Localized Spin Wave States in Ferrimagnets

Abstract

The localized spin wave states in ferrimagnets with a single substituted impurity spin are investigated within the linear spin wave theory. The cases both of a ferromagnetically coupled impurity spin and of an antiferromagnetically coupled one are discussed. Numerical analysis is performed for the one-dimensional lattice, in which there exist two types of localized modes, s and p modes. We calculate the energies and spin deviation amplitudes of the localized spin wave modes. The criteria for the appearance of the localized modes are also examined. In particular we are interested in the localized mode in the energy gap between the acoustic and optical spin wave bands. There exist hole modes in the gap under a certain condition. It is found that the localized mode in the gap behaves differently from the modes in the other energy regions.