Magnetic Properties of
Fe/Cu Codoped ZnO Nanocrystals
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Abstract
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<sup>2+</sup> and Fe<sup>3+</sup> ions, with the concentration of the
trivalent species increasing in the presence of Cu doping, providing
direct evidence of the Fe<sup>2+</sup> + Cu<sup>2+</sup> ⇌
Fe<sup>3+</sup> + Cu<sup>+</sup> 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