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Charge-neutral muon centers in magnetic and non-magnetic materials : implications and applications Dehn, Martin Herbert


Spin polarized muons are widely known as extremely sensitive local probes of magnetism. Muon spin rotation (μSR) spectroscopy has made key contributions in the study of complex condensed matter systems such as frustrated and dilute magnetic systems and superconductors. Additionally, positively charged muons implanted into semiconductors and insulators often bind an electron to form muonium (Mu=[μ+e-]), a charge-neutral muon-electron bound state. Muonium has been studied extensively in a wide range of semiconducting and insulating materials, motivated by the fact that its electronic structure inside a material is virtually identical to that of isolated hydrogen defects, one of the most ubiquitous impurities in semiconductors. However, such measurements are thought to be limited to non-magnetic compounds; in magnetic materials, muonium is widely assumed to be unobservable, and charge-neutral muon states are generally not considered relevant. Here, we present strong evidence that charge-neutral muon centers do exist in magnetic compounds. Detailed μSR investigations of the prototypical antiferromagnets Cr2O3, Fe2O3 and MnF2 reveal that charge-neutral muon states can form and take on different shapes, including muon-polaron complexes and interstitial centers with large muon-electron hyperfine coupling. Crucially, we find that in magnetic materials, charge-neutral muon states do not display any signatures conventionally associated with muonium, effectively “hiding” their presence. Despite their inconspicuous signals, charge-neutral centers can significantly change how the muons interact with their host material and thus significantly alter the μSR signals. In addition, we clearly demonstrate for MnF2 that the charge-state of the muon and the magnetic properties measured by μSR are closely related, and both aspects have to be considered when using μSR to determine the intrinsic magnetic properties. These results indicate that μSR may be useful to study not only the electronic impact of hydrogen defects, but also their role as magnetic impurities in non-conductive magnetic compounds. For comparison, we also investigate charge-neutral muon-polaron complexes in non-magnetic TiO2 as well as vacuum-like muonium diffusing through the voids of an amorphous silica aerogel. These examples are used to highlight the differences and similarities between charge-neutral muon states in magnetic and non-magnetic materials.

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