Epitaxial Ni-Mn-Ga Films for Magnetic Shape Memory Alloy Microactuators

Active materials such as piezoelectrics are established in the field of microsystems application despite their low achievable strains which often require the integration of additional gear mechanisms. The ongoing search for new active materials has focused on magnetic shape memory (MSM) alloys such as Ni?Mn?Ga since they combine macroscopic strains of up to 10% with a cycling frequency well above the frequencies of conventional thermal shape memory alloys. The present review focuses on preparation and analysis of Ni?Mn?Ga films that can eventually be integrated in microsystems. Single crystal like films are prepared by epitaxial growth on suitable substrate materials. Since the magnetically induced reorientation of variants is blocked by a rigid substrate, we present different methods for releasing films from the substrates. We show that the sacrificial layer technology is the most promising approach. Further processing of the freestanding film requires a microtechnology which is adjusted to the film laminate structure. The properties of the freestanding films are compared with films on a rigid substrate. Although we observe stress-induced twin boundary motion, the twinning stress is too high to be overcome by an external magnetic field. Therefore, it is necessary to develop suitable training methods to reduce the twinning stress below 2?MPa to enable the activation of the material by means of an external magnetic field.