Martensitic transformation and magnetic properties of Heusler alloys Co-V-(Al, Ga, In)

Abstract

The electronic structure, magnetism and martensitic transformation in Co-V based Heusler alloys Co2V1.5Z0.5 (Z = Al, Ga, In) were investigated theoretically. In these alloys, the tetragonal martensitic phase has a higher stability compared with the cubic austenite. This can be explained based on their electronic structures. Here the energy difference ΔEM between the martensite and austenite is large in Co2V1.5Al0.5 and Co2V1.5Ga0.5, suggesting that they are possible candidates for magnetic shape memory alloys (MSMAs). This also agrees with the existing experimental observation in Co-V-Ga. In Co2V1.5Z0.5 a ferromagnetic to paramagnetic transition occurs together with the martensitic transition. The spin polarization ratio also decreases drastically from close to 100% in austenite to almost zero in martensite. All this makes Co2V1.5Z0.5 alloys a cross-over point of MSMA and spintronics, which can be controlled by magnetic/temperature/stress multi-field. In Co2V1.5Ga0.5 alloy, calculations suggest a degree of Co-V disorder in the experimental sample, which may introduce a weak ferromagnetic state in the martensite and lower the phase transition driving force ΔEM. This can explain the small saturation magnetization of the martensite reported in literature. To improve the properties of Co-V-Ga alloys, adjusting their atomic ordering can be of great importance.