The role of Sz-nonconserving interaction in the spin dynamics of impure antiferromagnetic insulators

In some antiferromagnetic insulators with dilute substitutional impurities, the magnon spectrum includes spatially localized states in the energy gap below the magnon band. With increasing magnetic field these may become degenerate with a piece of the continuum. If Sz is conserved, the band states and the localized states are orthogonal by symmetry, and the local mode cannot decay into the continuum. We examine here the contribution of Sz-nonconserving interactions to the magnetic resonance linewidth of the local mode as it enters the continuum. The effect of the site local orthorhombic anisotropy in Mn:FeF2 is calculated, in particular, and is shown to be a reasonable source of the observed width at the single available experimental point.