Muon level-crossing resonance in antiferromagnetic MnF2

We report the observation of a muon-nuclear level-crossing resonance (LCR) in the antiferromagnet MnF2. The resonance occurs when the spin transition frequency of an interstitial muon is matched to that of the nearest-neighbor F19 nuclei, allowing for a resonant transfer of polarization from the muon to the nuclei. The applied external field H0 at which the LCR occurs is a measure of the local magnetic field on the neighboring F19 nuclei. The observed shift relative to earlier F19 NMR results is attributed to the disturbing influence of the muon. A second resonance was observed when H0 cancels the longitudinal component of the local field at the muon.