Unravelling competing microscopic interactions at a phase boundary: A single-crystal study of the metastable antiferromagnetic pyrochlore Yb2Ge2 O7

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore Yb2Ge2O7. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat using the numerical linked-cluster expansion method. The exchange parameters so determined are strongly correlated to the values determined for the g-tensor components, as previously noted for the related Yb pyrochlore Yb2Ti2O7. To address this issue we directly determined the g tensor from electron paramagnetic resonance of 1% Yb-doped Lu2Ge2O7, thus enabling an unambiguous determination of the exchange parameters. Our results show that Yb2Ge2O7 resides extremely close to the classical phase boundary between an antiferromagnetic Γ5 phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on Yb2Ti2O7, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.