Atomic Topology and Magnetic Microstructure of Highly Mobile Type I and Supermobile Type II Twin Boundaries in 10M Ni-Mn-Ga Single Crystal

TL;DR

This study investigates the atomic structure and magnetic microstructure of highly mobile twin boundaries in Ni-Mn-Ga single crystals, revealing atomically sharp boundaries and magnetic domain behavior using advanced TEM techniques.

Contribution

It provides detailed atomic and magnetic microstructural characterization of Type I and Type II twin boundaries in Ni-Mn-Ga martensite, challenging previous diffuse boundary assumptions.

Findings

Type I twin boundary is atomically sharp along (101) plane.

Type II twin boundary is also atomically sharp with occasional steps.

Magnetic induction reorients sharply at twin boundaries, forming 90°-like domain walls.

Abstract

The atomic topology and magnetic microstructure of individual, highly mobile Type I and Type II twin boundaries in 10M Ni-Mn-Ga martensite were investigated by transmission electron microscopy (TEM). The twin boundaries established in a bulk single crystal showed twinning stresses of ~1 MPa for Type I and ~0.1 MPa for Type II twin boundaries. TEM lamellae with a (010) cross-section, their c-axis (easy-magnetization direction) lying in-plane, were prepared by focused ion-beam milling, each containing a single twin boundary of specific type. High-resolution TEM confirmed an atomically sharp Type I twin boundary oriented along the rational (101) plane. The Type II boundary was also atomically sharp, apart from occasional single-atomic-plane steps. This contrasts with previous suggestions of its diffuse nature. Lorentz TEM showed 180{\deg} domain walls within martensite variants. The magnetic induction reorients sharply on both twin boundaries, forming 90{\deg}-like magnetic domain walls that follow the c-axis easy-magnetization direction.