Mesoscopic twin boundaries in epitaxial Ni-Mn-Ga films

TL;DR

This paper investigates mesoscopic twin boundary microstructures in Ni-Mn-Ga thin films, revealing how different orientations of 14M twin boundaries relate to martensitic nucleation, which is crucial for magnetic shape memory applications.

Contribution

It identifies and explains the formation of two distinct mesoscopic twin boundary patterns in Ni-Mn-Ga films based on martensitic nucleation orientations.

Findings

Type X twin boundaries are tilted from the surface.

Type Y twin boundaries are perpendicular to the surface.

Both patterns are formed by 14M twin boundaries.

Abstract

Twin boundaries play an essential role in the use of magnetic shape memory alloy Ni-Mn-Ga as active material. Only if twin boundaries can be moved by an external magnetic field, high strain values of up to 10 % can be obtained. Therefore, understanding the observed twin microstructure of thin films is crucial for future application. We exemplarily present two different microstructural pattern, type X and type Y microstructure, using the example of two Ni-Mn-Ga films with similar film thickness and composition. The analysis of microstructure and structure of the thin films shows that both, type X and type Y pattern, are formed by mesoscopic 14M twin boundaries. The mesoscopic 14M twin boundaries are either tilted from the substrate surface in case of type X pattern or they are perpendicular to the surface for type Y pattern. Based on a recently proposed scenario for the nucleation of martensite in austenite, we can trace back the difference in twin boundary orientation to differently oriented martensitic nuclei.