Substitution effect on the electronic structure, magnetic anisotropy and thermoelectric behavior of Heusler-type Co2Mn1-zXzSi (X = V, Cr, Fe; 0≤z≤1) compounds: A density functional study

Here we explore the effect of partial substitution by transition-metal elements on the elastic, electronic, magnetic and thermoelectric properties of Heusler-type alloys Co2Mn1-zXzSi (X = V, Cr, and Fe; 0 <= z <= 1), within the framework of density functional theory (DFT). Although the optimized structures at z = 0.5 crystallize in tetragonal with space group P4/mmm, incorporation of the on-site Coulomb interaction (U) allows for a wider direct bandgap in the minority-spin channel for all the ternary and quaternary Heusler alloy compounds (HACs) leading to half-metallicity. The enthalpy of formation along with our estimate of elastic constants further reveals that all the HACs under study can remain stable thermodynamically even with a considerable amount of substitutional defects. Compared to other systems, Co2Mn0.25V0.75Si displays rather a high Poisson's ratio of 0.36, indicating its ductile profile with the Vickers hardness of similar to 3. We find that the ternary and quaternary HACs can retain the half-metallic (HM) character for the minority-spin channels with conduction band minima due to Co-d orbitals. Bandgap tuning has also been observed in Co2Mn1-zXzSi with X = V/Cr at z = 0.5, signifying how the substitutional defects can influence the electronic structure of HACs. The tetragonal structures (c/a not equal 1) at z = 0.5 are found to display higher in-plane magnetocrystalline anisotropy energy (e.g. MAE similar to 70 mu eV, for X = V) than the cubic structures (c/a = 1) where it ceases eventually. This implies a strong dependence of MAE on the axial ratio, which can be tuned by way of uniaxial strain. The figure-of-merit analysis subsequently predicts Co2Mn0.25X0.75Si to possess better thermoelectric properties, especially with X = V/Cr. Such myriad abilities of HACs can potentially pave the way for designing better electrode materials in spintronic devices to generate controlled spin currents with resistance to high mechanical strain.