English: Magnetic domain walls in a non-stoichiometric copper-manganese-aluminium Heusler alloy with 15 percent of the copper atoms replaced by aluminium quenched from 700 and annealed at 290 for 2 ½ hours to allow the anti-phase domains to grow.

The specimen is a thin foil of the alloy only about a micron thick viewed by transmission electron microscopy. The magnetic domain walls are pinned to the anti-phase boundaries (APB’s) – i.e. there is an antiferromagnetic layer at the surface of each APB.

(a) type APB’s viewed in high contrast in a dark-field electron micrograph using the <111> Bragg reflection selected with a small aperture in the back focal plane. The same area is repeated twice for comparison with the following two micrographs.

(b) The same area as in (a) viewed in a bright field electron micrograph so that the APB’s are out of contrast. Magnetic domains are made visible as light and dark areas by a small displacement an aperture in the back focal plane of the objective lens to cut off part of the main beam deflected by magnetic domains of a particular orientation (Foucault contrast). The two micrographs are obtained by displacing the aperture in two opposite directions – the domain contrast in one case changing from light to dark (and vice versa) in the other. It can be seen that the magnetic orientation of the domains is forced to curve and follow the APB structure seen in the dark field micrograph (a) so that the observed effect is one of topography.

(c) The same area as in (a) viewed in a bright field electron micrograph so that the APB’s are out of contrast. Magnetic domain walls are made visible as light and dark lines by defocusing the objective lens (Fresnel contrast). The two micrographs are obtained in one case by overfocussing and in the second by underfocussing – the domain wall contrast in one case changing from light to dark (and vice versa) in the other. It can be seen that the magnetic domain walls are all tightly bound to where the APBs appear in the dark field micrograph (a)