Magnetically Coupled Impurities in a Linear-Chain Heisenberg Ferromagnet

Abstract

Utilizing the technique of Green functions, the states of the linear-chain Heisenberg ferromagnet with a single substituted impurity are studied in the linear spin-wave approximation. The most important features are the appearances of localized states outside the spin wave band and resonant states in the band. For a ferromagnetically coupled impurity, the problem has been extensively treated. An approximate spin-wave analysis for the antiferromagnetically coupled impurity yields ground state energies up to 50% higher than those obtained in an exact solution of single-spin deviation states for the case of impurity spin ½. Allowing arbitrary host and impurity spins, exchange constants, and Landé factors, and an external magnetic field of arbitrary magnitude, we obtain the exact single-spin deviation states. This is achieved by using the fully aligned state as the state of no excitations. Because the spin-deviation excitation energies are negative for states below the spin-wave band for an extended range of parameters, a state of higher spin deviation can be the ground state. Considering the mixing of higher spin deviation states, a variational calculation of the ground state is being performed.