Anomalous thermal properties of the Heusler alloy Ni2+xMn1−xGa near the martensitic transition

We present the measurements of electrical resistivity , specific heat CP , Seebeck coefficient S , as well as thermal conductivity of the Heusler compounds Ni2+xMn1−xGa from 10 to 400 K. A series of Ni2+xMn1−xGa alloys were prepared with x=0, 0.04, 0.06, 0.10, 0.14, 0.18, and 0.24 to systematically study the effect of substitution on the martensitic and ferromagnetic transitions. Unusual sharp peaks were observed in both S and around the martensitic transition temperature TM , while a noticeable change of slope was seen in S and at the ferromagnetic transition temperature TC in Ni2+xMn1−xGa with x 0.14. For x 0.14 the structural and magnetic phase transitions were found to merge together, as noticed from the sharp changes in all measured physical quantities. Existence of premartensitic transition TP was unambiguously resolved in the stoichiometric compound Ni2MnGa by the CP measurement, although such a feature becomes less pronounced with increasing x and completely disappears for the compositions x 0.10. Significant thermal hysteresis loops between heating and cooling cycles appear in the vicinity of TM, which is associated with the presence of 7-layer and 5-layer modulated structures. In addition, huge peaks were observed in the thermal conductivity near TM. We connected this observation to the nesting of the Fermi-level density of states, which is appropriate for the Peierls transition.