Direct Observation ofMn55NMR in Antiferromagnetic CsMnF3

The direct nuclear magnetic resonance (NMR) of the ${\mathrm{Mn}}^{55}$ nuclei in the hexagonal antiferromagnet CsMn${\mathrm{F}}_3$ has been observed in the frequency range between 250 and 650 Mc/sec. The very strong temperature and field dependence of the NMR frequency in the liquid-helium temperature range can be explained by the nuclear-electron coupling theory of NMR in magnetic materials developed by de Gennes. The results of the NMR experiments are in good agreement with the two-sublattice model proposed for CsMn${\mathrm{F}}_3$ by Lee. The NMR frequency extrapolated to infinite nuclear temperature was found to be 666 Mc/sec. This suggests a zero-point spin-wave reduction of 3.0±1.0%; Davis's calculation predicts a 4.36% reduction in $〈S〉$.