Speaker
Description
Ni-Mn-Ga alloy is the most promising candidate as material for magnetic shape memory applications. In addition to austenite, it is also found in variety of martensite phases. Antiphase boundaries (APBs) are planar defects that play a critical role in strengthening Ni-based alloys, and their sensitivity to alloy composition offers a flexible tuning parameter for alloy design.
We combine experimental and theoretical (ab initio) methods to analyze the stability and magnetic ordering near the antiphase boundary (APB) in Ni$_2$MnGa with L$_{21}$ ordering. Ab initio electronic structure calculations based on density functional theory are used to investigate the magnetic ordering in a structure simulating APB's in cubic austenite. We show that the magnetic moments of Mn atoms in ferromagnetic austenite are oriented perpendicular to the APB planes. The total energy calculations indicate that at the antiphase boundary the ferromagnetic ordering changes its orientation in the opposite direction. This sudden change in magnetization highlights the APB's, which are then readily visible by magnetic force microscopy. We find that single APB's, when close together, tend to attract each other and form thicker antiphase boundary complexes. Images are shown and discussed.
Acknowledgements
This work was supported by the Czech Science Foundation project 24-11361S.
