Pressure induced modification of electronic and magnetic properties of MnCrNbAl and MnCrTaAl

Spin-gapless semiconductor (SGS) is a new class of material that has been studied recently for potential applications in spintronic devices. These materials behave as an insulator for one spin channel, and as a gapless semiconductor for the opposite spin. In this work, we present results of a comprehensive computational study of two quaternary Heusler alloys, MnCrNbAl and MnCrTaAl that have been recently reported to exhibit spin-gapless semiconducting electronic structure. In particular, using density functional calculations we analyze the effect of external pressure on electronic and magnetic properties of these compounds. It is shown that while these two alloys indeed exhibit nearly SGS behavior at optimal lattice constants and at negative pressure (expansion), a closer inspection reveals that they are half-metals at equilibrium, and magnetic semiconductors at larger lattice constant. At the same time, reduction of the unit cell volume has a detrimental effect on electronic properties of these materials, by modifying the exchange splitting of their electronic structure and ultimately destroying their half-metallic / semiconducting behavior. Thus, our results indicate that both MnCrNbAl and MnCrTaAl may be attractive materials for practical device applications in spin-based electronics, but a potential compression of the unit cell volume (e.g. in thin-film applications) should be avoided.