BUSCADOR RECOLECTA

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.

The effect of geometry on Giant Magnetoimpedance effect (GMI) in a meander structure composed of three amorphous magnetic ribbons ((Co0.94 Fe0.06)72,5 Si12.5 B15; 20 mm length) connected electrically in series is analyzed. The impedance behavior under the meander configuration is compared with the sum configuration, namely, the sum of the impedance of each ribbon measured individually. The geometry effect in GMI response is examined by changing the distance, = 0.5, 1 and 2 cm, between ribbons in the meander. The highest GMI ratio is found for = 0.5 cm, with a gradual decrease for increasing distances. The lowest ratio corresponds to the sum configuration. The analysis of the results shows that this behavior of the GMI ratio, dominated by inductance, is determined by the overall negative contribution of the mutual inductance established between ribbons, and not by any intrinsic modification of the GMI effect in the meander structure., The authors want to acknowledge the funding from the Spanish Government (grants MAT2017-83631-C3-2-R, PID2020-116321RB-C21 and PID2020-115704RB-C32 all funded by Ministerio de Ciencia e Innovación MCIN/AEI/10.13039/501100011033). Besides, we want to acknowledge the funding of the Public University of Navarre (project PJUPNA2005) and the Basque Government (grants IT1245-19 and doctoral fellowship of N. L).