PNNL

Abstract

We report the development of room-temperature ferromagnetism (FM), with coercivity Hc = 2850 Oe and saturation magnetization Ms ~ 0.01 emu/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 progressive incorporation of 0.58 Å 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 4A2(F) ? 2E(G), 4A2(F) ? 4T1(P) and 4A2(F) ? 2A1(G) of high spin (S = 3/2) Co2+ in 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 results from Co2+ attached to the surface sites and it is lost in well dispersed samples formed at TP > 150 °C.