Origin of the Magnetic Anisotropy Energy of the Europium Monochalcogenides

The origin of the magnetic anisotropy energy of the europium monochalcogenides has been investigated. The contribution from the magnetic dipolar interaction was considered, in addition to the single-ion anisotropy, and the expression for the temperature dependence of the cubic anisotropy constants $K_1$ and $K_2$ is derived for various spin values using the molecular-field approximation. It is found that for large spins both $K_1$ and $K_2$ increase with temperature, attain a maximum, and then decrease smoothly to zero at the Curie temperature. The origin of the above temperature dependence is discussed. The experimental results for the magnetic-anisotropy measurements are explained in terms of the dipolar interaction and the single-ion anisotropy. The agreement between theory and experiment provides the first clear example of the importance of the dipolar anisotropy energy in cubic ferromagnets.