Insulator-ferromagnetic metal transition in anatase Fe:TiO$_2$

Local Fe valence states in Fe-doped anatase TiO$_2$ (Fe:TiO$_2 $) were investigated in relation with transport and magnetic properties. Anatase Fe:TiO$_2$ films were deposited on LaAlO$_3 $ (100) substrates by pulsed laser deposition technique. Amounts of oxygen vacancies in the films were controlled by varying partial oxygen pressure during deposition ($P_{O_2}$). Magnetic and transport properties of the synthesized Fe:TiO$_2$ films measured by SQUID and conventional fourt probe measurements. An insulator-ferromagnetic metal transition was clearly observed between $P_{O_2}$= 1$\times 10^{-6}$ and $3 \times 10^{-6}$ Torr. X-ray photoemission spectroscopy (XPS) measurements have revealed that the local Fe valence state changes from 3+ to 2+, accompanied with the insulator- ferromagnetic metal transition. These results strongly suggest that carriers bound to Fe-oxygen vacancy pairs form magnetic polarons, and that mutual overlap of magnetic polarons triggers the insulator to ferromagnetic metal transition in Fe:TiO$_2$.