Magnetic properties of mechanically alloyed Mn-Al-C powders

We have prepared supersaturated-solution Mn-Al-C alloy powders by mechanical alloying using a planetary high-energy mill. The starting materials were pure Mn, Al and C powers. The mechanically-alloyed powders were subjected to a two-step heating. Although starting particles are Al and Mn with additive C, the Al peak disappears with MA time. With increasing MA time, transition from alpha-Mn to beta-Mn does not occur; the alpha-Mn structure maintains. At 100 h, a single phase of supersaturated-solution alpha-Mn is obtained. The lattice constant of alpha-Mn decreases with increasing MA time. From the Scherrer formula, the crystallite size at 500 h is obtained as 200 angstrom, which does not mean amorphous state. By two-step heating, high magnetization (66 emu/g) was obtained from short-time-milled powders (t=10 h). The precursor of the as-milled powder is not a single phase alpha-Mn but contains small amount of fcc Al. After two-step heating, the powder changes to tau-phase. Although the saturation magnetization increases, the value is less than that by conventional bulk MnAl (88 emu/g). Meanwhile, long-time-milled powder of single alpha-Mn phase results in low magnetization (5.2 emu/g) after two-step heating.