Efeitos de tamanho finito e interface em nanopartículas e nanocolóides magnéticos

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

In this work, we investigated magnetic nanoparticles based on copper ferrite, with mean sizes ranging between 3,5 nm e 10,7 nm. Such nanomaterials are chemically synthesized by hydrothermal coprecipitation and allow elaborating magnetic nanocolloids in aqueous and acidic medium thanks to a core-shell strategy that prevent the nanoparticles from dissolution. Consequently, they consist in a core made of stoichiometric copper ferrite surrounded by a maghemite shell. Magnetization measurements as a function of the temperature allow to separate the magnetically ordered core monodomain from the surface shell which presents at low temperature a spin-glass like disordered structure. Moreover, the freezing temperature is larger in the presence of magnetic dipolar interactions between particles (powder). Mössbauer spectroscopy measurements, in the presence and absence of an applied magnetic field confirm the existence of the magnetic core-shell model, with cation inversion in the ordered magnetic core, and lead to the determination of the anisotropy energy. Furthermore, the investigation of the Brownian rotation dynamics using an experimental setup of magneto-optical birefringence in crossed fields also permits to determine the magnetic anisotropy energy of the particles. For both determinations, the results suggest an intense surface contribution for smaller nanoparticles.

ASSUNTO(S)

efeitos de tamanho e interface fisica nanopartículas magnéticas

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