  Recently, the signs of both superconducting transition temperature (Tc)
beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on
SrTiO3 (STO) substrate, which suggests a strong interplay between
superconductivity and magnetism. With the first-principles calculations, we
find that the substrate-induced tensile strain tends to stabilize the SDW state
in FeSe thin film by enhancing of the next-nearest-neighbor superexchange
antiferromagnetic interaction bridged through Se atoms. On the other hand, we
find that when there are oxygen vacancies in the substrate, the significant
charge transfer from the substrate to the first FeSe layer would suppress the
magnetic order there, and thus the high-temperature superconductivity could
occur. In addition, the stability of the SDW is lowered when FeSe is on a
defect-free STO substrate due to the redistribution of charges among the Fe
3d-orbitals. Our results provide a comprehensive microscopic explanation for
the recent experimental findings, and build a foundation for the further
exploration of the superconductivity and magnetism in this novel
superconducting interface.
