Exchange bias and the origin of room-temperature ferromagnetism in Fe-doped NiO bulk samples

A series of Ni(1-x)FexO (x=0, 0.015, 0.03, 0.05, and 0.1) bulk samples was synthesized by the chemical concentration-precipitation method. Phase composition analysis was carried out, which showed that trace amounts of ferromagnetic phase NiFe2O4 could not be detected by x-ray diffraction in these bulk samples with x <= 0.03. When x>0.03, NiFe2O4 ferrite is detected easily. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The results indicated that all the bulk samples sintered in air exhibited large room-temperature ferromagnetic behavior ascribed to a ferromagnetic impurity phase. Simultaneously, an exchange bias and training effect were also observed in all the bulk samples, suggesting the possibility of the existence of a strong ferromagnetic/antiferromagnetic exchange coupling in this kind of compound. Specifically, the exchange bias field could be tuned by changing the concentration of the Fe dopant. (C) 2008 American Institute of Physics.