Theory of Spin Waves in Strongly Anisotropic Magnets

A new infinite-order perturbation approach to the theory of spin waves in strongly anisotropic magnets is introduced. The system is transformed into one with effective two-ion anisotropy and considerably reduced ground-state corrections. A general expression for the spin-wave energy, valid to any order, is given in terms of a renormalized exchange interaction and two anisotropy parameters. The relation between them and the fundamental parameters is given explicitly to second order.