We present magnetization and Hall effect measurements on the pyrochlore iridate Nd${}_2$Ir${}_2$O${}_7$. Previous muon spin rotation measurements have shown that the system undergoes an unusual transition at $T_M\approx 110$ K into a magnetic phase lacking long-range order, followed by a transition at $T_{\mathrm{LRO}}\approx 6$ K into a state with long-range magnetic order. We observe a small remnant magnetization when cycling through a zero magnetic field at temperatures below $T_M$. Below $T_{\mathrm{LRO}}$, this remnant magnetization increases sharply, and additional hysteresis effects appear at a higher field $B_c$ $=$ 2.5 T, while the Hall resistance develops a nonmonotonic and hysteretic magnetic field dependence, with a maximum at $B_c$ and signatures of an anomalous Hall effect. The dependence on field sweep direction demonstrates a nontrivial transition into a magnetically ordered state with properties paralleling those of known spin-ice systems and suggests a spin reorientation transition across the metal-insulator transition in the $A$-227 series.

• Received 4 January 2013

DOI:https://doi.org/10.1103/PhysRevB.87.060403