Localized Spin Waves in Heisenberg Ferromagnets with an Impurity Spin

Abstract

The excitation energies of the localized modes in a Heisenberg ferromagnet with an impurity spin are calculated by the spin wave method. The impurity spin has a large uniaxial anisotropy and the different easy axis from that of host spins. Two localized modes are obtained. One of them has a positive energy irrespective of the anisotropy of the impurity spin, while the other has the energy whose sign depends upon the magnitude of the anisotropy of the impurity spin.