Muon level-crossing resonance in antiferromagneticMnF2

We report the observation of a muon-nuclear level-crossing resonance (LCR) in the antiferromagnet ${\mathrm{MnF}}_2$. The resonance occurs when the spin transition frequency of an interstitial muon is matched to that of the nearest-neighbor ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei, allowing for a resonant transfer of polarization from the muon to the nuclei. The applied external field $H_0$ at which the LCR occurs is a measure of the local magnetic field on the neighboring ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei. The observed shift relative to earlier ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ NMR results is attributed to the disturbing influence of the muon. A second resonance was observed when $H_0$ cancels the longitudinal component of the local field at the muon.