Origin of magnetism in cobalt-doped indium tin oxide thin films

We report an x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) study of a 5.4 at. % Co-doped indium tin oxide (ITO) thin film shown to exhibit ferromagnetism beyond room temperature. The XAS spectra at the Co L(2,3) edge reveal pronounced multiplet features characteristic of divalent octahedrally coordinated Co(2+) ions. The results suggest that the Co(2+) ions are nonmetallic and substitute for the In site in ITO. Magnetic field and temperature-dependent XMCD spectra imply that the Co(2+) ions give a paramagnetic contribution to the overall ferromagnetic response both at the near-surface region and throughout the bulk of the films. No magnetic polarization was detected at the In M(2,3) or Sn M(2,3) edges. We therefore presume that the ferromagnetism observed is a result of the sp-d exchange interaction between the sp band of the host ITO and that of the localized d electrons of the transition-metal Co dopants.