Observation of a charge-neutral muon-polaron complex in antiferromagnetic Cr$_2$O$_3$

TL;DR

This study uses muon spin rotation to reveal a charge-neutral muon-polaron complex in Cr$_2$O$_3$, providing insights into impurity behavior and local magnetoelectric effects in antiferromagnetic oxides.

Contribution

It reports the first observation of a muon-electron polaron complex in Cr$_2$O$_3$, advancing understanding of muon behavior and impurity effects in magnetoelectric materials.

Findings

Identification of multiple muon interstitial sites.

Observation of a charge-neutral muon-polaron complex.

Implications for interpreting $bc$SR spectra in magnetic oxides.

Abstract

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.