V. Masheva

On the magnetic properties of nanosized CoFe2O4

Abstract Thermomagnetic properties of fine CoFeO 4 particles with average sizes of 43, 51, 66, 195 and 465 A were investigated between 5 and 800 K in fields up to 140 kOe. The particles with sizes of 43 and 51 A behave superparamagnetically at room temperature, but the others exhibit mixed superparamagnetic and ferrimagnetic behavior. The concentration of the ferrimagnetic phases and their magnetic properties were evaluated.

Fourier analysis of hysteresis loops and initial magnetization curves: application to the singular-point-detection method

Abstract Hysteresis loops and initial magnetization curves, obtained within the framework of the Stone-Wohlfarth model, and experimental ones for a system of single-domain barium ferrite particles, have been studied by Fourier analysis. A simple, but significant relation between the Fourier coefficients of the initial and hysteresis curves has been derived. The correctness of the Fourier description of magnetization curves permits many of the magnetization quantities to be easily calculated with high accuracy. It has been demonstrated that the anisotropy field can be obtained from the Fourier-described hysteresis loops, following the SPD method.

Interaction effects detected by Fourier analysis of hysteresis loops and initial magnetization curves

A new technique for the estimation of interparticle interaction effects in some magnetic systems is developed only on the basis of Fourier description of experimental hysteresis loops and initial magnetization curves, avoiding remanence measurements. This technique is applied for Ba-ferrite powders and for several currently used magnetic recording media. It is found that the results coincide fairly well with these obtained by the commonly accepted /spl Delta/I technique, using remanence curves. >

Fourier analysis of AC hysteresis loops

AC magnetization curves of a Stoner-Wohlfarth model system consisting of noninteracting uniaxial single-domain particles are described by Fourier expansion. Two types of hysteresis curves are considered-the first one with a magnetizing field changed in a sine periodical way, and the second one with the magnetization of the system changed in a sine periodical way.

Magnetization processes of small gamma-Fe2O3 particles in non-magnetic matrix

The magnetization processes of small (350 A) and nearly spherical particles of -Fe2O3 were studied at room temperature using the initial, hysteresis and remanence magnetization curves. The effective interparticle interactions were estimated for a number of virgin and alternating current demagnetized samples of -Fe2O3-SiO2 mixtures with different magnetic phase concentrations. A non-monotonous dependence of the interactions on the magnetic phase concentration was observed. It was related to the changes in the domain state of some of the particles from the single-domain to the multi-domain.

The influence of the second anisotropy constant on the Fourier description of hysteresis loops for a Stoner-Wohlfarth system

Abstract The influence of the second anisotropy constant K 2 on the Fourier coefficients of hysteresis loops for Stoner-Wohlfarth systems is studied. The corresponding diagrams over a broad range of K 2 and K 1 are built. A method for estimation of K 1 and K 2 for real systems is proposed.

Temperature dependence of magnetic anisotropy of textured polycrystalline materials

Abstract A method for the determination of the temperature dependence of several magnetic anisotropy constants (for example K 1 ( T ), K 2 ( T ) etc.) for a cubic ferromagnetic is proposed. The measurements were carried out on textured polycrystalline samples. The effective constants of the magnetic anisotropy for low-carbon cold rolled steels were determined by the torque curves for rotation about the perpendicular axis of the rolling plane for several temperatures. The K 1 ( T ) relative temperature dependence for Fe is obtained. The temperature, above which material structural changes appear is found experimentally.

Effect of Cobalt Substitution on the Magnetic Properties of Ni‐Cu Ferrite

Single‐phase polycrystalline Ni0.85−xCu0.15CoxFe2O4 (x = 0, 0.02, 0.04 and 0.06) were synthesized by a standard ceramic technology. The mean particle sizes, obtained by SEM, vary between 2 and 8 μm. The main magnetic parameters such as saturation magnetization, remanence and coercivity were obtained from the entire hysteresis loops plotted in AC (2 kHz) magnetic field at room temperature. Hysteresis losses were evaluated from the same curves by using the Fourier decomposition. The initial permeabilities were obtained from the minor hysteresis loops, plotted in low enough magnetic fields. The remanence decreases almost linear with increasing Co‐concentration. Both saturation magnetization and initial permeability have maxima at x = 0.02, but the coercivity and hysteresis losses have minima at the same Co‐concentration. All the magnetic parameters, obtained at x = 0.02 are better than those of the starting composition with x = 0. The influence of the Co‐substitution on the magnetic parameters is discussed.

Magnetic Properties of Ni‐Cu‐Zn Ferrites with Co Substitution

Single‐phase polycrystalline Ni0.85−x−yCu0.15Znx Coy Fe2O4 (x = 0.2, 0.4, 0.6; y = 0, 0,02, 004, 006) were synthesized by a standard ceramic technology. The mean particle sizes range between 2 and 30 μm. The main magnetic parameters such as saturation magnetization, remanence and coercivity were obtained from the entire hysteresis loops plotted in AC (2 kHz) magnetic field at room temperature. Saturation magnetization decreases slightly with increasing Co‐concentration for x = 0.2, 0.4 only. The remanence decreases with increasing Co‐concentration for all the compounds, but the coercivity is practically constant for most of the samples. Hysteresis losses were evaluated from the hysteresis loops by using the Fourier decomposition. The initial permeabilities were obtained from the minor hysteresis loops in low enough fields. It was found that: only for x = 0.4 and y = 0.04, the initial permeability increases, hysteresis losses decrease and the changes of other magnetic parameters are negligible.

Hysteresis Loops and Remanence Curves of Small γ-Fe2O3 Particles with Different Packing Densities

Small (≈ 350A) and nearly spherical particles of γ-Fe2O3 were studied by XRD, TEM, Mossbauer spectra and RT magnetization measurements. The effective interparticle interactions for a number of γ-Fe2O3 - Al2O3 mixtures with different concentrations of magnetic phase were estimated from IRM and DCD remanence measurements, and from Fourier analysis of initial magnetization and hysteresis curves. The non-monotonous dependence of the interactions with decreasing concentration of magnetic phase was observed. It was related to possible changes in the domain status of some of the particles with increasing the distances between them.