ResearchCite this article: Li J et al. 2016 Controlled cobalt doping in the spinel structure of magnetosome magnetite: new evidences from element-and site-specific X-ray magnetic circular dichroism analyses. J. R. Soc. The biomineralization of magnetite nanocrystals (called magnetosomes) by magnetotactic bacteria (MTB) has attracted intense interest in biology, geology and materials science due to the precise morphology of the particles, the chain-like assembly and their unique magnetic properties. Great efforts have been recently made in producing transition metal-doped magnetosomes with modified magnetic properties for a range of applications. Despite some successful outcomes, the coordination chemistry and magnetism of such metal-doped magnetosomes still remain largely unknown. Here, we present new evidences from X-ray magnetic circular dichroism (XMCD) for element-and site-specific magnetic analyses that cobalt is incorporated in the spinel structure of the magnetosomes within Magnetospirillum magneticum AMB-1 through the replacement of Fe 2þ ions by Co 2þ ions in octahedral (O h ) sites of magnetite. Both XMCD at Fe and Co L 2,3 edges, and energy-dispersive X-ray spectroscopy on transmission electron microscopy analyses reveal a heterogeneous distribution of cobalt occurring either in different particles or inside individual particles. Compared with nondoped one, cobalt-doped magnetosome sample has lower Verwey transition temperature and larger magnetic coercivity, related to the amount of doped cobalt. This study also demonstrates that the addition of trace cobalt in the growth medium can significantly improve both the cell growth and the magnetosome formation within M. magneticum AMB-1. Together with the cobalt occupancy within the spinel structure of magnetosomes, this study indicates that MTB may provide a promising biomimetic system for producing chains of metal-doped single-domain magnetite with an appropriate tuning of the magnetic properties for technological and biomedical applications.

Magnetic anisotropies and crystallographic structures of cobalt-iron nanospinels obtained with the co-precipitation synthesis process are revealed using X-ray Absorption Spectroscopy and X-ray Magnetic Circular Dichroism. The chemical process allows to obtain nanoparticles of various sizes and chemical composition, but it is the site symmetry environment of Co 2+ that is found to be the crucial parameter that governs the magnetic anisotropies of the nanospinels. The distribution of Co 2+ among the crystallographic sites of the structure is directly linked to the temperature of the synthesis process. In parallel, the results also revealed that a superficial rich shell of iron is formed for the iron-cobalt nanospinels stable in acidic medium leading to chemically inhomogeneous nanoparticles.