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 20Å/Co x Å)₂₀ multilayer structure where x = 1. 5, 3, 4. 3, 6 and 9 Å. As the thickness of the Co sub-layer increased, the ZnO/Co multilayer changed its magnetic state from diamagnetism at Co 1. 5 Å and 3 Å to weak ferromagnetism at Co 4. 3 Å and strong ferromagnetism at Co 6 Å and 9 Å. The heat treatment, in a vacuum at 400°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.