Observation of muon level-crossing resonance in antiferromagnetic MnF2

We report the observation of μ+ Level-Crossing Resonances (LCR's) in the ordered phase of an antiferromagnetic material. Two LCR's were observed in MnF2 as a function of longitudinal magnetic field in the temperature range between 10K and 65K. Both are attributed to a muon in an interstitial octahedral-like site. The low field resonance is attributed to a muon-nuclear spin flip-flop transition involving the two nearest neighbour19F nuclei. The high field resonance occurs when the applied field cancels the local hyperfine field on the muon. The positions and widths of the LCR's were seen to scale with the sublattice magnetization.