Magnetic Moments and Ordered States in Pyrochlore Iridates Nd₂Ir₂O₇ and Sm₂Ir₂O₇ Studied by Muon-Spin Relaxation

Abstract

Magnetic-ordered states of the pyrochlore iridates Nd 2 Ir 2 O 7 (Nd227) and Sm 2 Ir 2 O 7 (Sm227), showing metal-insulator transitions at 33 and 117 K, respectively, were studied by both the muon-spin-relaxation (SR) method and density functional theory (DFT) calculations. A long-range magnetic ordering of Ir moments appeared in conjunction with the metal insulator transition, and additional long-range-ordered states of Nd=Sm moments were confirmed at temperatures below about 10 K. We found that the all-in all-out spin structure most convincingly explained the present SR results of both Nd227 and Sm227. Observed internal fields were compared with values derived from DFT calculations. The lower limits of the sizes of magnetic moments were estimated to be 0.12 B and 0.2 B for Ir and Nd moments in Nd227, and 0.3 B and 0.1 B for Ir and Sm moments in Sm227, respectively. Further analysis indicated that the spin coupling between Ir and Nd=Sm moments was ferromagnetic for Nd227 and antiferromagnetic for Sm227.