Chemical disorder and functionalities of Ni₂MnGa alloy thin films

Full text: In this work we investigate the chemical disorder in thin films of Ni₂MnGa alloy thin films grown by molecular beam epitaxy and DC sputtering techniques on crystalline GaAs substrates. The chemical and structural disorder induced during growth can be separated through X-ray diffraction measurements, whereas the surface stoichiometry is determined by in situ photoelectron spectroscopy analyses. Polycrystalline and crystalline thin films of Ni₂MnGa alloys are ferromagnetic and shape memory material (SMM) with rather distinct magnetic moments on Ni and Mn sites. In a SMM thin film with nanometric thickness, total magnetic moment per formula unit and lattice parameters become strongly dependent on the structural and chemical disorder. Residual strain commonly induced in the films due to the lattice mismatch with substrate or epitaxial relationships can be relaxed by thermal cycling and post-annealing procedures. However, intrinsic antisite disorder is quite difficult to mitigate in these Heusler alloys which adopt the L2₁-type crystal structure with four equivalent interpenetrating face-centered cubic structures, where Ni has two equivalent sites. Ex-situ X-ray diffraction experiments were used to probe the chemical and structural disorder by comparing the relative intensity of Bragg reflections corresponding to the crystalline planes of families (nnn), (0nn) and (n00), considering the atomic and structure factors for perfectly ordered alloys structurally and chemically. Essentially, the statistical relation between the lattice site occupations by the three possible atoms is modeled to better reproduce the ratio of diffraction peak intensities. Magnetization measurements using a vibrating sample magnetometer were also used to evaluate the changes in the saturation magnetization of the films. (author)