Investigation of local structural environments and room-temperature ferromagnetism in (Fe,Cu)-codoped In2O3 diluted magnetic oxide films
The local structural, optical, magnetic and transport properties of (In0.95−xFexCu0.05)2O3 (0.06 ≤ x ≤ 0.20) films deposited by RF-magnetron sputtering have been systemically studied by different experimental techniques. Detailed structural analyses using XRD, XPS, EXAFS and full multiple-scattering ab initio theoretical calculations of Fe K-edge XANES show that the (In0.95−xFexCu0.05)2O3 films have the same cubic bixbyite structure as pure In2O3. The doped Fe ions exist at both +2 and +3 oxidation states, substituting for the In3+ sites in the In2O3 lattice and forming a FeIn + 2VO complex with the O vacancy in the first coordination shell of Fe. However, the co-doped Cu atoms are not incorporated into the In2O3 lattice and form the Cu metal clusters due to high ionization energy. UV-Vis measurements show that the optical band gap Eg decreases monotonically with the increase of Fe concentration, implying an increasing s–pd exchange interaction in the films. All the films display intrinsic room-temperature (RT) ferromagnetism and the saturated magnetization (Ms) increases monotonically with Fe doping. The temperature dependence of the resistivity data suggests the conduction mechanism of Mott variable-range hopping (VRH) at low temperature, confirming that the carriers are localized. It can be concluded that the observed RT ferromagnetism in the films originates from the overlapping of polarons mediated by oxygen vacancies based on the bound magnetic polaron (BMP) model. The variation of the localization effect of carriers with Fe doping can obviously adjust the magnetic exchange interaction in the (In0.95−xFexCu0.05)2O3 films.