XY antiferromagnetic ground state in the effective S=12 pyrochlore Yb2Ge2O7

We report neutron scattering and muon spin relaxation measurements (μSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=72Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to mJ=±12 with local XY anisotropy, consistent with an Seff=12 description for the Yb moments. μSR measurements reveal the presence of an ordering transition at TN=0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k=(000)Γ5 ground state with an ordered magnetic moment of 0.3(1)μB per Yb ion. We compare and contrast this phenomenology with the low-temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical Seff=12 XY pyrochlore magnets.