Abstract
We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides and . These two materials possess related, but different ground states, with displaying “soft” spin ice order below K, while enters a hybrid, glassy spin ice state below K. Our neutron measurements, performed at and 30 K, probe the crystal field states associated with the = 6 states of Tb within the appropriate pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in and . While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of 2 increase in the crystal field bandwidth of the states in compared with . Our results are consistent with previous measurements on crystal field states in , wherein the ground-state doublet corresponds primarily to and the first excited state doublet to . In contrast, our results on differ markedly from earlier studies, showing that the ground-state doublet corresponds to a significant mixture of , , and , while the first excited state doublet corresponds to a mixture of , , and . We discuss these results in the context of proposed mechanisms for the failure of to develop conventional long-range order down to 50 mK.
- Received 10 October 2013
- Revised 25 February 2014
DOI:https://doi.org/10.1103/PhysRevB.89.134410
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