Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy:   magnetic, electrical and microstructural studies
  We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films. 
V.V. Strelchuk, K.A. Avramenko, V.P. Bryksa, P.M. Lytvyn, M.Ya.   Valakh, V.O. Pashchenko, O.M. Bludov, C. Deparis, C. Morha...
cond-mat.mtrl-sci