Regensburg 2010 – scientific programme

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MA: Fachverband Magnetismus

MA 8: Magnetic Shape Memory Alloys

MA 8.6: Talk

Monday, March 22, 2010, 16:30–16:45, H22

Magnetization processes during field and stress induced twin boundary motion in NiMnGa — •Andreas Neudert, Yiu-Wai Lai, Rudolf Schäfer, and Jeffrey McCord — IFW Dresden, Helmholtzstr. 20, 01069 Dresden

We have studied the twin boundary motion in bulk single crystals of the ferromagnetic shape memory alloy NiMnGa using polarized light microscopy. Magnetic domains were imaged by using magnetic indicator films that are placed on top of the sample. Those indicator films consist of a soft-magnetic garnet film that senses the out of plane stray field of the sample. Twin boundaries can be moved by either applying a magnetic field or a mechanical stress to the sample, but there are qualitative differences between the two mechanisms. After moving a twin boundary by applying an external magnetic field, the domain state consists of wide antiparallel domains with 180° domain walls. Moving the twin boundary by applying external stress results in a different domain state. Here the magnetization rotates as the twin boundary passes through and a patchy domain structure is created. After demagnetizing the sample in a decaying ac magnetic field the domain state consists of mainly 180° domain walls again. This suggests that the magnetization after stress-induced reversal is not in a global energy minimum and rather trapped in local minima. The involved energies and effective fields for the two mechanisms will be discussed in the presentation.