Lattice-site-specific spin-dynamics in double perovskite Sr2CoOsO6

We have studied the magnetic properties and spin-dynamics of the structurally-ordered double perovskite Sr${}_2$CoOsO${}_6$. Our neutron diffraction, muon spin relaxation and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature $T_{\mathrm{N}}=108$ K, cobalt (3$d^7$, $S=3\mathrm{/}2$) and osmium (5$d^2$, $S=1$) moments fluctuate dynamically, while their $\mathit{\text{average}}$ effective moments undergo long-range order. In the second AFM phase with $T_{\mathrm{N}}=67$ K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at $T\approx 5$ K. Our $\mathit{\text{ab}}\mathit{\text{initio}}$ calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin-dynamics, making Sr${}_2$CoOsO${}_6$ distinct from previously reported double-perovskite compounds.