Combination of Defects Plus Mixed Valence of Transition Metals: A Strong Strategy For Ferromagnetic Enhancement in ZnO Nanoparticles

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In spite of several studies investigating the effect of codoping ZnO nanoparticles with two different transition metals, there is no clarity on the combined effects of the transition metal (TM) dopants in modifying the properties of the host system such as enhancement in room-temperature ferromagnetism. We attempted to address this by experimentally investigating the chemical and physical properties of a series of sol–gel-synthesized Zn1–xTMxO (with x = 0, 0.01) and Zn0.98Fe0.01TM0.01O nanoparticles with TM = Cr, Mn, Fe, Co, Ni, and Cu. Interestingly, it was found that in comparison to ZnO nanocrystallites doped with a single TM, the ferromagnetic signal increases in a codoped system in which one of the TM ions increases the concentration of defects while the other ion exhibits two oxidation states, for example, Fe3+-(Co2+, Co3+), Fe3+-(Mn2+, Mn3+), and Fe3+-(Cu+, Cu2+) states as observed in the present study. On the contrary, the enhancement in magnetism was smaller or similar when TM ions exhibited only one oxidation state, either the same states as in (Fe3+–Cr3+, Fe3+–Fe3+) or different like in (Fe3+–Ni2+). The possible role of charge transfer ferromagnetism involving mixed valence ions and effects of the grain boundary were used to explain the room-temperature ferromagnetism.