Nucleation, seed-mediated growth and Integration of Magnetic nanoRods – NIMRod

The NIMRod project gathers basic research on magnetic nanorods (NR) from their synthesis in liquid phase to the engineering of their assembly into permanent magnets (PM) of controlled size in the submillimetre range to meet the microelectronics demand for integrated devices. The project relies on recent results that have shown that dense assemblies of single-domain and single-crystal magnetic NR make performant PM and that magnetophoresis can be implemented to fabricate localized magnets with tunable geometries. This method, fully compatible with the microelectronics workflow process, has been recently patented by the coordinator’s team. The ambition of the NIMRod project is to build a new generation of PM (i) by developing the syntheses of original magnetic NR by seed-mediated growth (SMG) methods, keeping in mind the full process sustainability by reducing the use of critical raw materials, and (ii) by increasing the performance of the PM prepared by directed assembly of NR under external fields thanks to in operando state-of-the-art experiments. The project gathers chemists, physicists and engineers specialized in the growth and assembly of nanoparticles, microfluidics and X-ray scattering and is organized in three complementary tasks.

The synthesis of metal seeds will be studied in situ and in real time to elucidate the nucleation mechanism, control their crystal habits and engineer reliably the stacking faults. Penta-twinned nanoparticles (PTNP), such as decahedra or bipyramid which are well-known to promote the anisotropic growth of noble metals of cubic structure, will be targeted more specifically. Microfluidic setups will be developed for time resolved spectroscopic (UV-visible and X-ray absorption) and X-ray scattering studies, from very short times (< ms) up to few hours, thus covering the pre-nucleation, nucleation, growth and aging stages. The experimental data will be used to build kinetic models including the role of coordinating ligands in the different steps and considering both the classical nucleation theory and non-classical mechanisms for the nucleation step.
Several SMG methods will be developed to grow NR of 3d metals, FeNi and FeCo alloys, from PTNP, using organometallic chemistry or polyol process, and using different strategies to overcome the possible issue of lattice mismatch. Ultrathin metal wires of polytetrahedral structure will also be used as seeds. Finally, the growth of cobalt NR by the polyol process with a precise control of the seeding stage will allow to spare 98% of noble metal compared to the current method. The seed/rod interface will be characterized by atomic-resolution electron microscopy and the magnetic properties of the final NR will be compared with micromagnetic modelling.
Magnetophoresis and dielectrophoresis experiments will be implemented to make PM from NR suspensions applying external magnetic or a.c. electric fields. In operando small and wide angle X-ray scattering (SAXS, WAXS) and polarized light microscopy studies will be carried out to describe each step of the process: NR alignment and formation of macroscopic fibres, migration toward areas of highest field gradient areas and finally densification during solvent evaporation. The 3D NR assembly will be characterized by its orientation distribution function (ODF) and order parameter. An important work will consist in developing the numerical tools to model the 2D SAXS diagrams of 3D NR assemblies. The ODF established from the SAXS and WAXS measurements, sensitive to the form and structure factors and to the crystallographic orientation, respectively, will allow for complete description of the PM microstructure. The ODF will be implemented in magnetic simulations to fit the PM magnetization loops. The knowledge of the influence of the NR properties and of their ODF will allow optimizing the synthesis and the assembly process and increasing the PM performance, assessed by the magnetic induction generated.