Ferromagnetic resonance of nanocrystal chains with competitive and cooperative anisotropy

Abstract:

The formation of cellular magnetic dipoles by chain assemblies of nearly equidimensional, stable single domain magnetite nanocrystals aligned along their [111] easy axes is a common property encountered in many magnetotactic bacteria (MTB). The development of such dipoles permits the navigation of MTB along the geomagnetic field towards favourable habitats, a process also referred to as magnetotaxis. An important characteristic is the anisotropy within the chains, which mainly consists of the magnetocrystalline and the shape anisotropy. The two anisotropy contributions can be cooperative or competitive depending on the orientation with respect to the chain axis. The change in the relative orientation between the two anisotropy contributions caused by the Verwey transition T_V, can be used to unambigously detect MTB and their fossil remains. Ferromagnetic resonance spectroscopy (FMR) is a well-established method to probe magnetic anisotropy in absolute units.

Here, we use X- and Q-band FMR spectroscopy and numerical simulation to analyze the MTB species of Desulfovibrio magneticus RS-1 with elongated magnetosomes aligned along the [100] hard axis. In this special case, the magnetotaxis above T_V is strongly affected by the shape anisotropy of the nanocrystals and it is competitive to the magnetocrystalline anisotropy. Below T_V, the change of the easy axis [111] to [100] generates a cooperative system, which can be considered as the optimal case for magnetotaxis, i.e., shape and magnetocrystalline anisotropies are nearly parallel to the MTB chain axis.

In summary, the nanocrystal assembly in RS-1 provides another step towards a better understanding of the physics behind magnetotaxis.