Indirect impurity and host mode excitation via far-infrared level-crossing spectroscopy in antiferromagnets

Through the field dependence of its linewidth, the ${\mathit{s}}_0$ local mode at 83 ${\mathrm{cm}}^{\mathrm{-}1}$ associated with a ${\mathrm{V}}^{2+}$ impurity in ${\mathrm{FeF}}_2$ is used as a novel probe of impurity shell modes, host magnon density of states, and the phonon mixing of the magnon branches at the magnon–acoustic-phonon level crossings. This is the first observation of individual shell modes. ${\mathit{S}}_{\mathit{z}}$-nonconserving and direct exchange interactions contribute to the ${\mathit{s}}_0$-local-mode relaxation. The method has general applicability to antiferromagnets.