Speaker
Description
Magnetic hybrid nano-architectures, which combine materials with distinct magnetic properties at the nanoscale, enable the development of advanced materials with novel functionalities arising from interface interactions [1]. In this context, exchange-coupled hard/soft nano-heterostructures, combining ferro(i)magnetic phases with high magnetic anisotropy and high magnetization, have garnered significant interest for applications in energy (e.g., permanent magnets), information storage, and biomedicine. However, synthesizing high-quality heterostructures remains complex due to the challenges in achieving well-defined interfaces and strong magnetic coupling. This work proposes a novel one-pot synthesis strategy, previously used to produce metallic nanoparticles, under milder conditions compared to conventional methods [2, 3], based on the reduction and subsequent oxidation of preordered precursor salts. To demonstrate the feasibility of the approach, the study focuses on a model Fe-Co/CoFe$_2$O$_4$ system, comprising a soft Fe-Co metal core with high saturation magnetization and a hard CoFe$_2$O$_4$ ferrite. Crystalline Co-nitroprusside complexes ($1:1$ and $2:1$ Fe:Co ratios) were reduced at $400$ °C in H$_2$ to form metallic cores, followed by controlled oxidation during the cooling process by varying the temperature ($T_i$) at which oxidation is induced from $400$ °C down to $250$ °C (with steps of $50$° C). The results reveal the formation of nanostructured Fe-Co powders dispersed in a CoFe$_2$O$_4$ matrix, exhibiting strong magnetic coupling, as evidenced by single-phase-like hysteresis loops at room temperature. Both coercivity ($H_c$) and saturation magnetization ($M_s$) vary as a function of $T_i$, with the best performance ($\mu_0H_c \simeq 0.1$ T, $M_s \simeq 140$ Am$^2$ kg$^{-1}$) achieved at $T_i= 350$ °C. These results highlight the potential of this approach for producing strongly exchange-coupled systems.
Acknowledgments
This work is supported by the project “Hybrid ferrite nanocomposites for novel rare-earth free permanent magnets” (P2022RRT4_PE5_PRIN2022PNRR)
References
[1] S. Laureti et al., “The role of chemical and microstructural inhomogeneities on interface magnetism,” Nanotechnology, vol. 32, no. 20. IOP Publishing, p. 205701, Feb. 22, 2021. https://doi.org/10.1088/1361-6528/abe260
[2] G. Varvaro et al., “Facile and fast synthesis of highly ordered L10-FeNi nanoparticles,” Scripta Materialia, vol. 238. Elsevier BV, p. 115754, Jan. 2024. https://doi.org/10.1016/j.scriptamat.2023.115754
[3] G. Varvaro et al., “Synthesis of L10 alloy nanoparticles. Potential and versatility of the pre-ordered Precursor Reduction strategy,” Journal of Alloys and Compounds, vol. 846. Elsevier BV, p. 156156, Dec. 2020. https://doi.org/10.1016/j.jallcom.2020.156156
