Phase transitions and corresponding magnetic entropy changes in Ni2 Mn0.75 Cu0.25-x Cox Ga Heusler alloys

Detailed studies of room temperature crystal structures, phase transitions, and related magnetic entropy changes (Δ Sm) in shape memory alloys Ni2 Mn0.75 Cu0.25-x Cox Ga (x=0.0, 0.01, 0.02, 0.025 0.03, 0.05) have been carried out by x-ray diffraction, magnetization, and thermal expansion measurements in magnetic fields of up to 5 T and in a temperature interval of 5-400 K. The high temperature austenitic cubic phase passes through a magnetic transition to ferromagnetic state and a structural transition to martensitic phase at the same temperature for all samples of the Ni2 Mn0.75 Cu0.25-x Cox Ga system. The first order magnetostructural transition temperature increases from 308 to 345 K with increasing Co concentration. All of the alloys in the Ni2 Mn0.75 Cu0.25-x Cox Ga system were found to possess large magnetic entropy changes. The maxima in the magnetic entropy changes ranged from Δ Sm max =-48 J/kg K to -64 J/kg K in a temperature range of 308-345 K. © 2007 American Institute of Physics