Impurity Exchange in Rutile Structure Antiferromagnets : Local Modes

Abstract

There is evidence that there is significant exchange coupling of an impurity in a rutile structure antiferromagnet to spins other than its next nearest neighbors, in contrast to the situation in the pure materials. In this paper two modifications of the coupling scheme are considered and an analysis is made of the resultant alterations in the localized magnon spectrum. The numbers and symmetries of these local modes are relatively unambiguous indicators of the from of the exchange, and their energies will then determine experimentally the coupling strengths. Thermodynamic Green's function techniques are used; decoupling in the RPA gives approximate predictions of the temperature dependence of the local mode energies. However, numerical results are given here only for zero temperature, and these are discussed in terms of a simple classical picture.