Observation of a Localized Magnon in Co-Doped MnF2

Abstract

Measurements have been made with neutron scattering techniques of the spin‐wave frequencies at 4.2°K in a doped single crystal of MnF₂ containing 5% of substituted Co impurity. The triple‐axis crystal spectrometer at the C5 facility of the NRU reactor was used throughout in its constant‐Q mode of operation. The low‐frequency acoustic spin‐wave branches for the [110] and [001] directions agree closely with the measured dispersion for pure MnF₂ and are largely unaffected by the presence of Co impurities. Additional scattering has been observed at a frequency of 3.57×10¹² sec⁻¹, which appears to be independent of the momentum transfer. This scattering is not observed above T_N at 110°K and no mode of this frequency has been observed in pure MnF₂. This additional scattering is believed to result from a localized spin‐wave mode associated with the Co impurities. The frequency agrees closely with calculations of the energy of the first excited state of Co in MnF₂ in the molecular‐field approximation.