Origin and orientation of the electric field gradient in ordered FeNi

Abstract
The electronic structure of tetrataenite, the ordered phase of FeNi, has been studied in the molecular cluster approximation using local-density theory. Clusters containing 13 and 19 atoms were embedded in the fcc host lattice, and spin-unrestricted potentials were iterated to self-consistency. Local moments, magnetic hyperfine fields, and electric field gradients (EFG) at the iron sites were determined for comparison with experiment. The orientation of the EFG principal axis is found to be parallel to the superstructure c axis.