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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.
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Lindgård et al. (1976) studied this question.
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