Formation and magnetic properties of the L10 phase in bulk, powder and hot compacted Mn–Ga alloys

Abstract

The formation and stability of the L10 phase in Mn–Ga binary alloys with compositions in the range 50–75at% Mn (in steps of 5at%) has been studied. Of these, single-phase L10 structure was successfully produced in the 55, 60 and 65at% Mn alloys by annealing the high temperature phases, which had been retained to room temperature following arc melting. Further annealing and thermal analysis were used to determine the phase transformation temperatures in the alloys and the results were used to guide further processing. The saturation magnetisation, Ms, and the anisotropy field, Ha, were determined in applied fields up to 14T. For Mn55Ga45, µ0Ms=0.807T and µ0Ha=4.4T were observed. Mechanically milled Mn55Ga45 powder had coercivity of µ0Hc=0.393T, which was a twentyfold increase compared to the bulk material but the magnetisation was reduced (cf. powder: µ0M5T=0.576T, bulk: µ0M5T=0.780T). Annealing the powder at 400°C led to recovery of the magnetisation but reduced the coercivity, which was still 10 times as high as the bulk value. A degree of texture of 0.45 was achieved by magnetic alignment of the powder particles, leading to a remanence of 0.526T. Furthermore, isotropic hot compacts of powders were produced with packing density from 83% to 99%, in which the improved coercivity of the powders was partially retained.