Room temperature ferromagnetism and crossover of negative magnetoresistance to positive magnetoresistance in the Co doped ITO films

The effects of Co doping on tuning optical, magnetic and transport properties of ITO films prepared by magnetron sputtering technique were investigated systematically by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-Vis, Hall effect, rho-T, magnetoresistance (MR) and magnetic measurements. The detailed structural analyses reveal that the Co-doped ITO films have cubic bixbyite structure of space group Ia (3) over bar and Co dopant atoms are substitutionally incorporated into In2O3 lattice with the high spin divalent state of Co2+. The Co-doped ITO films display clear room-temperature (RT) ferromagnetism and Mott variable range hopping (VRH) transport behavior with n-type conductivity. The saturated magnetization (M-s) decreases from 4.52 emu/cm(3) to 2.2 emu/cm(3) with increasing Co concentration from 3.7 at% to 6.5 at%. Crossover of negative MR to positive MR and the red shift of optical bandgap E-g are also observed with Co doping, reflecting the occurrence of spin polarization and stronger s-d exchange interaction. The observed RT ferromagnetism of the films can consider being due to the Co2+-vacancy complexes induced magnetic interaction by the overlapping of bound magnetic polarons. The Co doping can cause the variation of localization effect of carrier, which remarkably influences the ferromagnetic order of the Co-doped ITO films.