Martensitic transformation and magnetic properties of Co-Ni-Al shape memory alloy ribbons

We report on the magnetic and martensitic transformation properties of Co₃₉Ni₃₃Al₂₈ ferromagnetic shape memory alloy ribbons. We found that the phase formation of Co₃₉Ni ₃₃Al₂₈ is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the γ phase embedded in the primary β phase, while melt-spun ribbons contain the pure β phase. Co₃₉Ni₃₃Al₂₈ ribbons exhibit a perfect thermoelastic martensitic transformation from a cubic to a tetragonal structure at 240 K during cooling. The martensite structure can be well described by the L1₀ lattice, similar to that of Ni-Al alloys. The martensitic phase at 5 K exhibits a saturation magnetization of 48.67 emu g^-1 and saturates at about 8000 Oe. A large increase in coercive force after ageing at 500°C for 1 h has been found due to the change in atomic chemical ordering. The temperature dependence of the saturation magnetization indicates that magnetization can be well interpreted by spin-wave theory at temperatures lower than 200 K. The material shows a recoverable strain of 500 ppm upon the martensitic transformation.