Observation of a Charge-Neutral Muon-Polaron Complex in Antiferromagnetic Cr2O3

We report a comprehensive muon spin rotation (mu SR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (mu(+)) occupies several distinct interstitial sites and displays a rich dynamic behavior involving local hopping, thermally activated site transitions, and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of mu SR spectra in a wide range of magnetic oxides and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a mu(+) into a linear magnetoelectric and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.