Anomalous Nernst effect in Ni2⁢MnGa/MgO (001) shape-memory Heusler alloy thin films

Abstract

Ni2⁢MnGa is regarded as a prototypical shape memory Heusler alloy (SMHA) due to its large magnetic field-induced strain (∼10%) and multifunctional properties. The Ni2⁢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 Ni2⁢MnGa. However, the lack of a PM phase in the epitaxial thin films of Ni2⁢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 Ni2⁢MnGa where a clear PM phase transition was observed together with the martensite phase transition. A systematic analysis of the anomalous Nernst thermopower (𝑆ANE) and anomalous Nernst conductivity (𝛼ANE𝑥⁢𝑦) was performed across all the phases (austenite, PM and martensite). A sharp change in 𝑆ANE and 𝛼ANE𝑥⁢𝑦 at the PM phase is observed. With 𝛼ANE𝑥⁢𝑦 being sensitive to the changes at the Fermi surface, this establishes the link of soft phonon mode and structural modulation in Ni2⁢MnGa.