Highly spin-polarized electronic structure and magnetic properties of Mn(2.25)Co(0.75)Al(1−x)Ge(x) Heusler alloys: first-principles calculations

Highly spin-polarized half-metals (HMs) and spin-gapless semiconductors (SGSs) are the promising candidates in spintronic devices. However, the HM and SGS Heusler materials are very sensitive to the stoichiometric defects and lattice distortion, which will be not beneficial to the practical applications. Here, the electronic structure and magnetic properties of Mn(2.25)Co(0.75)Al(1−x)Ge(x) (x = 0, 0.25, 0.50, 0.75 and 1.00) Heusler alloys were investigated by first-principles calculations. Large negative formation energy, cohesive energy and phonon spectra confirm that the Mn(2.25)Co(0.75)Al(1−x)Ge(x) alloys are stable. It is found that Mn(2.25)Co(0.75)Al(1−x)Ge(x) with x = 0, 0.25, 0.75 and 1.00 show robust ferrimagnetic HM characteristics, while Mn(2.25)Co(0.75)Al(0.5)Ge(0.5) shows robust SGS characteristic. Under the hydrostatic and uniaxial strains, Mn(2.25)Co(0.75)Al(1−x)Ge(x) exhibit a series of rich electronic transitions. The magnetic anisotropy of Mn(2.25)Co(0.75)Al(1−x)Ge(x) turns from the in-plane [100] direction to the out-of-plane [001] one by applying the uniaxial strains. The results suggest that the complete spin polarizations of Mn(2.25)Co(0.75)Al(1−x)Ge(x) alloys are robust against the stoichiometric defects and lattice distortion, which have potential applications in spintronic devices.