About the origin of the magnetic ground state of Tb$_{2}$Ir$_{2}$O$_{7}$

TL;DR

This study investigates the origin of the unconventional magnetic ground state in Tb2Ir2O7, where Tb3+ moments acquire a planar Gamma5 component in the presence of an Ir-induced molecular field. By combining neutron diffraction, high- and low-energy inelastic neutron scattering, and specific-heat measurements with a crystal-field framework that includes Ir coupling and Tb–Tb exchange, the authors identify two representative crystal-field parameter sets that reproduce the observed Gamma3 (AIAO) order and the emergent Gamma5 component, as well as dispersive low-energy excitations. The minimal model captures the high-energy crystal-field spectrum and the overall magnetic ordering, but underestimates the Gamma5 ordering temperature and fails to reproduce some low-energy spectral details, indicating missing ingredients such as additional exchange channels (J_{z±}, J_{±±}), quadrupolar interactions, magnetoelastic couplings, and a more nuanced Tb–Ir coupling. Overall, the work highlights the vital role of both Ir-driven fields and Tb–Tb interactions in stabilizing a mixed magnetic ground state in Tb2Ir2O7 and points to richer physics beyond the current model for Tb-based pyrochlores.

Abstract

Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of Tb$_{2}$Ir$_{2}$O$_{7}$, where a component of the Tb$^{3+}$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at 1.5 K and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between Tb$^{3+}$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.

Figures (10)

• Figure 1: (a) Powder neutron diffractograms measured in G4.1 at $130K$ (grey dots) and $1.5K$ (red dots), along with the corresponding Rietveld refinement (black line). The difference between the two latter (blue line) illustrates the quality of the refinement (Bragg R-factor $R_{\mathrm{B}} = 1.51$, RF-factor $R_{\mathrm{F}} = 1.92$ and Magnetic R-factor $R_{\mathrm{M}} = 1.69$). Nuclear and magnetic Bragg peak positions are indicated by purple and gold vertical ticks, respectively. Inset: Measurements at $1.5K$ (red dots) and $26mK$ (purple dots) using a dilution refrigerator. Their difference (green) shows no significant evolution. (b) Temperature evolution of the refined Tb$^{3+}$ total ordered magnetic moment (black dots), its AIAO component (red dots), and its $\Gamma_{5}$ component (blue dots). (c) Temperature dependence of the specific heat (black dots). The magnetic contribution (green dots) is obtained by subtracting the lattice part (orange line), estimated from the scaled heat capacity of the non-magnetic rare-earth ion analogue Eu$_{2}$Ir$_{2}$O$_{7}$.
• Figure 2: (a-d) Scattering function intensity maps measured on IN4c at $T = 2$, $30$, $150$, and $300K$, respectively. (e-h) Corresponding momentum-integrated intensities over the range $Q = \SIrange{1}{3}{\angstrom^{-1}}$. Colored dots represent the experimental data, the thick black line is the fit, and the thinner colored lines indicate the different contributions to the fit.
• Figure 3: (a,b) Scattering function intensity maps measured on IN6 at $T = 1.5$ and $45mK$, respectively. (c-h) Scattering function intensity maps measured on IN5 at $T = 1.5$, $5$, $10$, $20$, $50$, and $100K$, respectively. (i) IN5 integrated intensity over the whole accessible momentum range.
• Figure 4: Comparison of experimental (dots) and calculated (lines) observables at the mean-field level for parameter set 1 on the left and parameter set 2 on the right. (a-h) high-energy neutron scattering magnetic intensity at $T=2$, $30$, $150$ and $300K$. (i,j) $\Gamma_{3}$ (AIAO) and $\Gamma_{5}$ components of the Tb$^{3+}$ ordered magnetic moment. (k,l) Magnetic specific heat.
• Figure 5: Calculated low-energy neutron scattering spectrum using the RPA approximation for parameter set 1 on the left and parameter set 2 on the right at $T=0.1$, $2$, and $10K$.