Compositional dependence of the electronic structures of Co2FeSi x Al1− x /Ge half-metal/semiconductor interfaces

The performances of Co-based Heusler alloys in spintronic devices are often degraded by interfacial effects. We used density functional theory to analyze the electronic structures of Co2FeSixAl1−x/Ge interfaces with Si content x from 0 to 1.0. Consistent with experimental data, interfaces with an antiparallel epitaxial relationship are more stable than those with a parallel epitaxy by ∼0.4 eV/supercell. The interfacial spin polarization of the more stable antiparallel interfaces varies significantly with composition, decreasing from ∼40% for an Al-rich interface (x = 0) to −24% at x = 1. Furthermore, local potential calculations show that the Co2FeAl/Ge interface has a better band alignment, with an energy barrier of 0.4 eV for majority spin-up electron injection, which increases to 0.5 eV for the Co2FeSi/Ge interface. These results suggest that Al-rich interfaces enable a more efficient spin injection into Ge, highlighting interfacial atomic control as a key strategy for optimization of spintronic heterostructures.