Materials Transactions Online

Materials Transactions, Vol.47 No.01 (2006) pp.20-24
© 2006 The Japan Institute of Metals

Phase Separation and Stability of L21-Type Phase in Co2(Cr1-xFex)(Ga1-yAly) Alloys

Kosei Kobayashi1, Ryosuke Kainuma1,2, and Kiyohito Ishida1,2

1Core Research for Evolutional Science and Technology (CREST)-Japan Science and Technology Agency (JST), Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
2Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

The B2/L21 order--disorder transition and ferromagnetic/paramagnetic transition temperatures in Co2(Cr1-xFex)(Ga1-yAly) 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(Ga1-yAly) alloys, leading to lowering of Curie temperature Tc. The transition temperature TtB2/L21 from the B2 to L21-type phase decrease with increasing y for both the Co2Cr(Ga1-yAly) and Co2Fe(Ga1-yAly) alloy systems. The Tc in the Ga-rich portion of the L21-type Co2Cr(Ga1-yAly) alloy system is almost constant at about 480 K, whereas that in the B2-type Co2Fe(Ga1-yAly) alloy system slightly increases from about 1100 K to about 1170 K. By extrapolation from the Ga-rich side of Co2Cr(Ga1-yAly) alloys, TtB2/L21 and Tc 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(Cr1-xFex)(Ga1-yAly) alloys is proposed.

(Received 2005/8/24; Accepted 2005/10/11; Published 2006/1/15)

Keywords: half-metallic ferromagnets, L21-type phase, order--disorder transition, spinodal decomposition, cobalt--chromium--iron--gallium--aluminum

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