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Colloidal solutions of magnetic nanoparticles (ferrofluids) exhibit intriguing macroscopic properties that find their origin in the magnetic dipole interparticle interactions taking place on the nanoscale. Here, we study experimentally the structuration and magnetic properties of a ferrofluid composed of 25 nm CoFe2O4 nanoflowers, which show different types of nanoparticle assemblies with and without an external magnetic field. A combination of advanced magnetometry (first-order reversal curve measurements combined with simulations), transmission electron microscopy (high-resolution and cryogenic transmission electron microscopy, electron tomography, and electron holography), and small-angle X-ray scattering techniques was used to build a comprehensive picture from the nanoscale to the macroscale of this ferrofluid, from the collective behavior down to the particle level. Our results provide an original, extensive set of experimental quantitative data and make a connection between the nanoscale organization and collective magnetic properties. Our findings also highlight the competition between the magnetic anisotropy of CoFe2O4 nanoflowers and magnetic dipole interactions.
Khelfallah et al. (Mon,) studied this question.