We perform detailed muon spin rotation ($\mu \mathrm{SR}$) measurements in the classic antiferromagnet ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and explain the spectra by considering dynamic population and dissociation of charge-neutral muon-polaron complexes. We show that charge-neutral muon states in ${\mathrm{Fe}}_2{\mathrm{O}}_3$, despite lacking the signatures typical of charge-neutral muonium centers in nonmagnetic materials, have a significant impact on the measured $\mu \mathrm{SR}$ frequencies and relaxation rates. Our identification of such polaronic muon centers in ${\mathrm{Fe}}_2{\mathrm{O}}_3$ suggests that isolated hydrogen (H) impurities form analogous complexes, and that H interstitials may be a source of charge carrier density in ${\mathrm{Fe}}_2{\mathrm{O}}_3$.

• Received 14 August 2020
• Accepted 4 December 2020

DOI:https://doi.org/10.1103/PhysRevLett.126.037202