Composition dependence of the ordering transitions and magnetic properties of β Cu3-x-Al-Mnx alloys

Abstract

The experimental study of four alloys Cu-Al-Mn with a constant 25 at.% Al and from 6 to 8 at.% Mn is presented. The critical temperatures for long-range atomic ordering, decomposition into an L21+DO3 two-phase structure, and magnetic ordering were obtained. Atomic ordering proceeds by a two-step mechanism A2→B2 and B2→L21, for all the studied samples, although both transition lines seem to join at Mn contents slightly below 6 at.%. The decomposition of the β phase into a DO3+L21 two-phase structure and the ferromagnetic ordering of the resulting L21 phase occur in the same range of temperatures (530-555 K). The use of complementary techniques allowed discrimination between the processes, confirming that they occur successively rather than simultaneously. The influence of different thermal treatments on the magnetic properties was studied. At room temperature, alloys quenched from 1173 K exhibit paramagnetic behavior, while samples cooled at 20 K/min are soft magnets, with higher saturation magnetization as the Mn content increases.