Stress-induced Martensitic transformation in epitaxial Ni-Mn-Ga thin films and its correlation to optical and magneto-optical properties

TL;DR

This study investigates how stress-induced martensitic transformation in epitaxial Ni-Mn-Ga thin films affects their optical and magneto-optical properties, revealing correlations between structural phase changes and electronic behavior.

Contribution

It provides new insights into the correlation between stress-induced martensitic transformation and optical/magneto-optical properties in epitaxial Ni-Mn-Ga films, including spectral permittivity analysis.

Findings

Martensitic transformation occurs in films thicker than 80 nm.

Magneto-optical spectra evolve with film thickness, indicating electronic structure changes.

Substrate-induced strain significantly impacts the electronic and magnetic properties.

Abstract

The optical and magneto-optical properties of thin epitaxial Ni-Mn-Ga films, with thicknesses ranging from 8 to 160 nm, were investigated across the spectral range of 0.7-6.4 eV. The films were deposited by DC magnetron sputtering on MgO substrate with stress-mediating Cr buffer layer. Structural and magnetic characterization revealed an stress-induced martensitic transformation for the films thicker than 80 nm, while thinner films remained in austenite structure deformed by substrate constraint. Both X-type and Y-type twin domains were observed in martensitic samples. Magneto-optical polar Kerr effect spectra showed notable evolution with film thicknesses, demonstrating changes in electronic structure of Ni-Mn-Ga. A combination of spectroscopic ellipsometry and magneto-optical Kerr spectroscopy allowed for the deduction of the spectral dependence of full permittivity tensor of investigated samples. Three magneto-optical transitions were fitted to the spectra of non-diagonal permittivity using semiclassical theory, showing strong correlations with in-plane coercivity. Observed correlations highlight the impact of substrate-induced strain on Ni-Mn-Ga films and provide insights into the electronic structure changes associated with the martensitic transformation.