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

Abstract

We present resistivity, magnetization, and zero field muon spin relaxation (<em>μ</em>SR) data for the pyrochlore iridate materials Nd_{2–<em>x</em>}Ca<em>x</em>Ir₂O₇ (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 <em>T_M</em> 105<em>K</em> and 137<em>K</em>, respectively. However, zero-field <em>μ</em>SR measurements show that long-range magnetic order of the Ir⁴⁺ sublattice sets in at much lower temperatures (<em>T_LRO</em> ~ 8<em>K</em> for Nd227 and 70<em>K</em> for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at <em>T_M</em> in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd_{2–<em>x</em>}Ca<em>x</em>Ir₂O₇ samples, but the muon precession is fully suppressed for both.