Surface structures of Co2FeAl0.5Si0.5(001) Heusler alloys: A first-principles study

We have studied the stable surface structures of the full-Heusler alloys Co2FeAl0.5Si0.5Co2FeAl0.5Si0.5 in the [001] direction by first-principles calculations. Only the pure Al-terminated surface is found to be the most stable surface in the whole allowed chemical potential range of the Co2FeAl0.5Si0.5Co2FeAl0.5Si0.5 bulk. The bulk truncated FeAlSi and pure Co surface structures are unstable. For the eight calculated surface terminations, the missing neighbor effects in surfaces result in: (1) the oscillatory relaxation behaviors of the surfaces, (2) the enhancement of magnetic moments of surface Co, Fe, and Si atoms, (3) the reduction of the hybridization of 3d-orbitals for surface transition metals or the weakening of the bonding–antibonding splitting. Surface states appear at the Fermi level and the half-metallic properties are lost at surfaces for the eight surface terminations. For the pure Al-terminated surface, the surface and subsurface layers keep spin polarizations of 98% and 17% at the Fermi level, respectively.