The effect of Cu on phase stability and magnetic properties of τ-MnAl: A comparative study of Mn55-xAl45Cux and Mn55Al45-xCux (x = 0.0, 2.0, 2.5) alloys

The hard ferromagnetic τ-phase in Mn-Al material system continues to attract attention since it has potential to be developed as permanent magnet without the use of critical/expensive elements. However, this phase is metastable, requiring controlled processing techniques and/or dopants to improve phase stability. The impact of copper (Cu) doping on phase formation and enhancing the coercive field of the material is attributed to the beta phase, was explored in melt-spun ribbons subjected to varying annealing temperatures for Mn55-xAl45Cux and Mn55Al45-xCux with x = 0.0, 2.0 and 2.5. In general, the best magnetic properties were obtained for samples annealed at 450 °C with 2.0 % and 2.5 at.% of Cu. In the Mn55-xAl45Cux system, the coercive field (Hc) values were 134.1 kAm−1 and 145.2 kAm−1, with a remanent magnetization (Mr) of 41.0 and 35.6 Am2kg−1 for 2 and 2.5 % Cu, respectively. When partially substituting Al per Cu, Mn55Al45-xCux system, the coercive field (Hc) values were 178.2 kAm−1 and 179.8 kAm−1, with a remanent magnetization of 4.2 and 12.7 Am2kg−1, respectively. However, for samples annealed at higher temperatures, it was observed that the metastable τ-phase partially decomposed into gamma (γ2) and beta (β) phases, resulting in a reduction of the material's magnetic properties.