Localized magnon-pair modes in FeF2:Mn

Abstract

From a combined experimental and theoretical study of Mn impurity pairs in the dilute Heisenberg antiferromagnet, ${\mathrm{Mn}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{F}}_{2} (xl{10}^{\ensuremath{-}2})$, we report the observation and identification of magnon-pair modes in an ordered magnetic substance. Magnetic resonance spectra were obtained at 4.2 K with the use of a high-resolution, far-infrared (FIR) laser spectrometer. The spin dynamics of magnon excitations, localized in the vicinity of pairs of substitutional impurity spins in a two-sublattice antiferromagnet, is investigated theoretically by applying Green's-function techniques to a spin Hamiltonian which includes both inter-(${J}_{2}^{\ensuremath{'}}$) and intra-(${J}_{1}^{\ensuremath{'}}$) sublattice impurity-host Heisenberg exchange terms and an anisotropy term of the form, $\ensuremath{-}\mathrm{DS}_{z}^{2}$. Theoretical predictions of six distinct magnon-pair energies, for various combinations of the exchange parameters ${J}_{1}^{\ensuremath{'}}$ and ${J}_{2}^{\ensuremath{'}}$, are compared to experiment. Good agreement is obtained with ${J}_{1}^{\ensuremath{'}}$ and ${J}_{2}^{\ensuremath{'}}$ both antiferromagnetic: ${J}_{2}^{\ensuremath{'}}\ensuremath{\simeq}1.8 $ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${J}_{1}^{\ensuremath{'}}\ensuremath{\lesssim}0.5 $ ${\mathrm{cm}}^{\ensuremath{-}1}$.