Unravelling competing microscopic interactions at a phase boundary: A single-crystal study of the metastable antiferromagnetic pyrochlore Yb2Ge2O7

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore ${\mathrm{Yb}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$. 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 ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. To address this issue we directly determined the $g$ tensor from electron paramagnetic resonance of 1% Yb-doped ${\mathrm{Lu}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$, thus enabling an unambiguous determination of the exchange parameters. Our results show that ${\mathrm{Yb}}_{2}{\mathrm{Ge}}_{2}{\mathrm{O}}_{7}$ resides extremely close to the classical phase boundary between an antiferromagnetic ${\mathrm{\ensuremath{\Gamma}}}_{5}$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.