Mixed anisotropies in small ferromagnetic particles

Low temperature magnetic measurements have been carried out on a system of fine cobalt particles in a solid matrix. Samples frozen in a large applied field were found to exhibit the shifted hysteresis loop indicative of exchange anisotropy. The magnetic properties of the system were found to be determined by the mixture of the unidirectional exchange anisotropy and the predominant uniaxial shape anisotropy of the particles. The exchange anisotropy requires the presence of an antiferromagnetic surface layer on the particles and this is consistent with the observed differences in particle size obtained from electron microscopy and room temperature magnetic measurements. Changes in the coercivity factor and the interaction field factor on freezing the samples in a large applied field have also been measured. The observed changes were found to be consistent with the expected changes in the switching field distribution of the particles and the local order within the system. The coercivity factor and interaction field factor are thus shown to be useful parameters in characterising the properties of fine particle systems.