Ultrafine maghemite particles. I. Studies of induced magnetic texture

A sample with an orientational magnetic texture was prepared by freezing a ferrofluid containing maghemite particles in a magnetic field. The degree of alignment of the easy directions obtained by field-cooling was examined by Mossbauer spectroscopy for varying strengths of the freezing field. We compare the results with the predictions of a simple model assuming that the intrinsic magnetic anisotropy in the particles is uniaxial and that the particles are non-interacting. Good agreement between the experimental results and the theoretical model is found. This allows a determination of the magnetic anisotropy energy constant of the particles, which is an important parameter in the model. The anisotropy energy constant is also determined from the reduction of the 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 energy constant by the three methods is found.