Unstable spin-ice order in the stuffed metallic pyrochlore Pr_2+xIr_2-x O_7-δ

Specific heat, elastic neutron scattering, and muon spin rotation (μSR) experiments have been carried out on a well-characterized sample of "stuffed" (Pr-rich) Pr_2+xIr_2-x O_7-δ. Elastic neutron scattering shows the onset of long-range spin-ice "2-in/2-out" magnetic order at TM=0.93 K, with an ordered moment of 1.7(1)μB/Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 Å and 0.7 ns, respectively. μSR experiments yield an upper bound 2.6(7) mT on the local field Bloc4f at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7μB moments (120-270 mT, depending on muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr3+ ions by the μ+-induced lattice distortion. For this to be the only effect, however, ∼160 Pr moments out to a distance of ∼14 Å must be suppressed. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat, invokes slow correlated Pr-moment fluctuations in the ordered state that average Bloc4f on the μSR time scale (∼10-7s), but are static on the time scale of the elastic neutron scattering experiments (∼10-9s). In this picture, the dynamic muon relaxation suggests a Pr³⁺4f correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.