ATOMIC STRUCTURE AND MAGNETIC PROPERTIES OF COBALT DOPED ZnO THIN FILMS PREPARED BY THE SOL-GEL METHOD

The ability of a sol-gel route for the synthesis of cobalt doped ZnO (Zn 1-xCoxO, x = 0.04-0.11) films grown on Si (100) and glass substrates is investigated. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature magneto-optical Kerr effect (Faraday rotation) measurements of the magnetization versus field give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 50 Oe is superimposed, most probably due to metal cobalt nanoclusters.