Self-doping effect and successive magnetic transitions in superconducting Sr2VFeAsO3

We have studied a quinary Fe-based superconductor Sr2VFeAsO3 by the measurements of x-ray diffraction, x-ray absorption, Mossbauer spectrum, resistivity, magnetization, and specific heat. This apparently undoped oxyarsenide is shown to be self-doped via electron transfer from the V3+ ions. We observed successive magnetic transitions within the VO2 layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO2 planes are discussed based on mixed valence of V3+ and V4+.