Annealing Effects on Zn(Co)O: From Para- to Ferromagnetic Behavior

Zn1-xCoxO nanocrystals were produced by forced hydrolysis in diethyleneglycol. The as-produced crystals are paramagnetic. However, heating at 400 degrees C under a nitrogen atmosphere for 1 h induces ferromagnetism at room temperature, characterized by coercive field (0.38 kOe) and magnetization at 10 kOe (0-10 mu(B)/Co) similar to the values reported recently for nanocrystalline Co:ZnO aggregates. To understand this evolution, we have characterized the samples by chemical and physical analyses including X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible-IR absorption spectroscopy, magnetic measurements, and EPR experiments. These analyses show that the Co ions are incorporated into the wurtzite structure, forming a Zn1-xCoxO solid solution. X-ray diffraction and electron microscopy failed to detect secondary phases, neither before nor after the thermal treatment. However, the shape of the nanoparticles is modified and becomes more spherical upon annealing, which gives evidence of an important diffusion of atomic species. X-band electron paramagnetic resonance (EPR) reveals the presence of magnetic clusters in the annealed samples. Both the magnetic and optical properties show that these are Co clusters that have grown at the expense of the cobalt diluted in the host matrix. The thermal treatment survey performed by thermogravimetric analysis coupled with mass spectrometry and IR spectroscopy gives evidence of desorption of organic species and liberation of oxygen from the surface of the particles. This study then suggests that the origin of the magnetism believed to be intrinsic to the material in many prior works is extrinsic in nature and is due to the diffusion of cobalt and oxygen species at the surface of the particles, where Co forms nanoclusters while most of the oxygen is liberated in the annealing process.