Ferromagnetism in ZnO Nanocrystals: Doping and Surface Chemistry

Room temperature ferromagnetic ordering is observed in chemically grown ZnO nanocrystals. Nanocrystals are simultaneously capped by two different organic molecules inducing p-type and n-type defects. A saturation magnetization of 0.008 emu/g is achieved at 300 K. Incorporation of Mn2+ ions at substitutional sites in nanocrystals gives rise higher saturation magnetic moment at lower doping level. ZnO nanocrystals codoped with Mn2+ and Co2+ and prepared under identical conditions revealed an increase in saturation magnetization. However, the saturation magnetic moment remains lower than that obtained for Co2+-doped ZnO nanocrystals prepared by the same method. An increase in saturation magnetization was invariably associated with quenching of photoluminescence emission. These findings reaffirm that magnetism in nanocrystals is a defect induced phenomenon that can be controlled by choice of capping agent as well as incorporation of the transition metal impurity. Magnetization as a function of temperature [M(T)] curve is discussed in view of available reports on the global exchange mechanism in these ferromagnetic nanocrystals.