Beyond conventional half-metals: gapless states and spin gapless semiconducting behavior in X$_2$MnGa (X = Ti, Ir) Heusler compounds

Abstract: The search for high-performance spintronic materials motivates the exploration of Heusler alloys with unconventional electronic properties. Using density functional theory with Hubbard correction (DFT+$U$, $U = 4$~eV), we investigate X$_2$MnGa (X = Ti, Ir) alloys, which stabilize in the ferromagnetic L2$_1$-type structure with strong thermodynamic stability. Electronic structure calculations reveal contrasting behaviors: Ti$_2$MnGa transitions from a metallic L2$_1$-type phase to a spin gapless semiconductor (SGS) in the XA-type, while Ir$_2$MnGa exhibits gapless half-metallicity behavior in the L2$_1$-type but becomes half-metallic in the XA-type. The magnetic properties are governed by spd hybridization between Mn-3$d$ and X-$d$/Ga-$p$ states, which stabilizes ferromagnetism and tailors electronic states near the Fermi level. The Hubbard $U$ correction proves essential for accurately describing the correlated Mn-3$d$ electrons. These alloys combine structural stability with tunable electronic and magnetic properties, offering a promising platform for spin-polarized transport in next-generation spintronic devices.