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 Co39Ni33Al28 is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the gamma phase embedded in the primary,B phase, while melt-spun ribbons contain the pure beta 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 L1(0) 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 degreesC 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.