The Pair Approximation Method (PAM) is applied to study the thermodynamic properties of a threedimensional spin lattice in a cubic structure with coordination number z = 6. The used model consists of the anisotropic ferromagnetic Heisenberg with spins 1/2 and 1 under an external magnetic field.

Within this approximation based on Gibbs free-energy, we investigate the effect of both exchange coupling anisotropies and the spin magnitude on the thermodynamic quantities such as the magnetization, the entropy, the heat capacity and the entropy change. We also study the resulting Relative Cooling Power (RCP) for the two considered spin values.

This work shows that even small amount of exchange anisotropy and spin size have a significant influence on the magnetic properties and RCP.