Anisotropy and Magnetostriction in Magnetic Oxides

Recent theories of the anisotropy and magnetostriction due to the presence of ${\mathrm{Co}}^{\text{2+}}$ in spinel ferrites are reviewed and new work on the effects of ${\mathrm{Fe}}^{\text{2+}}$ is presented. The effects of these ions are great because the orbital angular momentum is not fully quenched by the crystal field. In the case of ${\mathrm{Co}}^{\text{2+}}$ , the orbital moment of the ground state is coupled strongly to the trigonal axis of the crystal field. The very large anisotropy and magnetostriction arise from spin-orbit coupling in first order. The theory is found to account for many of the effects of cobalt-iron ferrite and cobalt-manganese ferrite at higher temperatures. The contribution of ${\mathrm{Fe}}^{\text{2+}}$ to these effects is less extreme but still striking. Here the orbital moment is more nearly quenched but the spin is still coupled strongly to the trigonal axis. The coupling gives rise to cubic anisotropy in second order approximation. This term appears to account for the large compositional dependence of anisotropy in manganese-rich manganese-iron ferrite. A crystal-field calculation of the orthorhomic distortion of the low-temperature form of magnetite is found to be consistent with observations at about 80°K, assuming a high degree of Verwey ordering. However, the anisotropy calculated on the same assumption is found to be inconsistent with experiment. It is concluded that the one-ion crystal-field theory is largely applicable to the effects of ${\mathrm{Co}}^{\text{2+}}$ and ${\mathrm{Fe}}^{\text{2+}}$ in ferrites. However, it does not work when the concentration of ${\mathrm{Fe}}^{\text{2+}}$ is high.