Self doping effect and successive magnetic transitions in superconducting Sr${}_2$VFeAsO${}_3$

We have studied a quinary Fe-based superconductor Sr${}_2$VFeAsO${}_3$ by the measurements of x-ray diffraction, x-ray absorption, M"{o}ssbauer spectrum, resistivity, magnetization and specific heat. This apparently undoped oxyarsenide is shown to be self doped via electron transfer from the V${}^{3+}$ ions. We observed successive magnetic transitions within the VO${}_2$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO${}_2$ planes are discussed based on mixed valence of V${}^{3+}$ and V${}^{4+}$.