OBSERVATION OF A LOCALIZED MAGNON IN CO-DOPED MNF2

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 MnF 2 and are largely unaffected by the presence of Co impurities. Additional scattering has been observed at a frequency of 3.57×10 12 sec-1, which appears to be independent of the momentum transfer. This scattering is not observed above TN 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. © 1968 The American Institute of Physics.