Exploring the complex interplay of anisotropies in magnetosomes of magnetotactic bacteria

TL;DR

This study investigates how different anisotropies influence the magnetic responses of magnetosomes in magnetotactic bacteria, revealing temperature-dependent behaviors and the emergence of magnetocrystalline anisotropy below the Verwey transition.

Contribution

It provides a systematic analysis of anisotropy effects in magnetosomes with different shapes, highlighting the temperature-dependent interplay of shape and magnetocrystalline anisotropies.

Findings

Shape anisotropy dominates above 110 K, increasing coercivity.

Below 110 K, uniaxial anisotropy increases non-monotonically.

Simulations show magnetocrystalline anisotropy peaks at 22-24 kJ/m^3 at 5 K.

Abstract

Magnetotactic bacteria (MTB) are of significant interest for biophysical applications, particularly in cancer treatment. The biomineralized magnetosomes produced by these bacteria are high-quality magnetic nanoparticles that form chains through a highly reproducible natural process. Specifically, Magnetovibrio blakemorei and Magnetospirillum gryphiswaldense exhibit distinct magnetosome morphologies: truncated hexa-octahedral and truncated octahedral shapes, respectively. Despite having identical compositions (magnetite, Fe3O4) and comparable dimensions, their effective uniaxial anisotropies differ significantly, with M. blakemorei showing ~25 kJ/m^3 and M. gryphiswaldense ~11 kJ/m^3 at 300K. This variation presents a unique opportunity to explore the role of different anisotropy contributions in the magnetic responses of 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. H, up to 1 T). Above the Verwey transition temperature (110 K), the effective anisotropy is dominated by shape anisotropy, notably increasing coercivity for M. blakemorei by up to two-fold compared to M. gryphiswaldense. Below this temperature, the effective uniaxial anisotropy increases non-monotonically, significantly altering magnetic behavior. Our simulations based on dynamic Stoner-Wohlfarth models indicate that below the Verwey temperature, a uniaxial magnetocrystalline contribution emerges, peaking at ~22-24 kJ/m^3 at 5 K, values close to those of bulk magnetite. 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.