Muons and frustrated magnetism in NiGa₂S₄ and Pr ₂Ir₂O₇

Geometrically frustrated magnets are of interest because of the novel phenomena that arise from their exotic ground states and low-lying excitations. Muon spin rotation and relaxation (μSR) is a sensitive probe of magnetism on the local (atomic) distance scale, and is an attractive tool for the study of frustrated magnets. The muon carries a unit electric charge, however, which can have an appreciable effect on local properties. We discuss two cases where such an effect might be involved. In the 2D triangular antiferromagnet NiGa ₂S₄ the 'magnetic' specific heat is field-independent up to 7 T, suggesting nonmagnetic excitations, but μSR experiments reveal Ni spin freezing below ∼9 K and strong magnetic fluctuations down to 25 mK. Comparison with Ga nuclear quadrupole resonance data suggests, however, that the muon charge does not cause this discrepancy. In the pyrochlore iridate Pr ₂Ir₂O₇ muon spin relaxation due to a distribution of quasistatic fields is observed over a wide temperature range. The data strongly suggest hyperfine-enhanced ¹⁴¹Pr nuclear magnetism, which requires a nonmagnetic Pr³⁺ ground state. This may be due to lifting of the Pr³⁺ non-Kramers degeneracy by the muon electric field or, at least in part, a property of a spin-liquid-like many-body ground state.