Geometrically frustrated magnets are of interest because of the novel and complex phenomena that arise from their exotic ground states and low-lying excitations. The muon spin rotation and relaxation (µSR) technique is a sensitive probe of static and fluctuating magnetism on the local (atomic) distance scale, and as such is an attractive tool for the study of frustrated magnets. The positive muon (µ+) used in µSR experiments carries a unit electric charge +e, however, which can have an appreciable effect on local properties. We discuss a case where such an effect is involved.

Thermodynamic and transport properties of the Kondo-lattice pyrochlore Pr2Ir2O7 prepared with excess Pr reveal a well-defined phase transition at 0.8 K at ambient pressure in zero magnetic field. This transition is not found in stoichiometric samples, and is suppressed by both applied field and pressure. Neutron Bragg diffraction studies on a well-characterized sample (PIOneu) show the onset of long-range “2-in 2-out” antiferromagnetic (AFM) order, with an ordered moment of 1.7µB. µSR experiments on the same sample yield an upper bound (~3 mT) on the dipolar field Bdip at the muon site due to Pr3+ AFM ordered moments. This is much smaller than the expected dipolar field (0.1–0.2 T depending on muon site).

At least in part this is due to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr3+ ions by the µ+-induced lattice distortion. However, if this were the only effect a very large number of Pr moments (~300) within a distance of ~20 Å must be suppressed. We know of no mechanism for such a suppression. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat of PIOneu, invokes ultra-slow correlated Pr-moment fluctuations in the ordered state that average Bdip on the µSR time scale (~10^(-6) s), but are static on the time scale of the neutron diffraction experiments (~10^(-9) s).