Experimental study of magnetic interactions between individual biogenic magnetite nanocrystals
Abstract
We present an electron holographic study of magnetostatic interactions between magnetite nanocrystals from wild-type and mutant magnetotactic bacteria of the Magnetospirillum gryphiswaldense (M. gryph) strain. The results are discussed with reference to their implications for the superparamagnetic limit, magnetic moments of arrays of magnetic nanocrystals, and the threshold of magnetotaxis. The size and spacing of magnetite nanocrystals in M. gryph was carefully adjusted through control of iron provision and growth time. Examination of these cultured cells provided data on the magnetic properties ranging from isolated superparamagnetic crystals to fully assembled magnetosome chains. A magnetic state phase diagram was created showing empirically how the superparamagnetic threshold is influenced by the effect of inter-particle interactions within the chains. This experimental data was compared with analytical models of superparamagnetism which account for size, spacing and crystal shape. Further to this, we also studied two-dimensional arrays of magnetite particles produced by bacteria whose mamJ protein had been removed through genetic modification. In unmodified bacteria this protein is responsible for producing magnetosomes in linear arrangements. The mutant bacteria's non-linear assemblages had much lower magnetic moments per unit volume than bacteria with intact chains, and also displayed more complex magnetic microstructures such as vortex states. These complex magnetic states were metastable, as the states were sensitive to subtle changes in experimental conditions. The magnetic moments of all the samples examined, both wild-type and mutant cells, were compared to that necessary for alignment in the geomagnetic field, showing the threshold at which the crystal sizes, arrangement and numbers led to effective magnetotaxis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMGP31C0103S
- Keywords:
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- 1505 Biogenic magnetic minerals;
- 1540 Rock and mineral magnetism;
- 1594 Instruments and techniques