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

Abstract

We report an x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) study of a $5.4\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ Co-doped indium tin oxide (ITO) thin film shown to exhibit ferromagnetism beyond room temperature. The XAS spectra at the $\text{Co}\text{ }{L}_{2,3}$ edge reveal pronounced multiplet features characteristic of divalent octahedrally coordinated ${\text{Co}}^{2+}$ ions. The results suggest that the ${\text{Co}}^{2+}$ ions are nonmetallic and substitute for the In site in ITO. Magnetic field and temperature-dependent XMCD spectra imply that the ${\text{Co}}^{2+}$ 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 $\text{In}\text{ }{M}_{2,3}$ or $\text{Sn}\text{ }{M}_{2,3}$ edges. We therefore presume that the ferromagnetism observed is a result of the $sp\text{\ensuremath{-}}d$ exchange interaction between the $sp$ band of the host ITO and that of the localized $d$ electrons of the transition-metal Co dopants.