Magnetic shape memory films
Magnetic shape memory (MSM) materials can change their shape in a magnetic field. The overall strain can be as high as 12%, which is much larger than in any common magnetostrictive material. These materials are interesting for all actuation applications.
The magnetic-field induced strain
In the animation above, a single crystal of the magnetic shape memory material Ni-Mn-Ga is placed in an alternating magnetic field. When the field increases, the length of the crystal in the field direction decreases by about 6%. When the field is decreased, this effect is reversed, because a constant external force is applied perpendicular to the magnetic field.
The mechanism behind this outstanding effect is the magnetic-field-induced twin boundary motion. The stripes that grow and shrink at the surface of the sample are signs of twin boundaries. These are interfaces inside the crystal where the crystal axes and also the direction of the easy axis of magnetization change (Figure (a) above). When a magnetic field is applied, the twin boundaries move in a way that the areas which have their easy axes in field direction grow. Since the crystal directions are coupled to the easy axis of magnetization, this leads to a decrease of the sample size. An external stress can reset the sample again by moving the twin boundaries.
Why thin films?
Thin films are suitable model systems to understand the behavior of the material and the influence of restrictions in size. Epitaxial films are also interesting to build micro-actuators. We study films of the prototype materials Ni-Mn-Ga and Fe-Pd.
M.E. Gruner, S. Faehler, P. Entel: Magnetoelastic coupling and the formation of adaptive martensite in magnetic shape memory alloys, Physica Status Solidi B 251 (2014) Nr. 10, S. 2067-2079 http://dx.doi.org/10.1002/pssb.201350397
M. Kohl, M. Schmitt, A. Backen, L. Schultz, B. Krevet, S. Faehler: Ni-Mn-Ga shape memory nanoactuation, Applied Physics Letters 104 (2014) Nr. 4, S. 43111/1-5 http://dx.doi.org/10.1063/1.4863667
R. Niemann, J. Kopecek, O. Heczko, J. Romberg, L. Schultz, S. Faehler, E. Vives, L. Manosa, A. Planes: Localizing sources of acoustic emission during the martensitic transformation, Physical Review B 89 (2014) Nr. 21, S. 214118/1-11 http://dx.doi.org/10.1103/PhysRevB.89.214118