BUSCADOR RECOLECTA

In this work the authors have performed the synthesis of FexCo100-x (0<x<100) alloy nanoparticles (NPs) with different compositions, as well as pure Fe and Co NPs for comparison, by a chemical reduction technique. The subsequent characterization demonstrated excellent quality NPs with the expected bcc cubic (for Fe and FeCo alloys) and hcp hexagonal (for Co NPs) structures showing a room temperature magnetization as high as 235 emu/g for the Fe66Co34 composition alloy. Nevertheless, this soft magnetic character is accompanied by determined values of effective anisotropy as high as 2 MJ/m3. Aiming to deep into the properties of these FeCo alloys as well as to unveil the origin of that observed high anisotropy value, we now present an extensive study at the nanoscale of the synthesized Fe, Co and FexCo100-x alloy nanoparticles by using nuclear techniques as neutron powder diffraction, EXAFS and XANES spectroscopies. Mössbauer spectroscopy revealed that the FeCo alloys are in an A2 disordered solid solution. The obtained results, combined with AFM/MFM images, have demonstrated that despite the cubic bcc structure observed for all FeCo alloys (in excellent concordance with the pure Fe one) the NPs show a "flaky" shape of 50–60 nm size (diameter) but only 3–4 nm thickness, giving rise to a strong shape anisotropy contribution to the observed total effective anisotropy., J. Gutiérrez and M. Insausti gratefully acknowledge the financial support of the Basque Government under Research Groups Programme (IT1479-22 and IT1546-22, respectively) projects and research project MMASINT (KK-2023/00041, Elkartek Program). J. Gutiérrez, J. Alonso and M. Insausti also want to acknowledge grants No. PID2022–138108OB-C33, PID2020–115704RB-C3 and PID2022–136993OB-I00, respectively, funded by MCIN/AEI/ 10.13039/501100011033 and, as appropriate, by “ERDF A way of making Europe”, by the “European Union” or by the “European Union NextGenerationEU/PRTR”. The Authors acknowledge the Spanish Ministry of Science Innovation and Universities as well as SpINS for the beam time allocation at D1B instrument at ILL (Grenoble, France), and the financial support of CERIC for the experiment at the LISA beamline (codes 08011082 and 20212115). Technical and human support provided by the General Research Services of the UPV/EHU (SGIker) is gratefully acknowledged., Peer reviewed

Magnetotactic bacteria (MTB) are at the forefront of interest for biophysics applications, especially in cancer treatment. Magnetosomes biomineralized by these bacteria are high-quality magnetic nanoparticles that form chains inside the MTB through a highly reproducible, naturally driven process. In particular, Magnetovibrio blakemorei and Magnetospirillum gryphiswaldense MTB exhibit distinct magnetosome morphologies: truncated hexa-octahedral and cuboctahedral shapes, respectively. Despite having identical compositions (magnetite, Fe3O4) and dimensions within a similar size range, their effective uniaxial anisotropies significantly differ at room temperature, with M. blakemorei exhibiting ∼25 kJ/m3 and M. gryphiswaldense ∼ 11 kJ/m3. This prominent anisotropy variance provides a unique opportunity to explore the role of magnetic anisotropy contributions in the magnetic responses of these magnetite-based nanoparticles. This study systematically investigates these responses by examining static magnetization as a function of temperature (M vs T, 5 mT) and magnetic field (M vs μ0H, up to 1 T). Above the Verwey transition temperature (∼110 K), the effective anisotropy is dominated by the shape anisotropy contribution, notably increasing the coercivity for M. blakemorei by up to twofold compared to M. gryphiswaldense. However, below this temperature, the effective uniaxial anisotropy rapidly increases in a nonmonotonic way, significantly changing the magnetic behavior. Computational simulations using a dynamic Stoner–Wohlfarth model provide insights into these phenomena, enabling careful interpretation of experimental data. According to our simulations, below the Verwey temperature, a uniaxial magnetocrystalline contribution progressively emerges, peaking around 22–24 kJ/m3 at 5 K. Our study reveals the complex evolution of magnetocrystalline contributions, which dominate the magnetic response of magnetosomes below the Verwey temperature. This demonstrates the profound impact of anisotropic properties on the magnetic behaviors and applications of magnetite-based nanoparticles and highlights the exceptional utility of magnetosomes as ideal model systems for studying the complex interplay of anisotropies in magnetite-based nanoparticles., This work has been funded by the Spanish Government (grants PID2020-115704RB-C3 and PID2023-146448OB-C2 funded by MICIU/AEI/10.13039/501100011033/FEDER, UE) and the Basque Government (grant IT1479-22). L.G. would like to acknowledge the financial support provided through a postdoctoral fellowship from the Basque Government (POS_2022_1_0017). L.M. thanks the Horizon Europe Programme for the financial support provided through a Marie Sklodowska-Curie fellowship (101067742) and the BBVA Foundation for the Leonardo Fellowships for Researchers and Cultural Creators 2022. E.M.J. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Actions grant agreement 101081455 - YIA and from the Institute for Advanced Studies (IAS) of the University of Luxembourg for a postdoctoral fellowship. The authors thank SGIker (UPV/EHU/ERDF, EU) for technical and human support.