Magnetic nanoparticles have been the issue of continuous and growing interest, from both fundamental and technological points of view, in the last decades. Their unique physical objects with remarkable magnetic properties differ significantly from their parent massive materials. These properties are due to finite size effects of the magnetic core, related to the reduced number of spins cooperatively linked within the particle, and to surface and interface effects, related to the lack of coordination for the surface ions, inducing broken exchange bonds that can result in frustration and spin disorder. A description of ferrite magnetic properties requires a through characterization of the crystallographic structure down to atomic scale. Indeed, synthesis processes significantly alter the chemistry of the involved compounds, and then their physical properties.

In this work, experimental investigations on cobalt ferrite nanoparticles CoxFe3-xO4 (1 < x < 1.8) are reported, providing a comprehensive description of different surface and interface effects. Surface and exchange anisotropy in ferrite nanoparticles are investigated by means of several experimental techniques such as X ray diffraction, Transmission Electron Microscopy and magnetization measurements. A strong correlation between the structural and the magnetic properties has been revealed. It is shown that the proposed synthesis technique leads to the formation of a spinel nanoparticles well dispersed. Many of them are faceted having cub-octahedral polygonal growth forms with exclusively the {1 1 1} and {1 0 0} type faces. Rather than core-shell structures, a number of ordered defects were observed.

Figures:

HRTEM micrographs, showing cobalt ferrites nanoparticles observed by Tecnai (a, c) and Titan (b, d). The insets in both images (c, d) represent the corresponding simulated ones

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EMC Abstracts - https://emc-proceedings.com/abstract/surface-effects-in-nano-cobalt-ferrites/