  Using first-principles calculations, we have systematically investigated the
structural, electronic, and magnetic properties of facial (fac-) and meridional
(mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction
with ferromagnetic substrate. Our calculation results show that for the
isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; and both Feq3 isomers
have a high spin-state of 5 {\mu}B as the ground state when an on-site
Hubbard-U term is included to treat the highly localized Fe 3d electrons; while
the standard DFT calculations produce a low spin-state of 1 {\mu}B. These
magnetic behaviors can be understood by the octahedral ligand field splitting
theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co
surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between
Fe and Co substrate, originating from the superexchange coupling between Fe and
Co mediated by the interface oxygen and nitrogen atoms. These findings suggest
that Feq3 molecular films may serve as a promising spin-filter material in
spintronic devices.
