Development and processing temperature dependence of ferromagnetism in Zn0.98Co0.02O

Abstract

We report the development of room-temperature ferromagnetism (FM), with coercivity Hc=2000Oe and saturation magnetization Ms∼0.01emu∕g, in chemically synthesized powders of Zn0.98Co0.02O processed at 150 °C, and paramagnetism with antiferromagnetic interactions between the Co2+ spins (S=3∕2) in samples processed at higher temperatures 200⩽TP⩽900°C. X-ray diffraction data show a decrease in the lattice parameters a and c with TP, indicating a progressive incorporation of 0.58Å sized tetrahedral Co2+ at the substitutional sites of 0.60 Å sized Zn2+. Diffuse reflectance spectra show three well defined absorption edges at 660, 615, and 568 nm due to the d-d crystal field transitions A24(F)→E2(G),A24(F)→T14(P), and A24(F)→T12(G) of high spin (S=3∕2)Co2+ in a tetrahedral crystal field, whose intensities increase with processing temperature. X-ray photoelectron spectroscopy shows that the doped Co2+ ions in the 150 °C processed samples are located mostly on the surface of the particles and they disperse into the entire volume of the particles when processed at higher temperatures. The observations suggest that the FM most likely results from Co2+ attached to the surface sites and it is lost in well dispersed samples formed at TP>150°C.