An anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modelled using density functional theory

Muon spin relaxation measurements on some quantum spin ice candidate materials, the insulating pyrochlores Pr2B2O7 (B = Sn, Zr, Hf), have been performed for temperatures in the range 0.05-280 K. The results are indicative of a static distribution of magnetic moments which appears to grow on cooling and whose size at low temperatures is significantly larger than that expected for Pr nuclear moments. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other frustrated magnetic materials.