PROBABLY the most interesting examples of the L21 structure are the copper-based Heusler alloys Cu2MnX (X = aluminium, indium or tin), which are ferromagnetic in the ordered state. Recent additions to this structural type are Cu2MnSb (ref. 1), which is antiferromagnetic, and Au2MnAl (ref. 2), which is ferromagnetic. With the possible exception of Cu2MnSb a difficulty common to all these from the point of view of an experimental study of order ing effects in ternary systems is that the L21 structure is stable only at high temperatures. Low-temperature anneals or slow cooling, while though initially increasing the degree of order and enhancing the magnetic proper ties, finally lead to the breakdown of the structure or the precipitation of a second phase.