Chemical Ordering in Bimetallic FeCo Nanoparticles: From a Direct Chemical Synthesis to Application As Efficient High-Frequency Magnetic Material

Single-crystalline FeCo nanoparticles with tunable size and shape were prepared by co-decomposing two metal-amide precursors under mild conditions. The nature of the ligands introduced in this organometallic synthesis drastically affects the reactivity of the precursors and, thus, the chemical distribution within the nanoparticles. The presence of the B2 short-range order was evidenced in FeCo nanoparticles prepared in the presence of HDAHCl ligands, combining 57 Fe Mössbauer, zero-field 59 Co ferromagnetic nuclear resonance (FNR), and X-ray diffraction studies. This is the first time that the B2 structure is directly formed during synthesis without the need of any annealing step. The as-prepared nanoparticles exhibit magnetic properties comparable with the ones for the bulk (M s = 226 Am 2 ·kg -1 ). Composite magnetic materials prepared from these FeCo nanoparticles led to a successful proof-of-concept of the integration on inductor-based filters (27% enhancement of the inductance value at 100 MHz)

On the advantages of spring magnets compared to pure FePt: Strategy for rare-earth free permanent magnets following a bottom-up approach

International audienceNanostructured magnets benefiting from efficient exchange-coupling between hard and soft grains represent an appealing approach for integrated miniaturized magnetic power sources. Using a bottom-up approach, nanostructured materials were prepared from binary assemblies of bcc FeCo and fcc FePt nanoparticles and compared with pure L10-FePt materials. The use of a bifunctional mercapto benzoic acid yields homogeneous assemblies of the two types of particles while reducing the organic matter amount. The 650 °C thermal annealing, mandatory to allow the L10-FePt phase transition, led to an important interdiffusion and thus decreased drastically the amount of soft phase present in the final composites. The analysis of recoil curves however evidenced the presence of an efficient interphase exchange coupling, which allows obtaining better magnetic performances than pure L10 FePt materials, energy product above 100 kJ m−3 being estimated for a Pt content of only 33%. These results clearly evidenced the interest of chemically grown nanoparticles for the preparation of performant spring-magnets, opening promising perspective for integrated subcentimetric magnets with optimized properties

Carbon Coating, Carburization and High Temperature Stability Improvement of Cobalt Nanorods

International audienceThe reactivity of highly crystalline hcp cobalt nanorods (NRs) with organic solvents at high temperature was studied. Cobalt NRs with a mean diameter of 15 nm were first synthesized by the polyol process and then heated at 300 °C in octadecene (ODE), oleylamine (OA) or mixtures of these two solvents. The surface and structural modifications of the Co NRs were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM). A disordered carbon shell was formed at the cobalt rod surface, the thickness of which can be tuned from 5 to 25 nm by increasing the amount of oleylamine in the solvent mixture. This carbon shell partially reduced the native cobalt oxide observed at the surface of the NRs and drastically improved their temperature stability as inferred from in-situ XRD study and TEM. The shape anisotropy and the crystallite anisotropy of the hcp phase are both preserved up to 400 °C for the carbon coated cobalt rods whereas the uncoated NRs lose their anisotropy at 225 °C. Treatments at 300 °C in ODE/OA mixtures for different durations allowed the progressive carburization of Co to Co2C. The crystallographic orientation of the Co2C grains within the cobalt NRs combined with the different carbon shell thickness on the {10-10} and (0001) facets of the rods suggested a preferential carburization from the lateral facets of the hcp cobalt rods

Co–Fe Nanodumbbells: Synthesis, Structure, and Magnetic Properties

International audienceWe report the solution phase synthesis, the structural analysis, and the magnetic properties of hybrid nanostructures combining two magnetic metals. These nano-objects are characterized by a remarkable shape, combining Fe nanocubes on Co nanorods. The topological composition, the orientation relationship, and the growth steps have been studied by advanced electron microscopy techniques, such as HRTEM, electron tomography, and state-of-the-art 3-dimensional elemental mapping by EDX tomography. The soft iron nanocubes behave as easy nucleation centers that induce the magnetization reversal of the entire nanohybrid, leading to a drastic modification of the overall effective magnetic anisotropy