Role of Co Clusters and Oxygen Vacancies in the Magnetic and Transport Properties of Co-Doped In 2 O 3 Films

The (In1-xCox)(2)O-3 films with x = (0.055, 0.08, 0.10, 0.15) have been prepared by a radio frequency magnetron sputtering technique and investigated by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, Hall effect, and room-temperature magnetic measurements. The detailed structural analyses and full multiple-scattering ab initio calculations indicate that most Co2+ ions substitute for In3+ sites of In2O3 lattice and form Co-In(2+) + V-O complexes with the O vacancy in the nearest coordination shell, whereas a portion of the Co atoms form the precipitate of Co metal clusters for all the (In1-xCox)(2)O-3 films. Despite the formation of Co clusters, magnetic characterizations show that the saturated magnetization M-s of films first increases and then decreases with the increase of Co concentration, suggesting that the small Co clusters are superparamagnetic. The electronic conducting mechanism is dominated by Mott variable range hopping behavior for all the films. The strong localization of carriers suggests the bound magnetic polarons scenario. It can be concluded that the observed room-temperature ferromagnetism in the (In1-xCox)(2)O-3 films is intrinsic and originates from electrons bound in defect states associated with oxygen vacancies. There exists an optimal localization radius xi of variable range hopping for achieving the largest M-s in the (In1-xCox)(2)O-3 films.