Effect of point defects and lattice distortions on the structural, electronic, and magnetic properties of Co2MnAl Heusler alloy

The effects of various point defects and lattice distortions have been studied on the structural, electronic, and magnetic properties of the Co₂MnAl alloy, considering the thermodynamically feasible range for the disorder concentrations. The AlMn binary antisite-disordered structures (Co₂Mn0.9375Al1.0625 and Co₂Mn0.875Al1.125) and (CoMn+AlMn)-ternary antisite-disordered structure (Co2.0625Mn0.875Al1.0625) may be formed spontaneously during the synthesis of Co₂MnAl due to their negative relative formation energies. The Al-deficient structures resulting from CoAl and MnAl binary antisite disorder (Co2.0625MnAl0.9375, Co2.125MnAl0.875, Co₂Mn1.0625Al0.9375, and Co₂Mn1.125Al0.875) and from (CoAl+MnAl)-ternary antisite disorder (Co2.0625Mn1.0625Al0.875) exhibit perfect half-metallicity. All other antisite disorders have a marginal effect on the half-metallic properties, and the disordered Co₂MnAl maintains the high spin polarization (≥70%) along with nearly the same magnetization as the L2₁-ordered structure. Conversely, vacancy defects seriously affect the electronic and magnetic properties with rapid shrinking of the minority pseudogap. Also, the effect of vacancy occurrence on the structural, electronic, and magnetic properties is found to be very susceptible to their concentrations. For lattice distortions, the uniform strain (ΔV/V₀) has a minimal effect on spin polarization and magnetization. Under negative strain within the range -10%≤ΔV/V₀≤7% (for 5.500.16em0ex≤a≤5.580.16em0ex), the strained structures showed perfect half-metallicity. For uniform strain >7%, spin polarization decreases monotonically; however, it remains high, reaching ~60% at ΔV/V₀=10% (a=5.880.16em0ex). Contrarily, the tetragonal distortions, except for small distortion values (Δc/a≤~±0.10), lead to significant degradation in half-metallic behavior. Moreover, the elongation along the c axis (with compression of the $ab$ plane) is found to be the favored condition for the occurrence of tetragonal distortion over compression along the c axis (with elongation of the $ab$ plane). Thus, the resulting perfect half-metallicity in disordered Co₂MnAl suggests that the occurrence of disorder could also be beneficial sometimes for spintronic applications.