Enhancement of Coercivity in α-Mn(Al) Magnetic Nanopowders Processed by High-Energy Ball Milling

We report the achievement of a high coercivity (Hc) value of 3 kOe for Mn₅₄Al₄₆ powders produced by high-energy ball milling without any heat treatment. The ferromagnetic α-Mn(Al) alloy powders were prepared with Mn (54 at%) and Al (46 at%) as the constituent metal elements. Analysis and discussion were held regarding the potential causes of the observed enhancement in the magnetic properties of the ball-milled powders in relation to their phase composition and microstructure. The process of milling causes partial solubilisation of Al atoms in the α-Mn phase and a change in the powder magnetic characteristics from paramagnetic to ferromagnetic. The existence of γ₂ and β phases was identified by XRD after long hours of the milling process. In addition, it was found that a longer milling time resulted in a smaller crystallite size, which also resulted in a decrease in magnetic saturation (Ms) but an increase in coercivity (H_C). Without applying any extra heat treatment, the Mn-Al powders milled for 6 h had optimum magnetic properties: Hc = 3.0 kOe, Mr = 0.05 emu/g, Ms = 0.19 emu/g, and (BH)max = 432 J/m³.