Field and stress induced twin boundary motion in NiMnGa

Twin boundaries in NiMnGa can be moved by applying either magnetic fields or mechanical stresses, which favour one of the two variants next to the twin boundary. We have used polarized light microscopy to study the two different ways of twin boundary motion in bulk NiMnGa. By placing magnetic indicator films on top of the sample surface we were able to detect the magnetic domain structure of the sample. Without the indicator films, the different variants can be seen by using polarized light illumination.
We found qualitative differences in the resulting magnetic state after a twin boundary has been moved by magnetic field or mechanical stress. By applying a magnetic field along the easy axis of one variant this variant is magnetically saturated and upon reducing the magnetic field the domain state consists of wide anti-parallel domains with 180° domain walls. Moving the twin boundary by applying mechanical stress results in a different domain state. Here the magnetization rotates by about 90° as the twin boundary passes through the area and we found the domain state to consist mainly of smaller patchy domains. The domain state with wide anti-parallel domains can be reached by demagnetizing the sample in an ac magnetic field. This suggests that the magnetic state after stress induced twin boundary motion is not in its global energy minima but rather trapped in a higher energy state