Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L2(1)) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retains half-metallic character in Cr-excess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half-metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. This information should help in controlling these potential spintronic materials.