Ferromagnetic Properties of Ni-Doped Rutile TiO2-¥ä Young Ran Park , Seung-li Choi , Jung Han Lee , Kwang Joo Kim and Chul Sung Kim J. Korean Phys.Soc. 50,638 [doi: 10.3938/jkps.50.638 | PDF Download] Ni-doped rutile TiO$_{2-\delta}$ thin films grown by using the sol-gel method exhibited ferromagnetic and semiconducting properties at room temperature. The ferromagnetic strength of the TiO$_{2-\delta}$:Ni films was found to vary with Ni doping ($x$). The Ni-doped films had $p$-type electrical conductivity for small $x$ ($\le$5 at.\%) with the hole density increasing with $x$ while the undoped ones had $n$-type conductivity. Formation of Ni clusters was detected for large $x$ ($\ge$6 at.\%) by using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and Hall effect measurements. The Ni ions in TiO$_{2-\delta}$:Ni were found to have valences of +2 and +3 by using XPS, with the latter having a larger density. The room-temperature ferromagnetism for $x$ $\le$ 5 at.\% is not attributable to mobile holes but to magnetic polarons formed around electrons trapped in oxygen vacancies.