$d^0$-$d$ half-Heusler alloys: A potential class of advanced spintronic materials
The possibility of ferromagnetic half-metallicity in lithium-based half-Heusler alloys, such as MnPLi, has been theoretically explored before [Damewood et al., Phys. Rev. B 91, 064409 (2015)], although at optimized lattice constants such lithiated manganese pnictides are predicted to be ordinary ferromagnets and therefore not suitable for spintronic applications. In the following, however, it is shown by first principles density functional calculations that half-Heusler alloys formed by $3d$ transition metals and $d^0$ alkali or alkaline-earth metals, which are defined by the valence electronic configuration $ns^{1,2},(n-1)d^0$, can produce all kinds of half-metallic behavior at optimized lattice constants including the elusive Dirac half-metallicity that is theoretically predicted for the first time in a three-dimensional prototype MnPK. Together with the predicted magnetic and mechanical stability, this could pave the way for massless and dissipationless spintronics of the future. Among other technologically important features of the prototype Dirac half-metal MnPK, are the maximal moment $d$-shell ferromagnetism, high Fermi velocity of Dirac fermions that is comparable with that in graphene, and a wide half-metallic gap. Furthermore, by considering the prototype conventional half-metal VSbSr, the introduction of $d^0$ metals is shown to substantially reduce the hull distance and produce true metastability in the otherwise instable chemical structure of zinc-blende transition metal pnictides--in this case zinc-blende VSb--without altering the `$p$-$d$ exchange' that is mainly responsible for their half-metallicity, thus bringing their realization and potential applications in spintronics a step closer to reality.