Exchange coupled core–shell nanoparticles present high potential to tune adequately the magnetic properties for specific applications such as nanomedicine or spintronics. Here, we report on the design of core–shell nanoparticles by performing the successive thermal decomposition of Fe and Co complexes. Depending on the thermal stability and the concentration of the Co precursor, we were able to control the formation of a hard ferrimagnetic (FiM) Co-ferrite shell or an antiferromagnetic (AFM) CoO shell at the surface of a soft FiM Fe_3−δO₄ core. The formation of the Co-ferrite shell was also found to occur through two different mechanisms: the diffusion of Co or the growth at the iron oxide surface. The structural properties of core–shell nanoparticles were investigated by a wide panel of techniques such as HAADF, STEM and XRD. The distribution of Fe and Co elements in the crystal structure was described accurately by XAS and XMCD. The operating conditions influenced significantly the oxidation rate of Fe²⁺ in the core as well as the occupancy of O_h sites by Fe²⁺ and Co²⁺ cations. The structural properties of nanoparticles were correlated with their magnetic properties which were investigated by SQUID magnetometry. Each core–shell nanoparticle displayed enhanced effective magnetic anisotropy energy (E_eff) in comparison with pristine Fe_3−δO₄ nanoparticles because of magnetic coupling at the core–shell interface. The Co-ferrite FiM shells resulted in better enhancement of E_eff than a CoO AFM shell. In addition, the magnetic properties were also influenced by the core size. The coercive field (H_C) was increased by core reduction while the blocking temperature (T_B) was increased by a larger core. Element-specific XMCD measurements showed the fine coupling of Fe and Co cations which agree with Co-ferrite in each sample, e.g. the formation of a Co-doped interfacial layer in the Fe_3−δO₄@CoO nanoparticles.

We show that thin layers of EuO, a ferromagnetic insulator, can be achieved by topotactic reduction under titanium of a ${\mathrm{Eu}}_2{\mathrm{O}}_3$ film deposited on top of a graphene template. The reduction process leads to the formation of a 7-nm-thick EuO smooth layer, without noticeable structural changes in the underlying chemical vapor deposited graphene. The obtained EuO films exhibit ferromagnetism, with a Curie temperature that decreases with the initially deposited ${\mathrm{Eu}}_2{\mathrm{O}}_3$ layer thickness. By adjusting the thickness of the ${\mathrm{Eu}}_2{\mathrm{O}}_3$ layer below 7 nm, we promote the formation of EuO at the very graphene interface: the EuO/graphene heterostructure demonstrates the anomalous Hall effect (AHE), which is a fingerprint of proximity-induced spin polarization in graphene. The AHE signal moreover persists above $T_c$ up to 350 K due to a robust super-paramagnetic phase in EuO. This original high-temperature magnetic phase is attributed to magnetic polarons in EuO: we propose that the high strain in our EuO films grown on graphene stabilizes the magnetic polarons up to room temperature. This effect is different from the case of bulk EuO in which polarons vanish in the vicinity of the Curie temperature $T_c=69$ K.

Co/Fe layered double hydroxides (LDHs) are among the most promising materials for advanced industrial and energy applications. Controlling the synthesis conditions of LDH materials is thus crucial to precisely tailoring cation composition and distribution, thereby regulating surface charge, ion sorption, and electron transfer required for optimal chemical and electrochemical performance. Therefore, characterizing Co/Fe precipitates (chemical composition, purity, morphology, and crystallinity) is also required to further exploit their controlled properties. Thus, solids with Co/Fe cation ratios between 2/2 and 10/2 were synthesized under an air atmosphere, at pH 8 or 11.5. For the first time, multiscale physicochemical techniques (FTIR, TEM-EELS, SEM, AAS, TGA, CHN elemental analysis, and XRD complemented by Rietveld refinement) were used to provide a fully documented characterization of the structure, texture, purity, chemical composition, and thermal properties of Co/Fe LDH-based materials. The combined interpretation of data from these complementary techniques enabled the precise identification and chemical characterization of the mineralogical phases formed. Both acid–base and redox reactions govern the overall Co^II/Fe^III LDH formation process. Well-crystallized LDHs were synthesized, except for the 2/2 ratio at pH 11.5, which led to the formation of α-Co(OH)₂, γ-Fe₂O₃, and Co₃O₄ byproducts. A pH value of 8.0 provides valuable LDH materials made of quasi-hexagonal particles with diagonal lengths between 200 and 500 nm. Rietveld refining showed the presence of LDH phases in the range of 95–98%. Multiple local chemical analyses using EDX on chosen particles demonstrated pure 4/2 and 6/2 LDHs. For the 2/2 ratio, the cumulative mass fraction of two LDH-type products consistently reached 97%, distributed between Co/Fe 1.5/2 (71%) and Co/Fe 4/2 (29%). For the 10/2 ratio, only partial Co precipitation was observed, forming 95% Co/Fe LDH phases distributed between Co/Fe 10/2 (72%) and 7/2 (28%).

Spark ablation is an attractive gas-phase route for producing ligand-free metal nanoparticles, yet control over agglomeration and phase evolution during deposition remains challenging. The influence of carrier gas flow rate and deposition time on the agglomerate morphology, size distribution, and oxidation behavior of copper nanoparticles deposited onto silicon and glass substrates is investigated. Copper nanoparticles were generated by spark discharge and transported by nitrogen (N₂) carrier gas under flow rates ranging from 0.5 to 3.5 L min⁻¹ prior to deposition onto silicon substrates for 2 h and 5 h. Scanning electron microscopy reveals a transition from compact, isotropic agglomerates at low flow rates to elongated, chain-like structures at higher flow rates, indicating a shift from coalescence- and restructuring-favored conditions toward increasingly kinetically constrained aggregation. Transmission electron microscopy reveals predominantly spherical nanoparticles with a lognormal size distribution centered at approximately 10 nm, consistent with gas-phase nucleation and coalescence. Structural and spectroscopic analyses suggest CuO as the dominant oxide phase with minor Cu₂O contributions, with Raman spectroscopy providing the primary evidence due to limitations in high-resolution TEM analysis from substrate effects and peak overlap. These findings show that carrier-gas flow rate systematically influences agglomerate structure under the investigated conditions, while oxidation may occur both during aerosol transport and following ambient exposure; the present measurements do not allow the relative contributions of these processes to be quantitatively separated.