Structure and magnetic properties of nanostructured MnNi alloys fabricated by mechanical alloying and annealing treatments

A nearly equiatomic MnNi alloy was fabricated from the elemental powders by means of mechanical alloying in a planetary ball milling apparatus. X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and measurements of magnetization were conducted to identify the structural states and properties of the prepared alloys. After ball milling for 20 h, a disordered face-centered cubic (f.c.c.) solid solution was formed which increased in lattice parameter by further milling up to 50 h. An exothermic reaction took place at around 300-400A degrees C during continuous heating of the disordered f.c.c. solid solution. This reaction is attributed to a structural ordering leading to the formation of a face-centered tetragonal (f.c.t.) phase with L1(0) type ordering. Examination of the magnetic properties indicated that the structural ordering increases remnant magnetization and decreases coerecivity.