Magnetic nanocomposite materials consisting of 5 and 10 wt% CoFe(2)O(4) nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe(2)O(4)-10wt% sample, bright-field scanning transmission electron microscopy (BF STEM) and high-resolution transmission electron microscopy (H REM) images showed distinct, rounded CoFe(2)O(4) nanoparticles, with typical diameters of roughly 8 nm. For the CoFe(2)O(4)-5wt% sample, BF STEM images and energy dispersive X-ray (EDX) measurements showed CoFe(2)O(4) nanoparticles with diameters of roughly 3 +/- 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe(2)O(4), and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe(2)O(4)-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than 0.5 nm in analysis position. BE STEM images obtained for the CoFe(2)O(4)-10wt% sample showed many "needle-like" nanostructures that typically have a length of 10 nm and a width of 1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 +/- 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials.
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