Effect of small changes in Mn concentration on phase transition temperatures and magnetic entropy variations in Ni2Mn0.75Cu0.25Ga Heusler alloys

The influence of small deviations of Mn from stoichiometric composition on the crystal structure and magnetic properties of Ni2Mn0.75(1−x)Cu0.25Ga Heusler alloys has been studied using X-ray diffraction, thermal expansion, and magnetization measurements. The samples were found to be of mixed phases containing cubic (austenitic) and tetragonal (martensitic) structures at room temperature. The temperatures of the martensitic transformations and ferromagnetic ordering of the austenitic phase coincide at T = TMS for all the samples, and vary from 310 to 333 K as the Mn deficiency (x) changes from 0 to 0.06. The magnetic entropy change (ΔSM) in the vicinity of TMS remains nearly constant for the entire Mn concentration interval with a value of about −45 J kg−1 K−1 for a magnetic field change of 5 T. The refrigeration capacity was found to change from 69 J kg−1 (for x = 0) to 88 J kg−1 (for x = 0.01) to 40 J kg−1 (for x = 0.06).