| Magnetic entropy changes and exchange bias effect associated with magnetostructural phase transitions of several Mn based ferromagnetic Heusler alloys have been investigated by x-ray diffraction, magnetization, thermal expansion, and electrical resistivity measurements. The alloys include Ni2+xMn1-xGa (0.16 ≤ x ≤ 0.20), Ni2Mn1-xCuxGa (0.245 ≤ x ≤ 0.26), Ni2Mn0.75Cu0.25-xCo xGa (0.245 ≤ x ≤ 0.26), Ni2Mn0.71Cu 0.27Fe0.02Ga, Ni2Mn0.70Cu0.30 Ga0.95Ge0.05, Ni50Mn25+xSb 25-x (0 ≤ x ≤ 15), and Ni50Mn50-xSn x (10 ≤ x ≤ 17).;The study of the Ga based Heusler alloys listed above shows that the alloys possess the cubic Heusler L21 structure or the lower symmetry martensitic structure (tetragonal/orthorhombic) at room temperature. Each alloy undergoes a first order martensitic structural phase transition and a second order ferromagnetic phase transition at the same temperature. This coupled magnetostructural phase transition occurs at different temperatures ranging from 295 K to 375 K. In the vicinity of each of these transitions large magnetic entropy changes have been observed. The maximum peak magnetic entropy change value, ΔSM, is found to be around -64 J/kg.K. Except the Ni2+xMn1-xGa alloys, all of the other alloys exhibit the large magnetic entropy changes at temperatures near room temperature.;The study of the Ni50Mn25+xSb25-x and Ni50Mn50-xSnx reveals many interesting properties of the alloys. For some critical Sb and Sn concentrations, martensitic transitions are observed in the samples, in the vicinity of which large inverse magnetic entropy changes have been observed. X-ray diffraction patterns of the alloys suggest that the martensitic phases possess 10M modulated orthorhombic structures, while the high temperature phase is purely cubic with the L21 structure. The martensitic phases in these alloys are found to host both ferromagnetic and antiferromagnetic coupling. Due to this coexistence of both ferromagnetic and antiferromagnetic coupling, exchange bias effects have been observed in the Ni50Mn25+xSb25-x and Ni50Mn 50-xSnx alloys in the lower temperature regions of the martensitic phase. Exchange bias fields, HE, of up to 248 Oe are observed in the 5 T field cooled samples. |
