Microstructure of epitaxial films of the magnetic shape memory material Ni2MnGa

磁性形状记忆合金可以通过施加磁场来改变它们的形状，因为孪晶边界通过晶体的运动的能量低于磁晶各向异性。在Ni_2MnGa单晶中，已经实现了巨大的磁致伸缩效应。对于作为致动器和传感器的许多应用，将需要这些材料的薄膜。然而，在单晶薄膜中，通常将存在与下面的衬底的相互作用。在这个研究项目中，我们一方面想制备单晶自支撑薄膜作为薄膜应用的理想材料。在这里，我们将使用可溶解的基板和缓冲层的牺牲层的选择性去除。另一方面，我们想利用薄膜-衬底相互作用来深入了解这些材料中磁场诱导的孪晶界运动的基本问题。对于马氏体薄膜，衬底相互作用被认为抑制了马氏体相变。在这里，临界厚度的确定是必不可少的。我们还希望使用具有单轴表面各向异性的衬底，以打破立方对称性并诱导具有单个孪晶方向的亚马氏体到马氏体的转变。磁性和结构表征将在内部以及与SPP 1239的合作伙伴合作完成。

Magnetic shape memory alloys can change their shape by application of a magnetic field, since the energy for movement of twin boundaries through the crystal is lower than the magneto crystalline anisotropy. In single crystals of Ni2MnGa huge magnetostrictive effects have been achieved. For many applications as actuators and sensors thin films of these materials will be needed. In a single crystal thin film, however, there will in general be an interaction with the underlying substrate. In this research project we want on one hand to prepare single crystalline free standing films as ideal material for thin film applications. Here we will use solvable substrates and buffer layers for a selective removal of the sacrificing layers. On the other hand we want to exploit the film substrate interaction to gain insight into basic questions concerning the magnetic field induced twin boundary motion in these materials. For ultrathin films the substrate interaction is expected to suppress the martensitic phase transition. Here a determination of the critical thickness is essential. Also we want to use substrates with uniaxial surface anisotropy in order to break the cubic symmetry and to induce an austenite-to martensite transition with a single twin direction. Magnetic and structural characterization will be done in house as well as in cooperation with partners from the SPP1239.