Among various candidates for the rare-earth free permanent magnets, the ferromagnetic τ-MnAl-C (L10, P4/mmm) shows great potential to replace ferrites and bonded Nd-Fe-B magnets for its attractive magnetic properties and the non-critical nature of the raw elements [1, 2]. Further magnetic properties enhancement will depend on understanding of the effect of various crystallographic defects in the magnet, e.g. twin boundaries, and on developing novel processing routes for microstructure optimization.

Twin boundaries are frequently observed in both the as-transformed and hot deformed τ-MnAl-C magnets [3]. Three different types of twin boundaries have been discovered in the τ-MnAl-C magnet and they are described as true twins, order twins and pseudo twins [4]. Considering the tetragonal structure of the chemically ordered τ-MnAl-C magnet, it is reasonable to assume that a different atomistic structure exists at these three types of twins, as well as its local magnetic properties.

In this study, aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) was used to investigate the atomistic structure and chemical composition at various twin boundaries in τ-MnAl-C [5]. The results show differing levels of structural disorder at the various types of twin boundaries and EELS data reveal the presence of a Mn-enriched layer at the twin boundary surrounded by Al-enriched layers. The thickness of these layers and the magnitude of the chemical segregation vary with the level of structural disorder. Micromagnetic simulations based closely on the experimental results showed that the coercivity tends to increase with increasing structural and chemical disorder at the twin interface. These results suggest that targeted doping of interfaces in τ-MnAl may be a promising strategy to increase the coercivity of the material for applications.