Semiconducting and ferromagnetic behavior of sputtered Co-doped TiO2 thin films above room temperature

We have investigated Co-doped ${\mathrm{TiO}}_2$ thin films grown by reactive co-sputtering. X-ray diffraction showed a single phase polycrystalline rutile structure, without any segregation of Co into particulates within the instrumental resolution limit. The atomic content of Co ranged from 1% to 12%. The temperature dependence of resistivity showed an extrinsic semiconducting behavior. From optical absorption measurements, the band gap $E_g\text{≈3.25±0.05}$ eV was found, independent of the Co concentration, and in agreement with a literature value. Room temperature M-H loops showed a ferromagnetic behavior for Co content higher than 3%. The magnetic moment per Co atom was estimated to be about ${\text{0.94\hspace{0.17em}μ}}_B,$ suggesting a low spin configuration of Co ions. The temperature dependence of remanent magnetization revealed a Curie temperature higher than 400 K for Co content of 12%.