Bose Operator Expansions of Tensor Operators in the Theory of Magnetism

For pt.I see ibid., vol.7, p.1523 (1974). The matching of matrix element method is used to find a new self-consistent Bose operator expansion for tensor operators in spin systems with isotropic exchange interaction plus anisotropy. Tables are given for all tensor operators relevant for cubic and hexagonal symmetry. A discussion of renormalized spin-wave theory for a system with planar anisotropy shows that the Goldstone theorem is rigorously fulfilled to the considered order of perturbation. It is finally shown that the new expansion introduces wavevector-dependent terms from the single-ion anisotropy into the dispersion relation.