Superexchange interaction in K2NiF4: an ab initio Hartree-Fock study

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Abstract
The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K2NiF4 have been investigated using the ab initio periodic Hartree-Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is more stable than the ferromagnetic phase by 0.0216 eV per Ni pair, which is almost exactly two thirds of that found in KNiF3, in agreement with the hypothesis of additivity of the superexchange interaction with respect to the number of Ni-Ni neighbours. K2NiF4 turns out to be a two-dimensional antiferromagnet, the calculated interlayer superexchange interaction being at least three orders of magnitude smaller than that within the layers; the latter is shown to obey a d-12 (d is the shortest Ni-Ni distance) power law as suggested in the literature, and as verified in a previous study on KNiF3. The two apical F ions of the NiF6 octahedra not involved in the Ni-F-Ni superexchange path play an important role in determining the ferro magnetic-antiferromagnetic energy difference, which on the other hand is insensitive to large geometrical modifications that leave the octahedra unaltered. Charge- and spin-density maps are used to illustrate the electronic structure of the system and the superexchange mechanism.