Phase Separation and Stability of L2₁-Type Phase in Co₂(Cr_1−xFe_x)(Ga_1−yAl_y) Alloys

抄録

The B2⁄L2₁ order–disorder transition and ferromagnetic/paramagnetic transition temperatures in Co₂(Cr_1−xFe_x)(Ga_1−yAl_y) alloys were determined by differential scanning calorimetry and transmission electron microscopy. It was found that spinodal decomposition cannot be suppressed by quenching in the Al-rich portion of Co₂Cr(Ga_1−yAl_y) alloys, leading to lowering of Curie temperature T_c. The transition temperature T_t^B2⁄L2₁ from the B2 to L2₁-type phase decrease with increasing y for both the Co₂Cr(Ga_1−yAl_y) and Co₂Fe(Ga_1−yAl_y) alloy systems. The T_c in the Ga-rich portion of the L2₁-type Co₂Cr(Ga_1−yAl_y) alloy system is almost constant at about 480 K, whereas that in the B2-type Co₂Fe(Ga_1−yAl_y) alloy system slightly increases from about 1100 K to about 1170 K. By extrapolation from the Ga-rich side of Co₂Cr(Ga_1−yAl_y) alloys, T_t^B2⁄L2₁ and T_c of the metastable Co₂CrAl alloy were determined as 800 and 470 K, respectively. An optimal concentration area for obtaining excellent half-metallic properties and high stability of the L2₁-type phase in the Co₂(Cr_1−xFe_x)(Ga_1−yAl_y) alloys is proposed.