The origin of room temperature ferromagnetism in cobalt-doped zinc oxide thin films fabricated by PLD

We have fabricated Zn0.75Co0.25O thin films on sapphire (0001) substrates at different substrate temperatures (400-700 degreesC) in a fixed O-2 pressure of 1 x 10(-5) Torr using pulsed laser deposition technique and investigated their structural and magnetic properties in relation to the growth conditions. X-ray diffraction and transmission electron microscopy observations have revealed that hexagonal Co clusters are present in the films grown at 700 degreesC. At relatively low temperatures (400-600 degreesC), homogeneous alloy films of wurtzite structure are formed with an epitaxial relationship of (0001)(ZnCoO)parallel to(0001)(sap) and [01 (1) over bar0](znCoO)parallel to(1) over bar2 (1) over bar0](sap), which is the same as that for ZnO grown on (0001) sapphire. Observed results of the structural and magnetic properties collectively imply that the presence of the ferromagnetic Co clusters leads to ferromagnetism in Co-doped ZnO films. (C) 2003 Elsevier Ltd. All rights reserved.