Evidence for $J_{\mathrm{eff}}=0$ ground state and defect-induced spin glass behaviour in the pyrochlore osmate Y${}_2$Os${}_2$O${}_7$

Abstract

We present AC and DC magnetometry, heat capacity, muon spin relaxation ($\mu$SR) and resonant inelastic X-ray scattering (RIXS) studies of the pyrochlore osmate Y${}_2$Os${}_2$O${}_7$. We observe a non-zero effective moment governed by $\sqrt{f}{\mu }_{\mathrm{eff}}=0.417(1){\mu }_{\mathrm{B}}$ where $f$ is the fraction of Os sites which exhibit a spin, and spin freezing at temperature $T_{\mathrm{f}}\simeq 5$K, consistent with previous results. The field dependence of magnetisation data shows that the paramagnetic moment is most likely due to large moments ${\mu }_{\mathrm{eff}}\simeq 3{\mu }_{\mathrm{B}}$ on only a small fraction $f\simeq 0.02$ of Os sites. Comparison of single-ion energy level calculations with the RIXS data yields a non-magnetic $J_{\mathrm{eff}}=0$ 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.