Magnetic Properties of Fe Doped, Co Doped, and Fe+Co Co-Doped ZnO

The structural, electronic, and magnetic properties of Zn_0.95Co_0.05O, Zn_0.95Fe_0.05O, and Zn_0.90Fe_0.05Co_0.05O nanoparticles prepared by a sol-gel method are presented and discussed. X-ray diffraction and optical analysis indicated that high spin Co²⁺ ions substitute for the Zn²⁺ ions in tetrahedral sites. ⁵⁷Fe Mössbauer spectroscopy showed the presence of isolated paramagnetic Fe³⁺ ions in both Fe doped and Fe+Co co-doped ZnO, however, no evidence of ferromagnetically ordered Fe³⁺ ions is observed. In the Zn_0.95Fe_0.05O sample, weak presence of ZnFe₂O₄ was detected as an impurity phase, whereas Zn_0.90Fe_0.05Co_0.05O was impurity-free within detection limit in all those measurements. Results of these studies suggest that Fe and Co ions in the Fe+Co co-doped sample has a strong synergistic effect because they eliminated the presence of impurities and gave the strongest ferromagnetic signal. Possible role of charge transfer ferromagnetism involving mixed valence ions is considered as a potential mechanism in these nanoparticles. Presence of both Co²⁺ and Fe³⁺ might promote more efficient charge transfer in the co-doped Zn_0.90Fe_0.05Co_0.05O, leading to the enhanced ferromagnetism observed in this sample. However, more evidence is necessary to confirm the role of charge transfer ferromagnetism.