Thursday march the 27th in room 40.07 of BEM at 11am :
Ferrofluids, colloidal suspensions of magnetic nanoparticles dispersed in a solvent, exhibit intriguing properties and hold great potential for biomedical and industrial applications. Due to their nanoscale size, these particles can be approximated as single magnetic dipoles, or nano-magnets, making them a real-world example of a theoretical dipolar fluid. Furthermore, their magnetic dipole interactions and free mobility in a liquid allow them to spontaneously self-assemble, even in the absence of an external magnetic field. This process gives rise to complex topologies with different magnetic states, including closed loops, head-to-tail coupling and networked structures. Although this phenomenon has often been modelled theoretically, it has rarely been observed experimentally. A deeper understanding and control of the interparticle interactions could improve the efficiency of existing ferrofluids in biomedical applications and lead to the development of novel magnetically responsive materials.
Most experimental techniques used to study ferrofluids provide macroscopic measurements, making it difficult to determine key nanoscale parameters. These include particle and assembly morphology, interparticle proximity and heating efficiency. Although magnetic dipolar interactions are fundamental to self-assembly, many aspects remain unexplored.
This work focuses on the detailed characterisation of ferrofluids composed of flower-shaped nanoparticles of hard magnetic materials such as cobalt ferrite (CoFe₂O₄) and soft magnetic materials such as manganese ferrite (MnFe₂O₄) and maghemite (γ-Fe₂O₃). The magnetic properties of these ferrofluids were analysed using standard magnetometry techniques, revealing the significant influence of the chemical composition of the nanoparticles on the macroscopic ferrofluid behaviour. In addition, the structuring of nanoparticles in liquid n ferrofluids was investigated by observing both isolated particles and assembled aggregates. This was achieved using a cryogenic transmission electron microscopy technique developed specifically for this study. The influence of nanoparticle morphology on their magnetic properties was furthe rinvestigated by tomography in collaboration with the IPCMS laboratory in Strasbourg. At the nanoscale, the magnetic properties of these assemblies were assessed using electron holography in collaboration with the CEMES laboratory in Toulouse.
The study also explored binary ferrofluids, consisting of physical mixtures of hard and soft magnetic nanoparticles, to investigate new dipolar magnetic interactions. The organisation of nanoparticles within these binary ferrofluids was analysed in detail using chemically selective and spatially resolved transmission X-ray microscopy on the HERMES beamline at the SOLEIL synchrotron. This provided chemical mappings of the CoFe₂O₄ and MnFe₂O₄ nanoparticle assemblies. The separation of the magnetic contributions from both types of nanoparticles was achieved using the First Order Reversal Curve (FORC) magnetometry technique in collaboration with the IPGP laboratory. In addition, chemically selective magnetisation curves were obtained by spectroscopic measurements using a liquid cell for in-situ experiments carried out on the GALAXIES beamline at the SOLEIL synchrotron.
These results help to improve future simulations of interacting nanoparticles and provide new insights into the behaviour of similar ferrofluids under static external magnetic fields.