Martensitic transformation and accompanying magnetic changes in Ni–Fe–Ga–Co alloys

Abstract

Two series of cobalt substituted Ni–Fe–Ga ferromagnetic shape memory alloys with lower gallium content (<27 at. %) were studied by differential scanning calorimetry, x-ray diffraction, and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase in the Curie temperatures and a variation in the martensitic transformation (MT) temperatures, in accord with the alloy valence electron concentration change. For alloys with MTs below room temperature, a field dependent thermomagnetic hysteresis was evidenced and discussed in connection with the effect of cobalt substitution, on the magnetic hardness of the martensite phase. A direct interpretation of the evolution of the thermomagnetic hysteresis versus the applied field was provided.