Exploring the Complex Interplay of Anisotropies in Magnetosomes of Magnetotactic Bacteria

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 μ₀ H, 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.