Ni2Mn-based pseudo-Heusler alloys: Effects of Fe substitution, grinding and heat treatment conditions on magneto-structural properties

Fe-substituted N i 2 M n 1.6 S n 0.4 pseudo-Heusler alloys (HAs) were produced by arc melting and quenched in air, iced-water, and N 2 to study the effects of Fe content and quenching medium on atomic ordering and magnetic properties of the L 2 1 structure, which governs the structural phase transition. Air quenching favored a predominantly cubic L 2 1 (austenitic) phase, while liquid N 2 promoted orthorhombic and tetragonal (martensitic) phases. Increasing Fe content progressively lowered the structural transition temperature ( T M ), but the Curie temperature of the cubic L 2 1 ( T C A ) remained roughly unchanged until 7.5 at. % of Fe. The effect of powdering was also analyzed and showed a decrease in magnetic properties, an effect explained based on the increased atomic disorder, internal stress, and reduced grain size of the L 2 1 phase. However, post-annealing partially restored magnetic behavior by improving atomic order, confirmed by 57 Fe Mössbauer spectroscopy. These findings suggest a processing strategy for martensitic alloys: Fe substitution allows tuning of T M and enables composites suitable for magnetic refrigeration over a broad temperature range, while the quenching medium controls the degree of atomic ordering, with air being the most effective. In contrast, crushing introduces disorder, strain and grain refinement, features that significantly alter magnetic properties of these alloys. Therefore, post-annealing is essential to recover atomic order and performance in powdered samples. Overall, N i 2 M n-based pseudo-HAs are complex systems whose structural and magnetic properties strongly depend on processing history, which must be carefully reported to avoid misinterpretation.