Synthesis and magnetic properties of Co-doped wurtzite ZnS nanocrystals

We report on the room temperature ferromagnetism in Co-doped wurtzite ZnS nanocrystals with size of about 5 nm, which gradually becomes weak as the increase of doping concentration. The coercivities for all samples maintain at about 200 Oe, and the saturation magnetic moment per Co ion decreases sharply for high doping samples. Besides ferromagnetism and paramagnetism, the temperature dependence of the susceptibility indicates the strong antiferromagnetic exchange in Co-doped wurtzite ZnS nanocrystals. X-ray diffraction, high-resolution TEM and X-ray photoelectron spectroscopy results show the ferromagnetism observed cannot be induced by secondary phases.