Magnetization reversal observation and manipulation of chains of nanoscale magnetic particles using the magnetic force microscope

The magnetization reversal of chains of 40-50 nanometer, magnetic particles has been studied using Magnetic Force Microscopy (MFM) in an applied field. The field was stepwise increased and decreased in order to make hysteresis loops for the chains. The magnetic particles magnetosomes biomineralized by magnetotactic bacteria, are single crystal Fe3O4 with a narrow size distribution. A method for extracting Switching Field Distributions (SFD 's) from sets of MFM images of such chains was developed The coercivity was found to increase with the number of particles in a chain up to 7 or 8 particles and then decrease with increasing number of magnetosomes. After the initial reversal observations one of the chains was cut into smaller pieces using the MFM-tip, thus producing separated chain segments, which resulted in altered inter-particle interactions. Additionally, multiple hysteresis loops were made for the same chain, showing that the field from the tip affects the sample magnetization. (C) 1999 Acta Metallurgica Inc.