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The origin of ferromagnetism in insulating CoxTi1-xO2 anatase thin films

K. Griffin Roberts,1 M. Varela,2 S. Rashkeev,3,2,# S. T. Pantelides,3,2 S. J. Pennycook,2,3 Kannan M. Krishnan1

Phys. Rev. B 78, 014409 (2008)

The point defect complex responsible for room-temperature ferroelectricity in Co-doped anatase has been elucidated by a combination of atomic resolution Z-contrast imaging, electron energy loss spectroscopy (EELS) and first principles calculations. Z-contrast images revealed not only the O sites in the anatase structure but weak additional intensity from locations which were predicted by theory to be the stable sites for Co. Theory predicted that vacancies would be attracted to the interstitial Co and form an ordered superlattice, as seen experimentally. Theory further predicted that neighboring Ti would be reduced, which was confirmed by atomic-resolution EELS. Finally, theoretical calculation of the magnetic moment for the complex was in good agreement with experimentally determined values.

(A) Z-contrast image of the anatase lattice after maximum entropy deconvolution. Red circles mark O columns while gray circles correspond to Ti/O columns. (B) Intensity traces along the atomic planes marked with red arrows on (A), compared to traces from the raw image, before deconvolution. The blue lines represent an average of over five lateral pixels such as the white box on (A). The red dots represent the intensity traces from the same regions and averaged in the same way on the raw data image. It is seen that the bright O column actually comprises two peaks: one (red arrow) in the expected position for O and another (green arrow) displaced away from the O column. This peak is due to preferential segregation of Co interstitials.

1Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA
2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
3Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, 37235, USA
#Now at Idaho National Laboratory. Idaho Falls, ID, 83415, USA


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