Based on density functional calculations, we report on the orbital order and microscopic magnetic model of FeNCN, a prototype compound for orbital-only models. Despite having a similar energy scale, the spin and orbital degrees of freedom in FeNCN are only weakly coupled. The ground-state configuration features the doubly occupied d_{3z^2-r^2} (a1g) orbital and four singly-occupied d orbitals resulting in the spin S=2 on the Fe+2 atoms, whereas alternative (Eg') configurations are about 75 meV/f.u. higher in energy. Calculated exchange couplings and band gap are in good agreement with the available experimental data. Experimental effects arising from possible orbital excitations are discussed.
@article{arxiv.1106.3665,
title = {Orbital order and magnetism of FeNCN},
author = {Alexander A. Tsirlin and Klaus Koepernik and Helge Rosner},
journal= {arXiv preprint arXiv:1106.3665},
year = {2011}
}