The Effects of Heat Treatment on Room Temperature Ferromagnetism in a Digitally Co Doped ZnO Thin Film

The effects of heat treatment on room temperature ferromagnetism of Co-doped ZnO were investigated with a (ZnO 20 angstrom/Co x angstrom)(20) multilayer structure where x = 1.5, 3, 4.3, 6 and 9 angstrom. As the thickness of the Co sub-layer increased, the ZnO/Co multilayer changed its magnetic state from diamagnetism at Co 1.5 angstrom and 3 angstrom to weak ferromagnetism at Co 4.3 angstrom and strong ferromagnetism at Co 6 angstrom and 9 angstrom. The heat treatment, in a vacuum at 400 degrees C, changed the diamagnetic property into a ferromagnetic one, resulting in room temperature ferromagnetism for all Co sub-layer thicknesses. The improvement in ferromagnetism could be ascribed to the exchange coupling between dispersed Co atoms through XRD, optical transmittance and TEM analysis.