Magnetocrystalline Anisotropy of Low Temperature Phase of Magnetite

The magnetocrystalline anisotropy of low temperature phase of magnetite (Fe 3 O 4 ) was measured for a monoclinic single phase specimen by using computerized fully-automatic torque magnetometer. The anisotropy is expressed by \begin{aligned} E_{\text{a}}=K_{a}\alpha^{2}_{a}+K_{b}\alpha^{2}_{b}+K_{aa}\alpha^{4}_{a}+K_{bb}\alpha^{4}_{b}+K_{ab}\alpha^{2}_{a}\alpha^{2}_{b}-K_{u}\alpha^{2}_{111}, \end{aligned} where α a , α b and α 111 are direction cosines of the magnetization with respect to the monoclinic a -, b - and cubic [111] axes, respectively, the last of which coincides with the longest cube diagonal. The values of the anisotropy constants at 4.2 K are: K a =25.5, K b =3.7, K u =2.1, K a a =1.8, K b b =2.4 and K a b =7.0 in 10 5 erg/cm 3 . It was found that K a , K b and K u exhibit the temperature dependence of an activation type with the activation energy of about 0.02 eV. It was also found that the constants K a a , K b b and K a b are well expressed in terms of cubic K 1 . The mechanism of the anisotropy is also discussed.