Stray-Field-Induced Actuation of Free-Standing Magnetic Shape-Memory Films By Michael Thomas, Oleg Heczko, Jo ¨rg Buschbeck, Yiu Wai Lai, Jeffrey McCord, Stefan Kaufmann, Ludwig Schultz, and Sebastian Fa ¨hler* Magnetic shape-memory (MSM) alloys, such as NiMnGa, [1] reach maximum strains of close to 10% in single crystals. [2] This exceeds the strain obtainable from piezoelectric or magnetos- trictive materials, currently used in actuators, by more than one order of magnitude, and opens new opportunities for applica- tions. Advanced materials have been developed based on new compositions [3–5] or on innovative routes of fabrication resulting in foams, [6] fibers, [7] textured polycrystals, [8] or composites. [9] Thin MSM films with these high strains would be of significant benefit for use in microactuators. However, preparation of films exhibiting such high strains has not been achieved so far. In the present work, we demonstrate a new and enhanced thermal actuation mode, which is achieved by the use of freestanding, epitaxially grown NiMnGa films. This new mode utilizes a variant selection by magnetic stray-field energy within the martensite state. It allows for reversible actuation similar to the conventional two-way shape-memory effect, yet neither training nor an external magnetic field is required. A novel path to the realization of sub-micrometer, miniaturized microactuators based on MSM films is established. Due to the finite size of magnetic domains, this actuation mode is a unique feature of small systems. The novel mode uses both the ferromagnetic and martensitic properties of MSM alloys. Since it involves features of known actuation modes, these are described in the following. Shape- memory alloys exhibit a diffusionless transition from a high- temperature cubic austenite phase to a low-temperature martensite phase of lower symmetry. [10] The martensitic phase may, as in the present case, exhibit a tetragonal structure with two identical long crystal a axes and one short c axis. Consequently, three different orientations of the martensitic c axis with respect to the cubic austenite cell are possible. Neighboring unit cells of identical orientation form so-called martensitic variants. Adjacent variants with different orientations are connected by twin boundaries. This special microstructure allows an easy deforma- tion by changing the fractions of martensitic variants through twin-boundary motion. Heating to the cubic austenite state restores the original shape, which is the so-called one-way shape-memory effect. To obtain the two-way shape-memory effect, several cycles of mechanical training and heating are required. This presumably leads to the formation of micro- structural defects, which can act as nucleation sites during martensite formation, memorizing the uneven variant distribu- tion and hence the macroscopic shape. [11] For this actuation mode, temperature is the control parameter. Martensitic materials, which also exhibit ferromagnetic order, allow two additional actuation modes. The first type utilizes the coupling between crystal structure and spontaneous magnetiza- tion. In a magnetic field, the phase with the higher magnetic moment is energetically favored, leading to a field-dependent shift of the martensitic phase-transformation temperature T M . [12,13] As in well-trained thermal shape-memory alloys, this magnetically induced martensite (MIM) can be applied for actuation in the vicinity of T M for bulk [5] and thin films. [14,15] The second actuation mode is observed in single crystals within the martensitic state. It is based on a magnetically induced reorientation (MIR) of variants, also referred to as the MSM effect. The key intrinsic property for MIR is the magnetocrystal- line anisotropy energy. It describes the fact that magnetization is favorably aligned along a specific crystallographic direction. [16] For actuation, a magnetic field is used to modify the martensitic microstructure by increasing the fraction of those martensitic variants with their easy magnetization axis parallel to the external field. This reorientation occurs by twin-boundary motion, and results in a large change of length. The maximum strain is given by the difference of lattice constants of the martensitic unit cell. Herein, we present an alternative thermal actuation mode obtained in free-standing MSM films, which, in contrast to the effects discussed above, requires neither an external magnetic field as for MIR nor training as for a two-way shape-memory effect. Our analysis will show that the underlying fundamental mechanism can be described as a stray-field-induced micro- structure (SFIM). The preparation of epitaxial films [17–22] is considered to be the most promising route towards realization of functional films, since the highest strains up to now have been obtained in single crystals. As all actuation modes require variant redistribution, freestanding films are required to overcome the constraint by a rigid substrate. Despite complex fabrication processes [23] or COMMUNICATION www.advmat.de [*] Dr. S. Fa ¨hler, M. Thomas, Dr. O. Heczko, J. Buschbeck, Y. W. Lai, Dr. J. McCord, S. Kaufmann, Prof. L. Schultz IFW Dresden P.O. Box 270116, 01171 Dresden (Germany) E-mail: s.faehler@ifw-dresden.de Dr. S. Fa ¨hler, S. Kaufmann, Prof. L. Schultz Institute for Solid State Physics Department of Physics Dresden University of Technology 01062 Dresden (Germany) Dr. O. Heczko Institute of Physics Academy of Sciences of Czech Republic Na Slovance 8, Prague (Czech Republic) DOI: 10.1002/adma.200900469 3708 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 3708–3711