Spin-wave theory of impurity states in a Heisenberg antiferromagnet I. Positive impurity-host exchange coupling

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Abstract
The linear spin-wave theory of a single magnetic impurity in a body-centred Heisenberg antiferromagnet is presented. The impurity-host exchange is assumed positive and hence in the ground state the impurity spin is aligned parallel with the spins on the same sublattice. The calculation consists of deriving the two-time thermal Green functions for the sublattices from their equations of motion. From the poles of the Green functions the criteria for the impurity modes are obtained and calculations of the energies, widths and spin contractions for the modes are given. Because the inelastic neutron cross section can be expressed in terms of spin-spin correlation functions it is readily calculated from the Green functions. Particular attention is paid to the diffuse contribution to the cross section.