How to enable bulk-like martensitic transformation in epitaxial films

The present study is dedicated to the influence of different substrate and buffer layer materials on the martensitic transformation in sputter deposited epitaxial shape memory Heusler alloys. For this, the magnetocaloric Heusler alloy Ni-Co-Mn-Al \cite{Teichert2015b} is grown on MgO(001), MgAl$_{2}$O$_{4}$(001), and MgO(001)/V substrates, which exhibit a lattice misfit to the Ni-Co-Mn-Al between $-1.2\%$ and $3.6\%$. By temperature dependent X-ray diffraction measurements it is shown that the optimum buffer layer for shape memory Heusler films is not one with minimum lattice misfit, but one with minimum Young's modulus and moderate misfit because an elastic buffer layer can deform during the martensitic transformation of the Heusler layer. Furthermore, epitaxial strain caused by a moderate lattice misfit does not significantly change the martensitic transformation temperatures.