This paper demonstrates that, by using oil-in-water micelles, nonstoichiometric zinc ferrite nanoparticles are fabricated. Control in size and composition is obtained. The nanocrystals are characterized by a spinel structure with two sublattices in tetrahedral and octahedral sites. Repartition of Fe 3+ in the two sublattices is disordered. The particles are ferrimagnetic with a noncollinear structure (the magnetic moments are not strictly parallel). This is more pronounced with the smallest particles. Zinc ferrite nanocrystals exhibit unusual magnetic properties. The anisotropy constant, K, of nanocrystals differing by their sizes is determined by Mo ¨ssbauer spectroscopy and from zero field cooled (ZFC) curves. The field cooled (FC) curves show a peculiar behavior with a decrease in the magnetization at very low temperature. The ZFC curves are highly sensitive to the applied field, indicating that Langevin law cannot be applied for zinc ferrite nanocrystals. These unusual magnetic properties are attributed to the inclusion of nonmagnetic zinc ions in ferrite nanocrystals, which induces a magnetic disorder.
Maghemite nanocrystals, deposited by slow evaporation on HOPG (highly oriented pyrolytic graphite) substrate, form mesoscopic structures which strongly depend on the nanocrystal coating. When the coating consists of citrate ions and octanoic acid, tubelike structures are formed when the deposition process takes place under an applied magnetic field. Conversely, dense films are formed when the nanocrystals are coated with propanoic acid and dodecanoic acid. The magnetic behaviors markedly differ with the organization of nanocrystals and are attributed to the change in the anisotropy of the organized structures. Nanocrystals organized in tubes behave as nanowires.
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