  We investigated the origin of perpendicular magneto-crystalline anisotropy
(MCA) in L10 ordered FeNi alloy using first-principles density-functional
calculations. We found that the perpendicular MCA of L10-FeNi arises
predominantly from the constituent Fe atoms, which is consistent with recent
measurements of the anisotropy of the Fe orbital magnetic moment of L10-FeNi by
x-ray magnetic circular dichroism. Analysis of the second-order perturbation of
the spin-orbit interaction indicates that spin-flip excitations between the
occupied majority-spin and unoccupied minority-spin bands make a considerable
contribution to the perpendicular MCA as does the spin-conservation term in the
minority-spin bands. Furthermore, the MCA energy increases as the in-plane
lattice parameter decreases (increasing the axial ratio c/a). The increase in
the MCA energy can be attributed to further enhancement of the spin-flip term
due to modulation of the Fe d(xy) and d(x2-y2) orbital components around the
Fermi level under the compressive in-plane distortion.
