Cubic magnetic anisotropy of nonstoichiometric magnetite

The cubic magnetic anisotropy constants of ${\mathrm{Fe}}_{3\left(1\mathrm{-}\mathrm{\delta }\right)}$ ${\mathrm{O}}_4$ single crystals (with 0.000<δ<0.006), obtained by curve fitting of the respective magnetization curves, are reported between room temperature and the isotropic point (∼130 K), where the easy axis changes from [111] to [100]. Two distinct temperature dependences are observed for nonstoichiometries above and below ${\mathrm{\delta }}_{\mathit{C}}$=0.0039, in samples with second- and first-order Verwey transitions, respectively. The characteristic temperatures for the first-derivative curves are consistent with mean-field parameters for nearest-neighbor-interaction models.