A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

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 magnetostrictive materials, currently used in actuators, by more than one order of magnitude, and opens new opportunities for applications. Advanced materials have been developed based on new compositions [3][4][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. Shapememory alloys exhibit a diffusionless transition from a hightemperature 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 deformation 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 microstructural defects, which can act as nucleation sites during martensite formation, memorizing the uneven variant distribution 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 magnetization. In a magnetic field, the phase with the higher magnetic moment is energetically favored, leading to a field-dependent shift of the martensitic p...

In this article we report on a neutron interferometry technique based on diffraction gratings which was used to visualize the geometry-dependent magnetization processes in bulk ferromagnetic materials. The contrast origin is based on the refraction of unpolarized neutrons at magnetic domain walls and the obtained image is termed neutron dark-field image ͑DFI͒. The magnetization process is imaged by measuring the spatially resolved domain wall density distribution of the sample. The sample under investigation was a polycrystalline steel plate where the magnetization process was imaged for different sample orientations. The DFI results of the magnetization processes were verified on the one hand by complementary neutron small angle scattering ͑SANS͒ experiments and on the other hand by finite element method ͑FEM͒ simulations. The obtained SANS and FEM results verify the same magnetization process behavior as observed in the DFI results.

We use photorefractive two-wave mixing for coherent amplification of the object beam in digital holographic recording. Both amplitude and phase reconstruction benefit from the prior amplification as they have an increased SNR. We experimentally verify that the amplification process does not affect the phase of the wavefield. This allows for digital holographic phase analysis after amplification. As the grating formation in photorefractive crystals is just driven by coherent light, the crystal works as a coherence gate. Thus the proposed combination allows for applying digital holography for imaging through scattering media, after the image bearing light is coherence gated and filtered out of scattered background. We show experimental proof-of principle results.

A high-throughput characterization technique based on digital holography for mapping film thickness in thin-film materials libraries was developed. Digital holographic microscopy is used for fully automatic measurements of the thickness of patterned films with nanometer resolution. The method has several significant advantages over conventional stylus profilometry: it is contactless and fast, substrate bending is compensated, and the experimental setup is simple. Patterned films prepared by different combinatorial thin-film approaches were characterized to investigate and demonstrate this method. The results show that this technique is valuable for the quick, reliable and high-throughput determination of the film thickness distribution in combinatorial materials research. Importantly, it can also be applied to thin films that have been structured by shadow masking.