The Magnetic Anisotropy Constant in the Case of Strong Anisotropy

A correct perturbational approach to the calculation of the magnetic anisotropy energy is presented. The theory is applicable even to the case of extreme anisotropy. The anisotropy energy is to be evaluated from the total (free) energy in a given direction of the magnetization vector. It is emphasized that one has to take into account, particularly in the higher order perturbation calculation, the presence of a thermo-dynamic magnetic field which fixes the magnetization vector in the given direction, because otherwise the magnetization vector deviates from the assumed direction, producing a false energy change. As an application the anisotropy energy in FeF 2 is calculated. The present theory yields a consistent explanation of both the magnitude and the temperature dependence of the observed anisotropy energy in FeF 2 .