(M=Cr,Mn,Fe,Co) by LSDA and LSDA+U

We have implemented a systematic LSDA and LSDA+U study of the double perovskites A2FeReO6 (A=Ba,Sr,Ca) and Sr2MMoO6 (M=Cr,Mn,Fe,Co) for understanding of their intriguing electronic and magnetic properties. The results suggest a ferrimagnetic (FiM) and half-metallic (HM) state of A2FeReO6 (A=Ba,Sr) due to a pdd-π coupling between the down-spin Re5+/Fe3+ t2g orbitals via the intermediate O 2pπ ones, also a very similar FiM and HM state of Sr2FeMoO6. In contrast, a decreasing Fe t2g component at Fermi level (EF ) in the distorted Ca2FeReO6 partly accounts for its nonmetallic behavior, while a finite pdd-σ coupling between the down-spin Re5+/Fe3+ eg orbitals being present at EF serves to stabilize its FiM state. For Sr2CrMoO6 compared with Sr2FeMoO6, the coupling between the down-spin Mo5+/Cr3+ t2g orbitals decreases as a noticeable shift 1 up of the Cr3+ 3d levels, which is likely responsible for the decreasing TC value and weak conductivity. Moreover, the calculated level distributions indicate a Mn2+(Co2+)/Mo6+ ionic state in Sr2MnMoO6 (Sr2CoMoO6), in terms of which their antiferromagnetic insulating ground state can be interpreted. While orbital population analyses show that owing to strong intrinsic pd covalence effects, Sr2MMoO6 (M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts for the similar M -independent spectral features observed in them. PACS numbers: 71.20.-b, 75.50.-y, 71.15.Mb