Evidence by Lorentz Microscopy for Magnetically Active Stacking Faults in MnAl Alloy

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Abstract
In the ordered MnAl crystal containing magnetic atoms which are coupled ferromagnetically, stacking faults are postulated through which an antiferromagnetic coupling exists between the atoms on either side of the fault. These account for the anomalous magnetic behavior of fine MnAl powder. Such faults give rise to a domain structure the walls of which are strongly pinned to these crystallographic defects. A direct evidence for the occurrence of these stacking faults is obtained by using Lorentz electron microscopy, whereby shadow patterns of opaque MnAl particles show a characteristic distortion of their contours when the microscope is used out of focus. The distortion is interpreted in terms of stray fields due to the domain structure. By using either the normal objective lens, which is an immersion lens, or the diffraction lens, the pattern can be studied with or without a strong applied field. Fields up to 9000 Oe could be applied. Domain structures were observed that showed a remarkable indifference for these fields. This rigidity is explained by assuming that the walls are pinned to stacking faults. Domain structures with a high pole density on the walls were also observed. These structures are unfavorable owing to large magnetostatic energy. Their occurrence is also explained by the presence of stacking faults.