Effect of Point Defects and Lattice Distortions on the Structural, Electronic, and Magnetic properties of Co$_2$MnAl Heusler alloy

Abstract: The effects of various point defects and lattice distortions on the structural, electronic, and magnetic properties of Co$2$MnAl alloy are investigated using density functional theory calculations. For the point defects, six types of binary antisites, three types of ternary antisites, and three kinds of vacancies have been simulated with different disorder degrees, up to a maximum of 12.50%. For the lattice distortions, cubic strain within -10% $\leq$$\Delta{V/V_0}$$\leq$ 10% (corresponding to 5.50\r{A} $\leq$ a $\leq$5.88\r{A}) and tetragonal distortions with 0.5$\leq$$\textit{c/a}$$\leq$1.5 at three different unit-cell volumes - $\textit{V}_0$ and ($\textit{V}_0$$\pm5$%$\textit{V}_0$) have been considered. The Co${Al}$ and Mn${Al}$ binary antisite disordered structures (namely, Co${2.0625}$MnAl${0.9375}$, Co${2.125}$MnAl${0.875}$, Co$_2$Mn${1.0625}$Al${0.9375}$ and Co$_2$Mn${1.125}$Al${0.875}$) and (Co${Al}$+Mn${Al}$) ternary antisite disordered structure (Co${2.0625}$Mn${1.0625}$Al${0.875}$) exhibit perfect half-metallicity. The rest of the antisite disorders have a marginal effect on the half-metallic properties of Co$_2$MnAl, along with high spin polarization ($\textit{P}$ $\geq$ 70%) and nearly same magnetization ($\textit{M$_s$}$) as that for ideal structure. Conversely, the vacancy defects significantly affect the electronic and magnetic properties. The cubic strained structures exhibit high $\textit{P}$ and constant $\textit{M$_s$}$. Under negative strain within -10% $\leq$$\Delta{V/V_0}$$\leq$ -7% (for 5.50\r{A} $\leq$ a $\leq$ 5.58\r{A}), the strained structures have perfect half-metallicity. On the other hand, tetragonal distortions lead to significant degradation in half-metallic behavior, except for small distortion values $\Delta{c/a}$, irrespective of their volume.