First-Principles Studies of the Structural and Magnetic Properties of the Thin-Film Structures of Cobalt-Based Heusler Alloys

The effect of different atomic ordering on the saturation magnetization, magnetocrystalline anisotropy, and equilibrium parameters of crystalline bulk and thin-film structures of Heusler Co₂Ni_{1 + x}Z_1–x alloys (Z = Al, Ga) is considered within the framework of density functional theory. It has been revealed that stoichiometric Co₂NiZ compositions have an inverse (XA) tetragonal structure, whereas deviation from stoichiometry (0 ≤ x ≤ 0.5) leads to the change of XA type atomic ordering for a layerwise ordering of Co and Ni atoms (T^p) in a tetragonal structure. For bulk and thin-film structures, the magnetization is observed to grow from ~65 to 98 (A m²)/kg depending on the Ni content (0 ≤ x ≤ 0.5). The effect of the thin-film state on magnetocrystalline anisotropy in the Co₂Ni_{1 + x}Z_{1 – x} alloys has been studied. All the structures are characterized by in-plane magnetic anisotropy. The Co₂NiZ alloys have an anisotropy energy of ~0.65 meV/f.u., whereas deviation from stoichiometry leads to nearly a two-fold decrease in its value. It has been shown that the thin-film structure does not suppress the energy value for the bulk specimen and even slightly increases the anisotropy energy for the composition with х = 0.25.