Magnetostriction and Permeability of Magnetite and Cobalt-Substituted Magnetite

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The principal magnetostriction constants of magnetite were determined over the temperature range from 120\ifmmode^\circ\else\textdegree\fi{}K to 300\ifmmode^\circ\else\textdegree\fi{}K by the strain gauge technique. Since ${\ensuremath{\lambda}}_{111}$ and ${\ensuremath{\lambda}}_{100}$ are different in sign and practically constant over this entire range (${\ensuremath{\lambda}}_{111}\ensuremath{\approx}+80\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$; ${\ensuremath{\lambda}}_{100}\ensuremath{\approx}\ensuremath{-}20\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$) the change in direction of easy magnetization which occurs at 130\ifmmode^\circ\else\textdegree\fi{}K is manifested by a change in structure of $\ensuremath{\lambda}$ 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\ifmmode^\circ\else\textdegree\fi{}K. The temperature shift is practically linear with respect to cobalt ferrite content, the rate being $\mathrm{ca}$ 140\ifmmode^\circ\else\textdegree\fi{}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.