Muons and Magnetic Order in the Frustrated Pyrochlore Pr2Ir2O7
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).