Spin Waves in an Easy‐Axis Ferromagnet with a Finite Concentration of Impurities with Competing Anisotropy

The spin‐wave spectrum is investigated of a Heisenberg ferromagnet with an easy‐axis single‐ion anisotropy in which a number of host atoms is substituted by impurities with an easy‐plane singleion anisotropy. It is shown for both, the case of a weak local perturbation (when one impurity atom does not break the stability of the ground state with completely parallel spins) and the case of a strong local perturbation (when a single impurity creates one or even several localized spin deviations in the ground state) that there is a gap in the spectrum of low‐lying spin excitations and the spin‐wave damping Γ k is proportional to the wave vector k near the bottom of the quasicontinuous band. The critical concentration up to which the ferromagnetic state possessing a gap in the spin‐wave spectrum remains stable is estimated.