Anja Backen

Recent Progress in FSMA Microactuator Developments

The giant magneto-strain effect is particularly attractive for actuator applications in micro- and nanometer dimensions as it enables contact-less control of large deformations, which can hardly be achieved by other actuation principles in small space. Two different approaches are being pursued to develop ferromagnetic shape memory (FSMA) microactuators based on the magnetically induced reorientation of martensite variants: (1) the fabrication of free-standing epitaxial Ni-Mn-Ga thin film actuators in a bottom-up manner by magnetron sputtering, substrate release and integration technologies and (2) the top-down approach of thickness reduction of bulk Ni-Mn-Ga single crystals to foil specimens of decreasing thicknesses (200 – 40 μm) and subsequent integration. This review describes the fabrication technologies, procedures for thermo-mechanical training adapted to the quasi-two-dimensional geometries of film and foil specimens as well as the performance characteristics of state-of-the art actuators after processing and training.

Twin boundary energy of hierarchically twinned magnetic shape memory alloys

Magnetic shape memory alloys exhibit a hierarchy of “twins within twins” microstructures, covering all length scale from nanometer to millimeter. Modulated structures as nano-twinned adaptive martensite are the first generation of twinning. Using epitaxial Ni-Mn-Ga films as model system, we observe a regular twinning of 14M modulated variants, which represent the second generation of twinning hierarchy. We analyzed a thickness series from 30 nm to 2 μm and found a square-root dependency of the twinning period on film thickness. From the Landau-Lifshitz-Kittel scaling law, the twin boundary energy between mesoscopic 14M modulated variants is estimated the value of 0.7 eV/A2.

Magnetic nanostructures by adaptive twinning in strained epitaxial films.

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.

Reducing the nucleation barrier in magnetocaloric Heusler alloys by nanoindentation

Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fracti...

Ni-Mn-Ga shape memory nanoactuation

To probe finite size effects in ferromagnetic shape memory nanoactuators, double-beam structures with minimum dimensions down to 100 nm are designed, fabricated, and characterized in-situ in a scanning electron microscope with respect to their coupled thermo-elastic and electro-thermal properties. Electrical resistance and mechanical beam bending tests demonstrate a reversible thermal shape memory effect down to 100 nm. Electro-thermal actuation involves large temperature gradients along the nanobeam in the order of 100 K/μm. We discuss the influence of surface and twin boundary energies and explain why free-standing nanoactuators behave differently compared to constrained geometries like films and nanocrystalline shape memory alloys.

Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Abstract Shape memory alloys often show a complex hierarchical morphology in the martensitic state. To understand the formation of this twin-within-twins microstructure, we examine epitaxial Ni-Mn-Ga films as a model system. In-situ scanning electron microscopy experiments show beautiful complex twinning patterns with a number of different mesoscopic and macroscopic twin boundaries between already twinned regions. We explain the appearance and geometry of these patterns by constructing an internally twinned martensitic nucleus, which can take the shape of a diamond or a parallelogram, within the basic phenomenological theory of martensite. These nucleus contains already the seeds of different possible mesoscopic twin boundaries. Nucleation and growth of these nuclei determines the creation of the hierarchical space-filling martensitic microstructure. This is in contrast to previous approaches to explain a hierarchical martensitic microstructure. This new picture of creation and anisotropic, well-oriented growth of twinned martensitic nuclei explains the morphology and exact geometrical features of our experimentally observed twins-within-twins microstructure on the meso- and macroscopic scale.

Magnetic domain structure of epitaxial Ni–Mn–Ga films

For the magnetic shape memory effect, knowledge about the interaction between martensitic and magnetic domain structure is essential. In the case of Ni-Mn-Ga bulk material and foils, a staircase-like magnetic domain structure with 90{\deg}- and 180{\deg}-domain walls is known for modulated martensite. In the present paper we show that the magnetic domain pattern of thin epitaxial films is fundamentally different. Here we analyze epitaxial Ni-Mn-Ga films by atomic and magnetic force microscopy to investigate the correlation between the twinned martensitic variants and the magnetic stripe domains. The observed band-like domains with partially perpendicular outof-plane magnetization run perpendicular to the microstructure domains defined by twinning variants. These features can be explained by the finite film thickness, resulting in an equilibrium twinning period much smaller than the domain period. This does not allow the formation of a staircase domain patter. Instead the energies of the magnetic and martensitic microstructures are minimized independently by aligning both patterns perpendicularly to each other. By analyzing a thickness series we can show that the observed magnetic domain pattern can be quantitatively described by an adapted band domain model of Kittel.

Development of ferromagnetic shape memory nanoactuators

This paper presents the fabrication and actuation of first freestanding in-plane nanostructures of a ferromagnetic shape memory alloy (FSMA) by combining magnetron sputtering of epitaxial FSMA thin films, a Cr-based sacrificial layer technology, electron beam lithography, and ion beam etching. The viability of the process is demonstrated by mechanical deflection and thermo-elastic performance tests. Temperature dependent electrical resistance measurements of double-beam test structures with critical dimensions of 200 nm show a full hysteresis. A reversible out-of-plane actuation of 500 nm is achieved, which is based on the thermally induced martensitic transformation of the Ni-Mn-Ga.

Intermittent Deformation Behavior in Epitaxial Ni–Mn–Ga Films

This study is conducted to measure intermittent and continuum deformation behaviors in epitaxial Ni–Mn–Ga films under uniaxial tensile loading. First, epitaxial Ni–Mn–Ga film is prepared on a substrate by magnetron sputtering. Secondly, the constraint film is released from substrate by wet-chemical etching of Cr. Thirdly, we simultaneously measure phase transformation band nucleation and strain field arising in epitaxial freestanding Ni–Mn–Ga films. The band nucleations are measured using Stress Drop Analysis (SDA) in macroscopic stress–strain curve, and strain fields are measured using Digital Image Correlation (DIC) method. Results show that smaller size bands nucleated on the beginning stage of stress induced martensite transformation in stress–strain curve, then larger size bands did on the latter stage. The strain field shows macroscopic inhomogeneity under tensile loading. Especially in the latter stage of stress induced martensite transformation in stress–strain curve, the inhomogeneous region propagates along to loading direction although the shape is not distinct. Present Ni–Mn–Ga film microstructure has an order structure with some disorder structures consisting of some martensite arrangements. Their structures will affect intermittent and continuum deformation behaviors in epitaxial Ni–Mn–Ga film.