Anomalous thermal properties of the Heusler alloy Ni2+x Mn1-x Ga near the martensitic transition

We present the measurements of electrical resistivity (ρ), specific heat (CP), Seebeck coefficient (S), as well as thermal conductivity (κ) of the Heusler compounds Ni2+x Mn1-x Ga from 10 to 400 K. A series of Ni2+x Mn1-x Ga 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 κ around the martensitic transition temperature (TM), while a noticeable change of slope was seen in S and κ at the ferromagnetic transition temperature (TC) in Ni2+x Mn1-x Ga 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 Ni2 MnGa 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.