Free-standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe 2+ and Fe 3+ ions, with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe 2+ + Cu 2+ ⇌ Fe 3+ + Cu + redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations.
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Viswanatha et al. (2012) studied this question.
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