AVS 51st International Symposium
    Semiconductors Monday Sessions
       Session SC+MI-MoM

Paper SC+MI-MoM8
Intrinsic Versus Extrinsic Nature of Co Doped TiO@sub 2@ Diluted Magnetic Semiconductor Thin Films

Monday, November 15, 2004, 10:40 am, Room 304B

Session: Dilute Magnetic and Ferromagnetic Semiconductors
Presenter: S.R. Shinde, University of Maryland
Authors: S.R. Shinde, University of Maryland
S.B. Ogale, University of Maryland
J. Higgins, University of Maryland
T. Zhao, University of Maryland
S.E. Lofland, Rowan University
V.N. Kulkarni, University of Maryland
A.J. Millis, Columbia University
S. Das Sarma, University of Maryland
R.L. Greene, University of Maryland
R. Ramesh, University of Maryland
T. Venkatesan, University of Maryland
Correspondent: Click to Email
The issue of Co distribution in TiO@sub 2@ (in anatase and rutile forms), a widely studied oxide based diluted magnetic semiconductor (DMS) system, is still controversial. Although all the reported studies have discovered room temperature ferromagnetism in this system, some of the researchers claim that the material is intrinsic, whereas others have found that cobalt forms small clusters and therefore the material has extrinsic origin of ferromagnetism. In our work we have grown (by pulsed laser deposition) and characterized epitaxial thin films of Co:TiO@sub 2@ at different growth conditions and Co doping concentrations. We noticed that Co distribution strongly depends on the growth parameters. At lower growth temperature (~700C) there is a limited solubility of Co (up to ~2%) above which nanometer sized Co clusters are formed. When the films are grown in ultrahigh vacuum (10-8 Torr), the films have low resistivity and show the anomalous Hall effect. Although this could be interpreted as a signature of carrier induced DMS nature of these particular films, our detailed magnetic and structural analysis shows the presence of Co nanoclusters in these films. In the magnetization data of these films we observe superparamagnetism with a blocking temperature of 250K. This temperature corresponds to Co particles of 7nm diameter, the presence of which was further confirmed by transmission electron microscopy (TEM). On the other hand, when the films grown at lower temperature are annealed at high temperature (~900C) the clusters dissolve in titanium dioxide matrix leading to an intrinsic DMS with a Curie temperature ~650C. Similar properties are observed for films directly grown at high temperature and no indication of any clustering of Co is observed in TEM. We have also observed electric field induced reversible modulations, in the magnetization of these films in PbZr@sub 0.2@Ti@sub 0.8@O@sub 3@/Co:TiO@sub 2@/SrRuO@sub 3@ field effect transistor structure.