Origin of the ferromagnetism in ZnCoO from chemical reaction of Co3O4

We investigated conversion of Co3O4 to Co nanoclusters through hydrogen reduction. Quantitative analysis is performed on the conversion of Co3O4 to Co metal as a function of hydrogen-injection conditions. Our results reveal that Co3O4 must be completely eliminated to avoid formation of the metal phase in ZnCoO. We also propose a new M-T curve based method for detecting nano-sized Co clusters which are below the detection limit of diffraction techniques. It is also found that the Co phase can be transformed back to the Co3O4 phase through oxygen annealing and that, as a result, the ferromagnetism can be eliminated. These findings are discussed in the context of the origin of ferromagnetism in ZnCoO.