Comparing properties of substrate-constrained and freestanding epitaxial Ni-Mn-Ga films

In order to use the magnetic shape memory alloy Ni-Mn-Ga with its high achievable strain of up to 10% for microactuators, freestanding epitaxial films are required. Here we show that these conditions can be fulfilled when using chromium as a sacrificial layer. The low misfit towards Ni-Mn-Ga enables epitaxial growth. Furthermore, Cr can afterwards be removed selectively by wet-chemical etching and during deposition no significant interdiffusion is observed. The structure, microstructure and magnetic properties of micrometer thick films are not affected by the etching process. Films are ferromagnetic at room temperature and we observe the coexistence of non-modulated (NM) and seven layered modulated (14 M) martensite. Tensile stress-strain measurement of a freestanding bridge reveals a broad strain plateau of 12% at a twinning stress of 25 MPa, indicating reorientation of NM variants. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.