Magnetocrystalline Anisotropy of Low Temperature Phase of Magnetite

Abstract

The magnetocrystalline anisotropy of low temperature phase of magnetite (Fe₃O₄) was measured for a monoclinic single phase specimen by using computerized fully-automatic torque magnetometer. The anisotropy is expressed by
E_a=K_aα_a²+K_bα_b²+K_aaα_a⁴+K_bbα_b⁴+K_abα_a²α_b²−K_uα₁₁₁²,
where α_a, α_b and α₁₁₁ 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_aa=1.8, K_bb=2.4 and K_ab=7.0 in 10⁵ erg/cm³. 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_aa, K_bb and K_ab are well expressed in terms of cubic K₁. The mechanism of the anisotropy is also discussed.