Hybridized orbital states in spin-orbit coupled 3d−5d double perovskites studied by x-ray absorption spectroscopy

We investigated the orbital hybridization mechanism in $3d\ensuremath{-}5d$ double perovskites (DPs) of ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ and ${\mathrm{La}}_{2}{\mathrm{CoPtO}}_{6}$ using x-ray absorption spectroscopy. It is clearly evidenced by O $K$-edge and Co $K$-edge x-ray absorption spectra that the Co $3d$ orbitals hybridize not only with the half-filled Ir/Pt ${j}_{\mathrm{eff}}$ states but also with the fully empty (unpolarized) Ir/Pt ${e}_{g}$ states in both DPs. The Co $3d$ ${e}_{g}$-Ir $5d$ ${e}_{g}$ hybridization cannot contribute to the ferrimagnetic long-range order in ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ established by spin-selective Co $3d$ ${t}_{2g}$-Ir $5d$ ${j}_{\mathrm{eff}}$ hybridization through the intermediate oxygen $p$ state but could serve as an origin of paramagnetism. The strengths of such orbital hybridizations were found to be almost invariant to temperature, even far above the Curie temperature, implying persistent paramagnetism against the antiferromagnetic ordering in the spin-orbit entangled $3d\ensuremath{-}5d$ DPs.