Magnetism and magnetic order in the pyrochlore iridates in the insulator-to-metal crossover region

We present resistivity, magnetization, and zero field muon spin relaxation (μSR) data for the pyrochlore iridate materials Nd2-xCaxIr2O7 (x = 0, 0.06, and 0.10) and Sm2Ir2O7. While Nd2Ir2O7 (Nd227) is weakly conducting, Sm2Ir2O7 (Sm227) has a slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at TM = 105K and 137K, respectively. However, zero-field μSR measurements show that long-range magnetic order of the Ir4+ sublattice sets in at much lower temperatures (TLRO ~ 8K for Nd227 and 70K for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at TM in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd2-xCaxIr2O7 samples, but the muon precession is fully suppressed for both.