Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

The magnetic properties of biologically produced magnetite nanocrystals biomineralized by four different magnetotactic bacteria were compared to those of synthetic magnetite nanocrystals and large, high-quality single crystals. The magnetic feature at the Verwey temperature T_V was clearly seen in all nanocrystals, although its sharpness depended on the shape of individual nanoparticles and whether or not the particles were arranged in magnetosome chains. The transition was broader in the individual superparamagnetic nanoparticles for which T_B<T_V, where T_B is the superparamagnetic blocking temperature. For nanocrystals organized in chains, the effective blocking temperature T_B>T_V and the Verwey transition is sharply defined. No correlation between particle size and T_V was found. Furthermore, measurements of M(H,T,time) suggest that magnetosome chains behave as long magnetic dipoles where the local magnetic field is directed along the chain. This result confirms that time-logarithmic magnetic relaxation is due to the collective (dipolar) nature of the barrier for magnetic moment reorientation.