization reversal has been estimated for the cases of vortex and single-domain states.     

Domain walls, magnetization, and magnetoelectric effect in bismuth ferrite films

The specific features of the antiferromagnetic domain structure, magnetization, and polarization induced by an inhomogeneous micromagnetic distribution in films of bismuth ferrite multiferroics have been investigated. It has been shown that the magnetic domain structure correlates with the ferroelectric domain structure, and the character of the rotation of the antiferromagnetic vector depends on the type of ferroelectric domain walls. An asymmetry in the distribution of the antiferromagnetic vector has been observed for the cases of 109° and 71° ferroelectric domain walls. It has been demonstrated that there are differences in the distributions of the polarization and magnetization in bismuth ferrite films with ferroelectric domains separated by 109° and 71° walls. The basic mechanisms responsible for the magnetization in domain walls in multiferroics have been considered.