Magnetic order and the electronic ground state in the pyrochlore iridate Nd${}_2$Ir${}_2$O${}_7$

We report a muon spin relaxation/rotation, bulk magnetization, neutron scattering, and transport study of the electronic properties of Nd${}_2$Ir${}_2$O${}_7$. We observe the onset of strongly hysteretic behavior in the temperature-dependent magnetization below 120 K, and an abrupt increase in the temperature-dependent resistivity below 8 K. Muon spin relaxation measurements show that the hysteretic magnetization is driven by a transition to a magnetically disordered state, and below 8 K a magnetically ordered ground state sets in, as evidenced by the onset of spontaneous muon precession. Our measurements point toward the absence of a true metal-to-insulator phase transition in this material and suggest that Nd${}_2$Ir${}_2$O${}_7$ may lie within or on the metallic side of the boundary of the Dirac semimetal regime within its topological phase diagram.

Figure 1

(a) Magnetization versus temperature in 1000 G for field-cooled (solid symbols) and zero-field-cooled (open symbols) sample of Nd${}_2$Ir${}_2$O${}_7$. Inset: Expanded view of the susceptibility in the vicinity of $T$ = 120 K. (b) Resistivity versus temperature for Nd${}_2$Ir${}_2$O${}_7$ in applied fields of 0 T, 5 T, and 9 T. Inset: Temperature-dependent resistivity at low temperatures in zero applied field.

Figure 2

Magnetic field dependence of the resistivity and magnetization at $T$ = 1.8 K. The dashed line shows the fit to a Brillouin function (see text). Inset: Variation of the fractional change in resistivity with the square of the magnetization; the straight line is a guide to the eye.

Figure 3

Temperature-dependent muon depolarization rate extracted from data taken at ISIS and PSI, as described in the text. Inset: Temperature dependence of the stretching exponent β as described in the text.

Figure 4

Evolution of the short-time muon depolarization curves (PSI) with decreasing temperature. The upper four curves (top to bottom) are for Nd${}_2$Ir${}_2$O${}_7$, at temperatures of 8 K, 5 K, 3.5 K, and 1.6 K; curves have been offset from the 1.6 K curve by successive increments of 6 for clarity. The bottom curve is for Yb${}_2$Ir${}_2$O${}_7$ at 1.6 K (Ref. 5) and is offset by −12. The solid lines are fits to Eq. (1) for the $T$ = 8 K data and to Eq. (2) for the $T$ = 1.6 K data for the Nd${}_2$Ir${}_2$O${}_7$ curves; the solid line for the Yb curve is a fit described in Ref. 5.