Variations from Zn_1-xCo_xO Magnetic Semiconductor to Co--ZnCoO Granular Composite

We investigate the variations from as-deposited Zn_1-xCo_xO magnetic semiconductors to the post-annealed Co--ZnCoO granular composite. The as-deposited Zn_1-xCo_xO magnetic semiconductor deposited under thermal non-equilibrium conditions is composed of Zn_1-xCo_xO nanograins of high Co concentration. The room-temperature ferromagnetism with high magnetization and large negative magnetoresistance are found in the as-deposited samples. By annealing, the samples become of granular composite consisting of the Co metal grains and the remanent Zn_1-xCo_xO matrix. Although the magnetization is enhanced after annealing, the spin-dependent negative magnetoresistance disappears at room temperature. The magnetoresistance observed in the annealed samples in the high field region has no relation with the ferromagnetism, which in turn indicates that the room-temperature ferromagnetism and large negative magnetoresistance observed in the as-deposited are the intrinsic properties of the Zn_1-xCo_xO magnetic semiconductor.