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 λ₁₁₁ and λ₁₀₀ are different in sign and practically constant over this entire range (λ₁₁₁≈+80×10^-6; λ₁₀₀≈-20×10^-6) the change in direction of easy magnetization which occurs at 130^∘{}K is manifested by a change in structure of λ 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 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.