On the Magnetic Anisotropy Energy of FeF₂

The magnetic anisotropy energy of FeF2 deduced from the measurement of Stout and Matarrese on the magnetic susceptibility of the single crystal has a large value of 5.3(M/M0)2 cm-1 per Fe ion, where M, (M0) is the magnetization, (the one at 0° K), and this cannot be interpreted in terms of the magnetic dipole-dipole interaction between Fe ions in the crystal. The origin of the anisotropy energy can be explained by the spin-orbit interaction in the Fe++ ion. We have calculated the second order perturbation energy due to this interaction, taking account of the crystalline Stark effect of the ground state of the Fe++ ion. The crystalline field can be determined by the anisotropy of the g-factor deduced from the experiment of Stout and Matarrese. The calculated anisotropy energy per ion due to the spin-orbit interaction is 7.6 (M/M0)2 cm-1, in good agreement with the observed value.