AVS 49th International Symposium
    Magnetic Interfaces and Nanostructures Tuesday Sessions
       Session MI+EL+SC-TuM

Paper MI+EL+SC-TuM4
Epitaxial Growth and Properties of Co-doped TiO@sub 2@ Anatase on LaAlO@sub 3@(001)

Tuesday, November 5, 2002, 9:20 am, Room C-205

Session: Ferromagnetic Semiconductors
Presenter: S.A. Chambers, Pacific Northwest National Laboratory
Authors: S.A. Chambers, Pacific Northwest National Laboratory
T. Droubay, Pacific Northwest National Laboratory
C.M. Wang, Pacific Northwest National Laboratory
S.M. Heald, Pacific Northwest National Laboratory
S. Thevuthasan, Pacific Northwest National Laboratory
A.S. Lea, Pacific Northwest National Laboratory
C.F. Windisch, Jr., Pacific Northwest National Laboratory
R.F.C. Farrow, IBM Almaden Research Center
L. Folks, IBM Almaden Research Center
J.-U. Thiele, IBM Almaden Research Center
M.G. Samant, IBM Almaden Research Center
R.F. Marks, IBM Almaden Research Center
Correspondent: Click to Email
We are investigating Co-doped TiO@sub 2@ anatase heteroepitaxy on LaAlO@sub 3@(001) by oxygen plasma assisted molecular beam epitaxy. This material is of considerable interest because it is ferromagnetic well above room temperature. Thus, it may be a useful DMS for spintronics. The use of a higher growth rate (0.04 nm/sec) results in the nucleation of nanocrystals of rutile, the more stable form of TiO@sub 2@, within the continuous anatase film. The density of rutile nanocrytals increases as the quality of the substrate surface decreases. A lower growth rate (0.01 nm/sec) results in a much better film morphology, although a low density of smaller nanocrystals remains. Unlike the fast-grown films, these films show no evidence for any phase other than anatase. A number of techniques reveal that Co substitutes for Ti in the lattice and exhibits a +2 oxidation state; there is no evidence for elemental Co in any form. Each Co(II) substitution for Ti(IV) requires an O@super -2@ anion vacancy in order to maintain charge neutrality, and evidence for such a vacancy is forthcoming from preliminary Co K-shell EXAFS. Such vacancies do not generate free carriers because they are uncharged. Hall effect and XPS measurements show that the films are n-type, the most likely cause being the presence of O atom vacancies that form during growth. These vacancies are independent of the presence of Co, and are negatively charged, thereby providing a source of free electrons from shallow donor states. The magnetization depends critically on free carrier concentration, as expected for a DMS. The exact Curie temperature is currently being determined, but appears to be in excess of 700K.