Combined experimental and theoretical investigation of the premartensitic transition in Ni$_2$MnGa

TL;DR

This study combines experimental measurements and theoretical calculations to investigate the premartensitic transition in Ni$_2$MnGa, revealing electronic structure changes and a pseudogap formation around 247K.

Contribution

It provides a comprehensive analysis of the electronic and structural changes during the premartensitic transition using combined experimental and first-principles approaches.

Findings

Detection of a pseudogap at 247K via UPS

Disappearance of the 0.3 eV peak below the martensitic transition

Fermi surface reconstruction associated with electronic state changes

Abstract

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K < T < 250K$. All measurements detect clear signatures of the premartensitic transition ($T_\mathrm{PM}\sim 247K$) and the martensitic transition ($T_\mathrm{M} \sim 196K$). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at $T_\mathrm{PM}$ located 0.3eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3eV in the UPS spectra for $T > T_\mathrm{PM}$ is due to the Ni-d minority-spin electrons. Below $T_\mathrm{M}$ this peak disappears, resulting in an enhanced density of states at energies around 0.8eV. This enhancement reflects Ni-d and Mn-d electronic contributions to the majority-spin density of states and is accompanied by significant reconstruction of the Fermi surface.