Photoemission and x-ray absorption studies of valence states in (Ni,Zn,Fe,Ti)${}_3$O${}_4$ thin films exhibiting photo-induced magnetization

By means of photoemission and x-ray absorption spectroscopy, we have studied the electronic structure of (Ni,Zn,Fe,Ti)${}_3$O${}_4$ thin films, which exhibits a cluster glass behavior with a spin-freezing temperature $T_f$ of $\sim 230$ K and photo-induced magnetization (PIM) below $T_f$. The Ni and Zn ions were found to be in the divalent states. Most of the Fe and Ti ions in the thin films were trivalent (Fe${}^{3+}$) and tetravalent (Ti${}^{4+}$), respectively. While Ti doping did not affect the valence states of the Ni and Zn ions, a small amount of Fe${}^{2+}$ ions increased with Ti concentration, consistent with the proposed charge-transfer mechanism of PIM.