Magnetocrystalline Anisotropy of Magnetite at Low Temperature

Abstract

The magnetocrystalline anisotropy of magnetite has been measured by the torque method between 4.2 and 110°K. The anisotropy energy of the orthorhombic phase which exists below 114.5°K is represented by the formula $E_a=K_a{{\alpha }_a}^2+K_b{{\alpha }_b}^2+K_{\mathrm{aa}}{{\alpha }_a}^4+K_{\mathrm{ab}}{{\alpha }_a}^2{{\alpha }_b}^2+K_{\mathrm{bb}}{{\alpha }_b}^4$, where ${\alpha }_a$ and ${\alpha }_b$ are the direction cosines of the magnetization with respect to the hard and intermediate axes, respectively. The five anisotropy coefficients were determined from torque values measured in the cubic (001) and (011) planes in fields as large as 25 000 G. The values of $K_a$ and $K_b$ confirm the conclusion of Slonczewski that the anisotropy of the orthorhombic phase cannot be accounted for simply by a consideration of magnetic dipole, spin-orbit, and intra-atomic spin-spin interactions. With the exception of $K_b$, the anisotropy coefficients show little temperature dependence. An increase of $K_b$ by more than 40% between 4.2 and 110°K is attributed to local disorder which is generated with an activation energy of 0.016 eV.