Evidence for J_ eff = 0 ground state and defect-induced spin glass behaviour in the pyrochlore osmate Y_2Os_2O_7

We present AC and DC magnetometry, heat capacity, muon spin relaxation (μSR) and resonant inelastic X-ray scattering (RIXS) studies of the pyrochlore osmate Y_2Os_2O_7. We observe a non-zero effective moment governed by √(f)μ_eff = 0.417(1) μ_B where f is the fraction of Os sites which exhibit a spin, and spin freezing at temperature T_ f≃ 5K, consistent with previous results. The field dependence of magnetisation data shows that the paramagnetic moment is most likely due to large moments μ_ eff≃ 3 μ_ B on only a small fraction f ≃ 0.02 of Os sites. Comparison of single-ion energy level calculations with the RIXS data yields a non-magnetic J_ eff = 0 ground state on the Os^4+ sites. The spin-orbit interaction, Hund's coupling and trigonal distortion of OsO_6 octahedra are all important in modelling the experimentally observed spectra. We are able to rule out impurity effects, leaving disorder-related effects such as oxygen non-stoichiometry or site interchange between Os and Y ions as the most plausible explanation for the magnetic response in this material.