Epitaxial growth and magnetic and electric properties of Co-doped TiO 2 thin films: Is nonequilibrium doping an essential for ferromagnetism?

Abstract

In this research, c-axis oriented epitaxial anatase TiO 2 thin films were grown on SrTiO 3(100) substrates using a ceramic Ti 0.95Co 0.05O 2 target by Pulsed Laser Deposition (PLD). The film growth processes were monitored by reflective high energy electronic diffraction (RHEED). Microstructure, conductivity, and magnetism of these doped films are found strongly affected by the oxygen pressure and substrate temperature T s. Grown at a T s around 750 °C in an oxygen pressure of 0.2 mbar, the dopants are found existing as oxide inclusions. The doped film thus behaves as an insulator and shows diamagnetism in a magnetic field parallel to the film surface. However, in the doped film grown at a reduced temperature of 630 °C in a vacuum, no impurity phase can be identified. The film shows a saturated magnetic moment of 0.16μ B/Co and a fairly good conductivity at room temperature. It is then concluded that nonequilibrium growth at lower temperatures in vacuum is essential for a high solubility of Co in the TiO 2 lattice and thus the ferromagnetism. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2003.