We present the measurements of electrical resistivity $\left(\rho \right)$, specific heat $\left(C_P\right)$, Seebeck coefficient $\left(S\right)$, as well as thermal conductivity $\left(\kappa \right)$ of the Heusler compounds ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ from $10\text{to}400\mathrm{K}$. A series of ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ 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 $\kappa$ around the martensitic transition temperature $\left(T_M\right)$, while a noticeable change of slope was seen in $S$ and $\kappa$ at the ferromagnetic transition temperature $\left(T_C\right)$ in ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ 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 $\left(T_P\right)$ was unambiguously resolved in the stoichiometric compound ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{Ga}$ by the $C_P$ 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 $T_M$, 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 $T_M$. We connected this observation to the nesting of the Fermi-level density of states, which is appropriate for the Peierls transition.

• Received 3 December 2004

DOI:https://doi.org/10.1103/PhysRevB.72.054116