A general method for the evaluation of lattice sums determining the effective parameters in the Hamiltonian of a dipolar magnetic system is described. The anisotropy of the Hamiltonian is examined for crystal structures of tetragonal and hexagonal type. The results are of particular relevance in systems where exchange and any other nondipolar interactions are isotropic. Applications to gadolinium are considered.

• Received 23 March 1987

DOI:https://doi.org/10.1103/PhysRevB.36.8512