Electronic structure and magnetism in Ru-based perovskites

The magnetic properties of ruthenates with perovskite-derived structures, particularly ${\mathrm{}(\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r})\mathrm{RuO}}_3$ and ${\mathrm{Sr}}_2{\mathrm{YRuO}}_6$, are studied within the context of band-structure-based Stoner theory. First principles calculations are used to demonstrate that in all cases the correct magnetic behavior and order can be obtained without recourse to strong correlation effects and that the insulating character of ${\mathrm{Sr}}_2$ ${\mathrm{YRuO}}_6$ is reproduced. The different magnetic states of ${\mathrm{SrRuO}}_3$ and ${\mathrm{CaRuO}}_3$ are shown to be due to the different structural distortions in these materials, most significantly the larger rotation of the octahedra in the Ca compound. ${\mathrm{CaRuO}}_3$ is found to be on the verge of a ferromagnetic instability, leading to the expectation of giant local moments around magnetic impurities and other anomalous effects in analogy with fcc Pd metal. Oxygen $2p$-derived states hybridize strongly with Ru $d$ states in all three compounds, and O, through this hybridization, plays an unusually large role in the magnetic properties. This involvement of O is responsible for the strong magnetostructural coupling that is found in the calculations. Transport properties of ${\mathrm{CaRuO}}_3$ and ${\mathrm{SrRuO}}_3$ are analyzed using the calculated Fermiology. Unusually large magnon and paramagnon couplings are found, which are consistent with reported measurements of the low-temperature specific heat and the resistivity coefficient.