Interplay of 3$d$-5$d$ interactions in high-$T_{\mathrm{C}}$ osmium-based double perovskites

In 3d-5d systems the strongly magnetic 3d orbitals and extended 5d orbitals with enhanced spin-orbit coupling lead to a range of high $T_{\mathrm{C}}$ magnetic states and novel behavior not present in systems consisting solely of 3d or 5d ions. The two distinct octahedral sites in double perovskites $A_{2}BB$'O$_6$ allow an ordered 3d-5d structure to form, providing a variety of systems to be investigated. Unravelling the interactions controlling these systems, however, is an open challenge. The highest known $T_{\mathrm{C}}$ in such a system, 725K, is found in insulator Sr$_2$CrOsO$_6$. This questions the theory for high-$T_{\mathrm{C}}$s in systems such as $T_{\mathrm{C}}$=400K Sr$_2$FeReO$_6$ which relies on half-metallic behavior. To unravel the nature of the interactions in 3d-5d systems, we have studied the series of compounds Sr$_2X$OsO$_6$. We have utilized elastic and inelastic neutron scattering to probe the spin states in the systems, and therefore test predictions that the magnetic interactions are controlled by a frustrated AFM Heisenberg model [1]. By studying the series, we are able to relate changes in the spin wave spectrum to dramatic changes in the magnetic order from $T_{\mathrm{N}} = 95$K antiferromagnetism to $T_{\mathrm{C}}=725$K ferrimagnetism.\\[4pt] [1] O. N. Meetei et al, PRL 110, 087203(2013)