SPIN WAVE IMPURITY STATES IN LINEAR CHAIN FERROMAGNETS AND ANTIFERROMAGNETS

A simple linear chain model is used to find the perturbed states in Heisenberg ferromagnets and antiferromagnets with a magnetic impurity. The defect is characterized by a different nearest neighbor exchange coupling, spin, anisotropy field and gyromagnetic ratio from the corresponding parameters for the host lattice. A semiclassical calculation demonstrates that for some values of the defect parameters, resonant states in the spin wave band can arise near the bottom of the band, while for other values, localized states will occur. The various physical conditions for the occurrence of those states are given. When an applied magnetic field is included in the model calculation, a gap mode instability can occur for some values of the gyromagnetic ratio of the impurity spin. For some combinations of defect parameters, a local mode can be transformed into a resonant mode and vice versa by the magnetic field. The impurity-induced absorption is also calculated. © 1969 Springer-Verlag.