Evidence of Ferromagnetic Signal Enhancement in Fe and Co Codoped ZnO Nanoparticles by Increasing Superficial Co3+ Content

In spite of the various theoretical and experimental efforts performed to understand the origin of ferromagnetism in Fe and Co codoped ZnO, there are still serious controversies in the reported data. While theoretical studies predicted the relative spin alignment and location of Co ²⁺ and Fe ²⁺ as the main source of magnetism, experimental studies have reported Co ²⁺ and superficial Fe ³⁺ . In this work, we performed a careful experimental study on Zn _{1–2 x} Fe _x Co _x O ( x = 0, 0.01, 0.03, and 0.05) nanoparticles prepared by a sol–gel method and have found new interesting results. We detected only Fe ³⁺ ions located in tetrahedral-core and pseudo-octahedral-surface sites. The Co ions displayed 2+ and 3+ oxidation states, with Co ²⁺ ions in high spin state located mostly in the tetrahedral-core sites, while Co ³⁺ in low spin states located presumably in pseudo-octahedral-surface sites. We detected isolated Fe ³⁺ ions and weakly ferromagnetic coupled Co ²⁺ ions. The most important finding is that the saturation magnetization ( M s) did not depend on the magnetic interactions involving the high spin Co ²⁺ or Fe ³⁺ ; but M _s and Co ³⁺ concentration increased systematically with x , indicating that multivalent ionic states may be playing a crucial role in the observed ferromagnetism.