${\mathrm{Ni}}_2\mathrm{MnGa}$ is regarded as a prototypical shape memory Heusler alloy (SMHA) due to its large magnetic field-induced strain ($\sim 10\%$) and multifunctional properties. The ${\mathrm{Ni}}_2\mathrm{MnGa}$ undergoes a transition from a high-temperature austenite phase to a low-temperature martensitic (MT) phase via a precursor or premartensite (PM) phase transition. The PM phase plays an important role in understanding the origin of the structural phase transition in ${\mathrm{Ni}}_2\mathrm{MnGa}$. However, the lack of a PM phase in the epitaxial thin films of ${\mathrm{Ni}}_2\mathrm{MnGa}$ left an issue of the origin of modulation unresolved. Here, we investigated the temperature-dependent anomalous Nernst effect (ANE) in an epitaxial thin film of ${\mathrm{Ni}}_2\mathrm{MnGa}$ where a clear PM phase transition was observed together with the martensite phase transition. A systematic analysis of the anomalous Nernst thermopower ($S_{\text{ANE}}$) and anomalous Nernst conductivity (${\alpha }_{xy}^{\text{ANE}}$) was performed across all the phases (austenite, PM and martensite). A sharp change in $S_{\text{ANE}}$ and ${\alpha }_{xy}^{\text{ANE}}$ at the PM phase is observed. With ${\alpha }_{xy}^{\text{ANE}}$ being sensitive to the changes at the Fermi surface, this establishes the link of soft phonon mode and structural modulation in ${\mathrm{Ni}}_2\mathrm{MnGa}$.

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• Anomalous Hall effect
• Magnetotransport
• Nernst effect
• Shape memory effect
• Heusler alloy
• Thin films