Properties of theMn55Nuclear-Magnetic-Resonance Modes in CsMnF3

A new NMR mode of the ${\mathrm{Mn}}^{55}$ nuclei in the hexagonal antiferromagnet CsMn${\mathrm{F}}_3$ has been observed directly between 673 and 676 Mc/sec. This mode results from the difference in the hyperfine couplings for nuclei of the Mn1 and Mn2 sites when the nuclei are strongly coupled by the Suhl-Nakamura (SN) interaction. The new NMR mode resembles an antiferromagnetic (AFM) exchange mode, while the NMR mode observed by Minkiewicz resembles an acoustic AFM mode. The linewidths of the acoustic and exchange NMR modes at 5000 Oe are 0.042 and 0.22 Mc/sec, respectively. These are a factor of ten narrower than predicted from the SN interaction. A four-sublattice model of CsMn${\mathrm{F}}_3$ is proposed which accounts for the field dependence of both NMR modes. The NMR frequencies extrapolated to infinite nuclear temperature are 666.0±0.2 Mc/sec for the Mn2 site and 676.85±0.1 Mc/sec for the Mn1 site. This suggests zero-point spin-wave reductions of (2.2±1.0)% and (3.2±1.0)% for the Mn1 and Mn2 sites; Davis's calculation predicts 2.49% and 4.36%, respectively. The temperature dependence of the electron-sublattice magnetization is determined from the temperature dependence of the exchange NMR mode. Four-sublattice-model spin-wave calculations account for this temperature dependence when an intrasublattice ferromagnetic exchange energy is included which is 32% of the antiferromagnetic intersublattice exchange energy. The ${\mathrm{Mn}}^{55}$ nuclear spin-lattice relaxation times have been determined for fields between 600 and 5000 Oe and for temperatures between 1.4 and 4.2°K. The field dependence and magnitude of the relaxation times are not understood, but at 5000 Oe, $T_1\propto T^{-4.96\mathrm{}\pm 0.03}$ with $T_1$ equal to 3.7 sec at 1.4°K. This temperature dependence indicates that three-magnon processes may be responsible for the relaxation.