RESUMO
Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni1-xFexO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.
RESUMO
Monophasic Zn1-xFexO nanoparticles with wurtzite structure were synthesized in the 0 ≤ x ≤ 0.05 concentration range using a freeze-drying process followed by heat treatment. The samples were characterized regarding their optical, structural, and magnetic properties. The analyses revealed that iron doping of the ZnO matrix induces morphological changes in the crystallites. Iron is substitutional for zinc, trivalent and distributed in the wurtzite lattice in two groups: isolated iron atoms and iron atoms with one or more neighboring iron atoms. It was also shown that the energy band gap decreases with a higher doping level. The samples are paramagnetic at room temperature, but they undergo a spin-glass transition when the temperature drops below 75 K. The magnetic frustration is attributed to the competition of magnetic interactions among the iron moments. There are a superexchange interaction and an indirect exchange interaction that is provided by the spin (and charge) itinerant carriers in a spin-polarized band situated in the vicinity of the Fermi level of the Fe-doped ZnO semiconductor. The former interaction actuates for an antiferromagnetic coupling among iron ions, whereas the latter constitutes a driving force for a ferromagnetic coupling that weakens, decreasing the temperature. Our results strongly contribute to the literature because they elucidate the controversies reported in the literature for the magnetic state of the Fe-doped ZnO system.