Understanding modulated Phases in Heusler Alloys

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Modulated phases seem to be a general feature of various functional materials. In particular for Ni-Mn-Ga Heusler alloys, the 14M modulated martensite reaches the highest strain close to 10% in external magnetic fields. Recently we showed that this phase can be built from nanotwinned variants of the non-modulated (NM) martensite phase following the concept of adaptive martensite. In combination with branching this concept suggests microscopic explanations of the key features of this phase.Here we aim to understand the origin of the differently modulated phases observed in Heusler alloys in relation to composition, temperature, stress and magnetic fields. Structure and phase transformation of Ni-Mn-Ga and other Heusler alloys are examined in form of epitaxial films and bulk single crystals. Since modulated structures affect the material from the atomistic to the macroscopic length scale, dedicated integral diffraction methods in combination with high resolution local probes are applied. Finally consequences of the modulation for the functional properties of Heusler alloys are analyzed.