We have implemented a systematic LSDA and LSDA+U study of the double
perovskites A2βFeReO6β (A=Ba,Sr,Ca) and Sr2βMMoO6β
(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 A2βFeReO6β (A=Ba,Sr) due to a pdd-Ο coupling between the
down-spin Re5+/Fe3+t2gβ orbitals via the intermediate O
2pΟβ ones, also a very similar FiM and HM state of Sr2βFeMoO6β.
In contrast, a decreasing Fe t2gβ component at Fermi level (EFβ) in the
distorted Ca2βFeReO6β partly accounts for its nonmetallic behavior,
while a finite pdd-Ο coupling between the down-spin
Re5+/Fe3+egβ orbitals being present at EFβ serves to
stabilize its FiM state. For Sr2βCrMoO6β compared with
Sr2βFeMoO6β, the coupling between the down-spin Mo5+/Cr3+t2gβ orbitals decreases as a noticeable shift up of the Cr3+ 3d
levels, which is likely responsible for the decreasing TCβ value and weak
conductivity. Moreover, the calculated level distributions indicate a
Mn2+(Co2+)/Mo6+ ionic state in Sr2βMnMoO6β
(Sr2βCoMoO6β), 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, Sr2βMMoO6β
(M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts
for the similar M-independent spectral features observed in them.Comment: 21 pages, 3 figures. to be published in Phys. Rev. B on 15th Se