Magnetostriction and Permeability of Magnetite and Cobalt-Substituted Magnetite

The principal magnetostriction constants of magnetite were determined over the temperature range from 120°K to 300°K by the strain gauge technique. Since ${\lambda }_{111}$ and ${\lambda }_{100}$ are different in sign and practically constant over this entire range (${\lambda }_{111}\approx +80\mathrm{}\times {10}^{-6\mathrm{}}$; ${\lambda }_{100}\approx -20\mathrm{}\times {10}^{-6\mathrm{}}$) the change in direction of easy magnetization which occurs at 130°K is manifested by a change in structure of $\lambda$ vs $H$ curves in a polycrystalline sample. The substitution of small amounts of cobalt for divalent iron in polycrystalline magnetite causes a marked shift upwards in the temperature of the maximum in initial permeability, which in the case of magnetite occurs at 130°K. The temperature shift is practically linear with respect to cobalt ferrite content, the rate being $\mathrm{ca}$ 140°C/mole percent. Magnetostriction vs magnetic field curves for these specimens indicate that the shifted permeability peak is still associated with a change in direction of easy magnetization. The predicted anisotropy of cobalt ferrite, obtained by extrapolation of these results, is in reasonable agreement with the values measured directly by other investigators.