Tuning macro-twinned domain sizes and the b-variants content of the adaptive 14-modulated martensite in epitaxial Ni-Mn-Ga films by co-sputtering

TL;DR

This study uses co-sputtering to tune the composition of epitaxial Ni-Mn-Ga films, revealing how composition influences the variant distribution of 14-modulated martensite and supporting the adaptive phases theory.

Contribution

It demonstrates the first experimental validation of adaptive modulated-martensites in Ni-Mn-Ga films across different compositions using co-sputtering.

Findings

b-variants form near the film surface to relieve stress

b-variant content decreases with larger macro-twinned domains

composition tuning affects variant distribution

Abstract

In order to obtain modulated-martensite in our epitaxial Ni-Mn-Ga films, we have tuned the composition by using a co-sputtering process. Here we present how the composition affects the variant distribution of the 14-modulated martensite at room temperature. The nature of such modulated-martensites is still strongly debated for magnetic shape memory alloys. It has been very recently demonstrated that the modulated-martensites in Ni-Mn-Ga are adaptive phases. The results presented here corroborate this theory for the first time, for three different compositions. Moreover, we demonstrate with the help of the adaptive modulations theory that b-variants of the 14-modulated martensite form close to the free-surface of the film to release the stress induced by branching of macro-twinned domains during the martensitic transformation on a rigid substrate. At room temperature, the content of such b-variants is found to strongly decrease when the macro-twinned domain sizes increase.