Study of Co, Ni and Co/Cu nanowires: Magnetic flux imaging by off-axis electron holography

The potential of electron holography for determining the remanent magnetisation state of small particles is illustrated by the study of nanometer-sized cylindrical model structures. The leaking flux was imaged for ferromagnetic Co and Ni wires of radius ranging from 20 to 150 nm and of a length of a few microns. For the smallest radius, the remanent magnetisation state was found to be uniform along the wire axis, with a value corresponding to the saturation magnetisation. For larger radii, nonuniform magnetisation configurations begin to appear, in general evidenced by the magnetisation value smaller than the saturation value. In a few cases multidomain wires were observed where a fraction showed an opposite magnetisation. Nanowires formed by a succession of elongated Co grains separated by Cu showed several magnetic configurations, from the parallel ferromagnetic alignment of the magnetic grains to the antiparallel configuration along the wire.