Magnetic anisotropy and Verwey transition of magnetosome chains in Magnetospirillum gryphiswaldense

Magnetotactic bacteria (MTB) are characterized by cellular magnetic dipoles formed by the 1-D assembly of magnetite and/or greigite particles aligned along their magnetic easy axes. This alignment creates strong interaction-induced shape anisotropy. Ferromagnetic resonance (FMR) spectroscopy is applied to study the changes in anisotropy of the MTB Magnetospirillum gryphiswaldense between room temperature and 10 K. The Verwey transition is found at about 100 K. The characteristic FMR signal of the cellular dipole at room temperature vanishes upon cooling to the isotropic point at Ti ≈ 130 K, where the magnetocrystalline anisotropy constant K1 becomes zero. Monitoring of the FMR response of intact MTB as a function of temperature is taken to discuss theoretically the reduction of the interaction-induced shape anisotropy in magnetofossils because of diagenetic processes. It is concluded that there is a similarity in the FMR response between magnetofossils at room temperature and intact MTB near Ti. This is because the critical effect of the magnetocrystalline anisotropy constant K1 and of the alignment of magnetic easy axes on the cellular dipole. Low-temperature FMR results of intact MTB can thus be used as a guideline for detecting magnetofossils in geological environment