ULTRAFINE MAGHEMITE PARTICLES .1. STUDIES OF INDUCED MAGNETIC TEXTURE

A sample with an orientational magnetic texture was prepared by freezi ng a ferrofluid containing maghemite particles in a magnetic field. Th e degree of alignment of the easy directions obtained by field-cooling was examined by Mossbauer spectroscopy for varying strengths of the f reezing field. We compare the results with the predictions of a simple model assuming that the intrinsic magnetic anisotropy in the particle s is uniaxial and that the particles are non-interacting. Good agreeme nt between the experimental results and the theoretical model is found . This allows a determination of the magnetic anisotropy energy consta nt of the particles, which is an important parameter in the model. The anisotropy energy constant is also determined from the reduction of t he magnetic hyperfine field relative to the saturation value caused by collective magnetic excitations, as well as from a decay of remanence measurement. Good agreement between the estimates of the anisotropy e nergy constant by the three methods is found.