ROOM TEMPERATURE FERROMAGNETISM IN Co-DOPED ZnO NANOPARTICLES: MILLING TIME DEPENDENCE AND ANNEALING EFFECT

We report on the occurrence of room temperature ferromagnetism in Co-doped ZnO nanoparticles (NPs). Doping is performed by ball milling of 3wt% of Co mixed with ZnO nanopowders (commercial) for durations of 2 - 8 h. X-ray diffraction data and high-resolution transmission electron microscopy (HRTEM) confirm the absence of metallicCo clusters or any other phase different from wurtzite-type ZnO. The magnetization (M - H curve) measured at room temperature exhibits the clear ferromagnetic characteristic with saturation magnetization (M-s) and coercivefield (H-c) of the order of 3 - 4 emu/g and 225 Oe, respectively. Post-growth annealing at 250 degrees C results in an increase of Ms by a small magnitude, while annealing at 500 degrees C results in reduction of Ms. UV - visible absorption spectra show small redshift in the absorption peaks in the Co-doped ZnO NPs due to the incorporation of Co atoms in ZnO lattice. Room temperature photoluminescence studies show enhanced near-band-edge emission at 378nm in the doped NPs as compared to the undoped NPs indicating low density of defects in the doped ZnO crystals. Contribution of intrinsic defects and magnetic impurities in the observed ferromagnetism is discussed.