AVS 52nd International Symposium
    Magnetic Interfaces and Nanostructures Wednesday Sessions
       Session MI+EM-WeA

Invited Paper MI+EM-WeA7
Growth and Magnetic Properties of Doped ZnO Epitaxial Films and Nanocrystal

Wednesday, November 2, 2005, 4:00 pm, Room 204

Session: Magnetic Semiconductors
Presenter: S.A. Chambers, Pacific Northwest National Laboratory and Univ. of Washington
Authors: S.A. Chambers, Pacific Northwest National Laboratory and Univ. of Washington
A.C. Tuan, Pacific Northwest National Laboratory
K.R. Kittilstved, University of Washington
D.R. Gamelin, University of Washington
Correspondent: Click to Email
Since 2001, researchers around the world have been involved in a vigorous search for new ferromagnetic oxide semiconductors with Curie temperatures above ambient. Such materials are vitally important for the practical realization of spintronics. Two wide bandgap oxide semiconductors have been of particular interest - TiO@sub 2@ anatase and ZnO. A number of claims of room temperature ferromagnetism (RTFM) in these host oxides with various dopants have been made. However, some of the results were based on poorly characterized material, often containing magnetic secondary phases, leading to illegitimate claims. Even for well characterized materials which are phase-pure magnetically doped oxides, the mechanism(s) of magnetism remain largely undetermined. We have used oxygen plasma assisted metal organic chemical vapor deposition along with direct wet chemical synthesis and spin coating to prepare Co@sub x@Zn@sub 1-x@O and Mn@sub x@Zn@sub 1-x@O epitaxial and nanoparticle films. Co(II) and Mn(II) substitute for Zn(II) in the wurtzite lattice in materials synthesized by both methods. Room temperature ferromagnetism in epitaxial Co:ZnO films can be reversibly activated by diffusing in Zn, which occupies interstitial sites and makes the material n-type. O-capped Co:ZnO nanoparticles, which are paramagnetic as grown, become ferromagnetic upon being spin coated in air at elevated temperature. Likewise, spin-coated N-capped Mn:ZnO nanoparticle films also exhibit room temperature ferromagnetism. However, the inverse systems, N-capped Co:ZnO and O-capped Mn:ZnO, are entirely paramagnetic when spin coated into films in the same way. Unfortunately, the nanoparticle films are not sufficiently conductive do perform magneto-transport measurements. Instead, we have carried out a detailed analysis of optical absorption, photovoltage, and magnetic circular dichroism spectra. This analysis reveals that the resonances Co(I) ↔ Co(II) + e@super -@@sub CB@ and Mn(III) ↔ Mn(II) + h@super +@@sub VB@ are energetically favorable, consistent with strong hybridization of Co (Mn) with the conduction (valence) band of ZnO. In contrast, the resonances Mn(I) ↔ Mn(II) + e@super -@@sub CB@ and Co(III) ↔ Co(II) + h@super +@@sub CB@ are not energetically favorable. These results indicate that Co(II)-derived states will strongly interact only with the conduction band, whereas Mn(II)-derived states interact strongly only with the valence band. These spectral results are consistent with the observed ferromagnetism in Co:ZnO (Mn:ZnO) being mediated by electrons (holes). @footnoteText@This work was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering Physics. Work at UW was funded by the NSF (DMR-0239325 and ECS-0224138) and the Research Corporation.