Room-temperature antiferromagnetic resonance in NaMnAs

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs – a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At $B=0$, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($k=0$) magnon mode. Its energy softens considerably with increasing $T$, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs in the range 0.1-0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.