Off-axis electron holography of magnetotactic bacteria: magnetic microstructure of strains MV-1 and MS-1

Off-axis electron holography in the transmission electron microscope is use d to characterize the magnetic microstructure of magnetotactic bacteria. Th e practical details of the technique are illustrated through the examinatio n of single cells of strains MV-1 and MS-1, which contain crystals of magne tite (Fe3O4) that are similar to 50 nm in size and are arranged in chains. Electron holography allows the magnetic domain structures within the nanocr ystals to be visualized directly at close to the nanometer scale. The cryst als are shown to be single magnetic domains. The magnetization directions o f small crystals that would be superparamagnetic if they were isolated are found to be constrained by magnetic interactions with adjacent, larger crys tals in the chains. Magnetization reversal processes are followed in situ, allowing a coercive field of between 30 and 45 mT to be measured for the MV -1 cell. To within experimental error, the remanent magnetizations of the c hains are found to be equal to the saturation magnetization of magnetite (0 .60T). A new approach is used to determine that the magnetic moments of the chains are 7 and 5X10(-16)Am(2) for the 1600-nm long MV-1 and 1200-nm long MS-1 chains examined, respectively. The degree to which the observed magne tic domain structure is reproducible between successive measurements is als o addressed.