DOMAIN ANALYSIS BY MEANS OF MAGNETOTACTIC BACTERIA

Magnetotactic bacteria (MB)-cocci as well as a spirillum of type Magnetospirillum gryphiswaldense-were investigated for their applicability for nondestructive domain analysis on soft magnetic materials. Solely magnetotactic cocci which show a definite swimming direction proved to be suitable for this purpose. The intensity of the domains' magnetic stray field H of coated SiFe sheets proved to be high enough to visualize main domains even in distances of 500 mum from the specimen surface, in comparison to about 5 mum for colloid particles. Thus it was possible to develop a specific ''domain viewer'' which allows simple handling and reuse of bacteria for repeated analysis. Based on theoretical considerations about the cocci's motion in a magnetic field, a computer program for the simulation of spatiotemporal bacteria distributions was developed. Results show that-in contrast to colloid techniques and nonmotile bacteria-the gradient of H does not influence the contrast in an essential way. Due to Brownian motion of motile bacteria, a minimum domain width is required for effective visualizations. This was confirmed by experimental results of both secondary domains of crystalline specimens and narrow surface domains of amorphous bands. However, for small domain structures of the latter, unmotile dead bacteria proved to be applicable. Finally, influences of bacteria concentrations on the contrast as well as the contrast mechanism as a function of time are discussed.