Thermal modification of magnetism in cobalt-doped ZnO nanowires grown at low temperatures

The optical and magnetic stability of hydrothermally grown Co-doped ZnO nanowires were investigated. The experimental results suggest that two different kinds of coordination of Co ions in the ZnO host, either with or without magnetic coupling, likely coexist in the as-grown samples. Low-temperature annealing resulted in the transformation of the isolated ions to magnetically coupled ones, leading to the enhancement of ferromagnetic properties. High-temperature annealing (900 degrees C) leads to paramagnetic behavior and improved crystallinity of the ZnO host materials due to Co migration and the formation of nonmagnetic oxide clusters, as evidenced by photoluminescence, Raman scattering, x-ray diffraction, and electron microscopy.