Observation of Ferromagnetism in Heavy Ion Bi-doped Nanocrystalline Zinc Oxide Prepared by Co-precipitation

Room temperature soft ferromagnetism has been detected in nanocrystalline ZnO while doping with Bi as a heavy non-magnetic atom. The ZnBiO (x = 0.0, 0.01, 0.03, and 0.05) system was synthesized by chemical co-precipitation. To investigate the origin of this enhanced ferromagnetism, characterization of crystal structure, lattice dynamics, and magnetic properties has been achieved. The presence of only one wurtizite single phase of nanocrystalline ZnO was confirmed through Rietveld analysis. The gradual increase of the c parameter also reflected complete solubility of Bi in the wurtizite lattice up to x = 0.05. Bi was detected to occupy the position 2b (1/3, 2/3, 0) of Zn. A drastic decrease of the crystallite size to less than its half value was also detected. Well-defined intercrystallite boundaries were imaged by HR-TEM. ESR studies revealed a moderate increase of the free spin density while the g values gave evidence of the formation of defects like V, V, and (Bi–V) complex. Considerable increase of magnetic moment was found as Bi content increased to 0.05. We believe that the enhancement of ferromagnetism in Bi-doped ZnO can be attributed to an ensemble of key factors: free spins, vacancy defects, and the formation of nanocrystallites with intergranular layers within a threshold limit.