Nanocrystalline CoMnFeNiGa high entropy alloys: Room temperature ferromagnetism bridging the gap from Bulk to Nano

Abstract

Nanocrystalline CoMnFeNiGa high entropy alloys (HEAs) were successfully synthesized and characterized across different length scales. Compositionally homogeneous single-phase FCC HEA micropowder particles with a nanocrystalline structure (~8 nm) were produced by short-term (190 min) high energy ball milling (HEBM). These powders were subsequently used as precursors for fabricating dense HEA bulk by spark plasma sintering (SPS) and HEA nanoparticles (NPs) by laser fragmentation in liquids (LFL) — both synthesis routes are not achievable by direct processing of elemental powder blends. We show that the single-phase FCC CoMnFeNiGa HEA micropowder partially transforms into a BCC phase upon consolidation by SPS at 1073 K. As a result, the HEA bulk consists of a mixture of FCC and BCC phases. In addition, Mn-rich BCC precipitates (10–50 nm) were formed in both HEA phases. The LFL of HEA micropowder leads to the formation of HEA NPs with two morphologies (spheres and quasi-2D platelets with 5–10 nm thickness and 40–150 nm lengths) with FCC, BCC, and hexagonal structures (birnessite-type layered δ-MnO2 structure). All three nanocrystalline CoMnFeNiGa HEAs exhibit soft ferromagnetic behavior at RT with a saturation magnetization (Ms) of 19.5–33.5  Am²/kg for the micropowder and NPs, while the Ms of HEA bulk is 2–4 times larger (88.8  Am²/kg). A short thermal treatment (1000 K, 30 s) significantly enhanced Ms and increased the Curie temperature of the micropowder to 105.6 Am²/kg and 785 K, of the NPs to 46.9 Am²/kg and 850 K, and of the bulk material to 106 Am²/kg and 793 K. The coercivity increased threefold to 1.8 kA/m only in NPs. Structure-property relationships in CoMnFeNiGa HEAs are herein systematically compared across all length scales, demonstrating that magnetic behavior can be effectively tuned by nanoscale structural control and rapid thermal treatment.