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 Cr$_2$O$_3$. 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.