Structure of Antiphase boundaries in Ni-M-Ga: multiscale modelling

TL;DR

This study combines atomic-scale DFT calculations with micrometer-scale magnetostatics to elucidate how antiphase boundaries in Ni-M-Ga influence magnetic contrast, revealing their role in coercivity and domain structure.

Contribution

It introduces a multiscale modeling approach linking atomic exchange interactions at APBs to magnetic contrast observed in MFM in Ni-M-Ga alloys.

Findings

APB pairs cause characteristic MFM contrast extending ~100 nm.

Nanoscale antiparallel magnetization region is only three Mn-Ga layers thick.

Extended antiparallel domains are prevented when APBs are more than 50 nm apart.

Abstract

Antiphase boundaries (APBs) are ubiquitous in ordered Heusler alloys and strongly influence magnetic coercivity in Ni-Mn-Ga, yet the link between their atomic-scale exchange interactions and micrometer-scale magnetic contrast measured by magnetic force microscopy (MFM) remains unclear. We combine density functional theory (DFT) and finite-element magnetostatics to bridge these scales in Ni-Mn-Ga. DFT calculations on supercells containing planar APBs show that the lowest-energy configuration comprises a pair of parallel APBs enclosing a nanoscale region - only three Mn-Ga atomic layers thick - whose magnetization is antiparallel to the surrounding matrix due to strong antiferromagnetic exchange across each APB (in contrast to ferromagnetic coupling in bulk martensite). According to our magnetostatic finite element model, this thin region with antiparallel magnetization generates the characteristic MFM contrast extending approx. 100 nm from the APB pair. When the APBs are further apart than 50 nm, dipole-dipole penalties outweigh exchange gains, preventing formation of an extended antiparallel domain, in agreement with experimental evidence. These results identify APB pairs as the origin of the observed MFM contrast and offer an interpretation of the modest strengths of domain-wall pinning by APBs, informing the design of magnetic shape-memory alloys with tailored coercivity.