Magnetic properties and martensitic transformation in Cu-Mn-Al alloys

CuMnAl alloys with compositions Cu3Al1-xMn2x, where 0<x<0.26, undergo chemical ordering reactions of the type A2-->B2-->DO3-L2(1) during quenching from 1123K into water at room temperature [1], retaining the metastable beta phase. At lower temperatures they transform to close packed 18R or 2H martensites [2]. From about 90K to room temperature Mn a toms respond to an applied magnetic field as clusters of ferromagnetically coupled atoms, behaving as superparamagnets. The A(s) (martensite-->beta) temperature is independent on the applied magnetic field up to 5 Tesla. Both, beta and martensite phases, exhibit at lower temperatures mictomagnetism, it is to say a mixture of ferromagnetism and antiferromagnetism. This can be understood in terms of the Mn-Mn distances. Antiferromagnetism in beta and martensite phases, is mainly due to inherited chemical disorder during quenching.