Fabrication and characterization of a Ni-Mn-Ga uniaxially textured freestanding film deposited by DC magnetron sputtering

TL;DR

This study reports the fabrication of Ni-Mn-Ga ferromagnetic shape memory alloy films via DC magnetron sputtering, analyzing their structural, magnetic, and shape memory properties with a focus on martensitic transformation.

Contribution

It introduces a method to produce uniaxially textured Ni-Mn-Ga films with controlled properties and models their magnetization behavior during phase transformation.

Findings

Films exhibit strong (022) out-of-plane texture.

Martensitic transformation occurs around 255K.

Martensite shows ~10% higher saturation magnetization than austenite.

Abstract

Homogeneous freestanding films have been obtained by the direct current (DC) magnetron sputtering technique using a sacrificial layer. After annealing, the films are crystallized with a strong out-of-plane texture along the (022) direction. The stoichiometry of the annealed films is close to the target composition and leads to a martensitic transformation around 255K. The annealed films demonstrate ferromagnetic behavior with a Curie temperature of about 362K. The magnetization process has been studied on the both states and during the martensitic transition. The saturation magnetizations have been determined by fitting the experimental data with a saturation approach law in the range 1-5T. Results show the saturation magnetization of the martensite is around 10% higher than that of the austenite. A model based on intrinsic magnetic properties of each state allowing the description of the magnetization M=f (H, T) of such polycrystalline films during the martensitic transformation is presented. The mass fraction of martensite inside the austenite phase can be determined using this model. The shape memory effect is analyzed both by scanning electron microscopy and by optical microscopy with in-situ measurement of the resistance temperature dependence.