Single crystal neutron scattering study of the pyrochlore Yb₂Ge₂O₇

Rare earth pyrochlore oxides have been found to be hosts for a wide breadth of exotic phenomena based on strong frustration. Previous single crystal studies of Yb₂Ti₂O₇ have established that it forms a ferromagnetic (FM) ground state, but lies near a classical phase boundary between FM and antiferromagnetic (AFM) states. Meanwhile, powder neutron scattering studies of Yb₂Ge₂O₇ show it selects the AFM state. Despite this difference, these Yb pyrochlores share the same unusual spin dynamics in zero field. Further experimental progress on Yb₂Ge₂O₇ has been stymied by a lack of large single crystal samples. We report inelastic neutron scattering measurements on single crystals of Yb₂Ge₂O₇ grown by a hydrothermal method. We have used the field-polarized spin waves to determine the exchange parameters for Yb₂Ge₂O₇. The g-tensor was independently determined from electron paramagnetic resonance on a Lu_1.98Yb_0.02Ge₂O₇ sample in order to enable an unambiguous determination of these parameters. Our analysis and theoretical modeling show that Yb₂Ge₂O₇ lies in extreme proximity to the classical phase boundary between the AFM and FM phase, lending strong support to the notion of phase competition as playing a key role in the Yb pyrochlore family.