Low-Temperature Transition of Magnetic Anisotropy in Nickel-Iron Ferrite

Abstract

A study of the magnetic properties of single crystal samples of nickel-iron ferrite with composition ${\mathrm{Fe}}_{1.00}^{3+}$[${\mathrm{Ni}}_{0.79}^{2+}$ ${\mathrm{Fe}}_{0.36}^{2+}$ ${\mathrm{Fe}}_{0.90}^{3+}$]${\mathrm{O}}_4$ has revealed an abrupt transition in the magnetic anisotropy characteristics at 10°K. Above this temperature the anisotropy energy is small and has cubic symmetry; below 10°K the anisotropy energy increases rapidly with decreasing temperature and contains a uniaxial annealing term in addition to the cubic terms. The uniaxial term is of the form $\Sigma i^{}{{\alpha }_i}^2{{\beta }_i}^2$, where ${\alpha }_i$ and ${\beta }_i$ are the direction cosines of the magnetization at the measuring temperature and at the annealing temperature, respectively. The observed characteristics of this material below the transition cannot be explained on the basis of long-range order. A model is proposed which assumes the excess cations of the sample to be located at normally vacant octahedral lattice sites. The theory of magnetic anneal introduced by Taniguchi and Yamamoto, and independently by Néel, is then applied to this model. The resulting predictions are in accord with our experimental findings.