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

A. M. H. R. Hakimi, F. Schoofs, R. Bali, N. A. Stelmashenko, M. G. Blamire, S. Langridge, S. A. Cavill, G. van der Laan, and S. S. Dhesi

Phys. Rev. B 82, 144429 – Published 20 October 2010

Abstract

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 $s p − d$ exchange interaction between the $s p$ band of the host ITO and that of the localized $d$ electrons of the transition-metal Co dopants.

Received 25 February 2010

DOI:https://doi.org/10.1103/PhysRevB.82.144429

Authors & Affiliations

A. M. H. R. Hakimi^*, F. Schoofs, R. Bali, N. A. Stelmashenko, and M. G. Blamire

Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB3 9EU, United Kingdom

S. Langridge

Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 OQX, United Kingdom

S. A. Cavill, G. van der Laan, and S. S. Dhesi

Diamond Light Source, Harwell Science and Innovation Campus, Diamond House, Chilton, Didcot, Oxfordshire OX11 0DE, United Kingdom

^*amh74@cam.ac.uk

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Vol. 82, Iss. 14 — 1 October 2010

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Images

Figure 1
(Color online) (a) $θ − 2 θ$ XRD diffractograms focusing on the ITO (222) and (400) for Co-ITO films of varying Co concentrations, demonstrating the shift in the peak position and intensity upon Co doping. Inset: Showing the steady decrease in the lattice parameter, $a$ $(Å)$ , as a function of Co doping. The solid line serves as a guide to the eyes.Reuse & Permissions
Figure 2
(Color online) (a) Magnetic hysteresis loops using a SQUID magnetometer for a $5.4 at . %$ Co-doped ITO film at varying temperatures, (b) magnified image showing the large change in coercive field between 2 K (solid black circles) and 300 K (solid green circles), (c) variation in coercive field as a function of temperature; the solid line is a guide to the eyes, and, (d) normalized field-cooled magnetization for a $5.4 at . %$ Co-doped ITO film as a function of temperature in an applied magnetic field of 0.5 T; the solid line fits the experimental data to the magnetic dual-phase model described in Eq. (1).Reuse & Permissions
Figure 3
(Color online) XAS and XMCD spectra in TEY mode at 2 K under an applied field of 4 T at the (a) $In M_{2}$ edge and (b) $Sn M_{2}$ edges, along with the raw XMCD data (solid circles) and a smoothed extrapolation (solid lines).Reuse & Permissions
Figure 4
(Color online) $Co L_{2, 3}$ edge for a $5.4 at . %$ Co-doped ITO film at 2 K under an applied magnetic field of 4 T; (a) TEY XAS for right $(ρ_{+})$ and left $(ρ_{-})$ circularly polarized light. Inset: FY XAS spectra for right $(ρ_{+})$ and left $(ρ_{-})$ circularly polarized light under the same conditions, (b) average experimental TEY XAS and the calculated XAS spectra along with the experimental XMCD spectrum and calculated XMCD spectrum for octahedral ${Co}^{2 +} (10 D_{q} = 0.7 eV)$ , (c) TEY XMCD spectra at the $Co L_{2}$ edge of a $5.4 at . %$ Co-doped ITO film as a function of temperature under an applied magnetic field of 4 T.Reuse & Permissions
Figure 5
(Color online) (a) XAS and XMCD spectra at the $Co L_{2, 3}$ edge in FY mode at remanence (0 T) and 300 K. The XMCD is shown in the lower panel (solid circle with solid line), (b) Magnetization curve of a $5.4 at . %$ Co-doped ITO measured at the $Co L_{2, 3}$ edge in FY mode at 2 K (solid circles), along with the fit to a Brillouin function (solid line), and (c) temperature dependence of the FY XMCD signal at an applied magnetic field of 4 T (solid circles), along with the fit to a modified Curie-Weiss law (solid line).Reuse & Permissions
Figure 6
(Color online) (a) Temperature dependence of calculated magnetic moment $(μ_{XMCD})$ using normalized TEY XMCD spectrum and the application of the XMCD sum rules, (b) temperature dependence of the orbital $(μ_{L})$ and spin $(μ_{S})$ magnetic moments, and (c) temperature dependence of the ratio of the integrated signals over the $L_{2, 3}$ edges, $R$ . The solid lines are a guide to the eyes.Reuse & Permissions

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