PHASE STABILITY OF L21 PHASE IN Co-BASED HEUSLER ALLOYS

Abstract

Order–disorder phase transition temperature from L21 to B2 phase, [Formula: see text], in Co -based Heusler alloys was systematically investigated based on the Braggs–Williams–Gorsky (BWG) approximation. Because height of [Formula: see text] closely correlates to phase stability of the L21 phase, confirmation of [Formula: see text] would be a key to obtain a highly ordered L21 phase. From modification with the BWG approximation it was shown that the degree of order indicated temperature dependence, namely, the degree of order decreased above around 0.6 [Formula: see text] and reached zero at [Formula: see text]. Therefore, it should be better to select the Co -based Heusler alloys with a higher value of [Formula: see text] if the higher degree of order in the L21 phase is desired by thermal annealing at as low temperature as possible. From powder neutron diffraction (ND) studies of Co 2Y Ga (Y = Ti , V , Cr , Mn and Fe ) alloys it was actually confirmed that materials with higher value of [Formula: see text] indicated higher degree of order even in the same annealing temperature. Studies for the phase state in Co – Ti – Fe – Ga films also showed that a height of [Formula: see text] concretely correlated to the phase stability of the L21 phases, that is, the specimen with higher value of [Formula: see text] was easy to obtain with a higher degree of order.