Comparative measurements of the effective anisotropy field Ha for barium ferrites

Abstract

BaFe 12O 19 powders were prepared by coprecipitation and by glass crystallization. Electron microscopic investigations revealed that the particles of the glass crystallized powder exhibit excellent uniform shapes and narrow size distribution in comparison to the coprecipitated one consisting of irregularly shaped, polycrystalline particles with a broad and inhomogeneous size distribution. The crystallites of the coprecipitated powder particles, however, are smaller (mean diameter D = 0.11 μm) and the size distribution is narrower than that of the particles of the glass crystallized powder with a mean diameter of D = 0.42 μm. The lattice perfection of the particles and the crystallites of both ensembles is quite good. In order to determine the magnetic properties — above all the effective anisotropy field — we used six different methods: transverse susceptibility χ t, singular point detection (SPD), ferromagnetic resonance (FMR), torsion pendulum (TP), rotational hysteresis losses (RHL), remanence curves (RC). It has been found that differences in coercivity values H cJ are not dramatic (about ±3%). Effective anisotropy fields H a measured with “switching” methods (RHL, RC) are smaller than those measured with “stiffness” ones (FMR, TP, SPD, χ t), by about 1700 Oe for the coprecipitated powder and about 2500 Oe for the glass crystallized one. The glass crystallized particle assemblies used for the present investigations do not exhibit pure Stoner-Wohlfarth behaviour, because of their relatively large particle diameters. The ratio H cJ/ H a ≈ 0.39 of the coprecipitated powder is relatively near the ideal value of 0.48, although the particles are considerably mechanically aggregated.