Effect ofSz-nonconserving interactions on local-mode dynamics in FeF2: Mn

In Mn-doped Fe${\mathrm{F}}_2$ a localized excitation exists below, but close to, the magnon continuum. With the application of an external magnetic field along the anisotropy axis, both the magnon band and the local mode undergo Zeeman splitting. When, with increasing field, the upgoing local mode becomes degenerate with the downgoing band, it broadens because of the opening of scattering channels into the continuum through $S_z$-nonconserving terms in the Hamiltonian (dipolar and orthorhombic anisotropy interactions). The contribution of each of these interactions to the local-mode linewidth is calculated, using standard Green-function techniques. Although comparison with the single available experimental point indicates that much of the linewidth arises from magnon-phonon decay, the signature of the field dependence of the contribution calculated here makes separate identification readily possible, once field-dependent experiments have been made.