Order-disorder phase transition temperature from L21 to B2 phase, TtL21=B2, in Co-based Heusler alloys was systematically investigated based on the Braggs-Williams-Gorsky (BWG) approximation. Because height of TtL21=B2 closely correlates to phase stability of the L21 phase, confirmation of TtL21=B2 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 TtL21=B2 and reached zero at TtL21=B2. Therefore, it should be better to select the Co-based Heusler alloys with a higher value of TtL21=B2 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 Co2Y Ga (Y = Ti, V, Cr, Mn and Fe) alloys it was actually confirmed that materials with higher value of TtL21=B2 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 TtL21=B2 concretely correlated to the phase stability of the L21 phases, that is, the specimen with higher value of TtL21=B2 was easy to obtain with a higher degree of order.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Atomic and Molecular Physics, and Optics
- Electrical and Electronic Engineering