Magnetic order in the pyrochlore iridates $A_2$Ir${}_2$O${}_7$ ($A$ = Y, Yb)

We present results from muon spin relaxation/rotation, magnetization, neutron scattering, and transport measurements on polycrystalline samples of the pyrochlore iridates Y${}_2$Ir${}_2$O${}_7$ (Y-227) and Yb${}_2$Ir${}_2$O${}_7$ (Yb-227). Well-defined spontaneous oscillations of the muon asymmetry are observed together with hysteretic behavior in magnetization below 130 K in Yb-227, indicative of commensurate long-range magnetic order. Similar oscillations are observed in Y-227 below 150 K; however, the onset of hysteretic magnetization at $T$ $=$ 190 K indicates a transition to an intermediate state lacking long-range order as observed in Nd-227. Our results also show that insulating members of the iridate family have nearly identical magnetic ground states, and that the presence of magnetic $A$-site species does not play any significant role in altering the ground-state properties.

Figure 1

XRD patterns for (a) Y-227 and (b) Yb-227 with Rietvald refinement. (c) Diffraction pattern from elastic neutron scattering of Y-227 at 3 K.

Figure 2

Temperature-dependent susceptibility of (a) Y-227 and (b) Yb-227 with inset showing enlargement of data around bifurcation of Yb-227 near 130 K. Temperature-dependent resistivity of (c) Y-227 and (d) Yb-227; insets show data on log-log scale.

Figure 3

(a) Representative short-time asymmetry data taken from GPS for Y-227 with curves offset for clarity and solid lines representing fits to Eq. (1). (b) FFT of the asymmetry data smoothed to highlight evolution of the peak.

Figure 4

(a) Representative short-time asymmetry data taken from GPS for Yb-227 with curves offset for clarity, and solid line representing fits to Eq. (1). (b) FFT of asymmetry data.

Figure 5

Temperature dependence of the parameters extracted from fitting Eq (1) for Y-227. Closed symbols represent data from GPS, while open symbols are from EMU. TOP: ${\omega }_{\mu }$/2π with η inset; the solid line is a mean-field fit, as described in the text. MIDDLE: Λ, solid lines are guides for the eyes. BOTTOM: λ from both GPS and EMU.

Figure 6

Temperature dependence of parameters extracted from fitting Eq (1) for Yb-227. Closed symbols represent data from GPS, while open symbols are from EMU. Top: ${\omega }_{\mu }$/2π with η inset; the solid line is a fit to Eq. (2), as described in the text. Middle: Λ solid lines are guides for the eyes. Bottom: λ from both GPS and EMU.

Figure 7

Field dependence of the initial asymmetry taken from the EMU spectrometer at 1.8 K for Y-227 and Yb-227. The solid line is the fit of Eq. (4) to the Y-227 data.

Figure 8

Field dependence of λ for Yb${}_2$Ir${}_2$O${}_7$ taken from the EMU spectrometer at 1.8 K. The solid line represents the fit of Eq. (5).