How to enable bulk-like martensitic transformation in epitaxial films

TL;DR

This study investigates how substrate and buffer layer choices influence martensitic transformation in epitaxial Heusler alloy films, revealing that elastic buffer layers with moderate misfit optimize transformation behavior.

Contribution

It demonstrates that buffer layers with low Young's modulus and moderate lattice misfit enable bulk-like martensitic transformation in epitaxial films, challenging the focus on minimal lattice misfit.

Findings

Elastic buffer layers facilitate martensitic transformation.

Moderate lattice misfit does not significantly alter transformation temperatures.

Optimal buffer layers are characterized by low Young's modulus.

Abstract

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.