Muon spin relaxation study of the (FμF)− ion in magnetic fluorides

A positive muon implanted in a solid fluoride insulator can form a hydrogen-bonded (FμF)− ion, in which the nuclear magnetic interactions cause a distinctive zero-field (ZF) muon spin relaxation (μSR) signal. In a fluoride material in which the anions have electronic magnetic moments, the muon’s interaction with nearby 19F nuclear moments must compete with its interaction with more distant but much larger anion moments. We report observation of this competition in ZF-μSR in MnF3 (TN=43 K) and CuF2 (TN=69 K). In both MnF3 and CuF2 characteristic ZF FμF oscillations are observed above the Néel temperature. As temperature drops below TN in MnF3 the relaxation rate of the (FμF)− signal increases dramatically. Below TN in CuF2 a small-amplitude high-frequency coherent oscillation appears on top of the low-frequency (FμF)− signal.