Abstract
The low-index (001), (010), and (110) surfaces of $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ are investigated using the ab initio generalized gradient (GGA) density functional method. For the bulk crystal, the GGA approach is found to predict correctly its electronic structure and the antiferromagnetic ordering along the [001] direction, with Mn magnetic moments close to 3${\ensuremath{\mu}}_{B}$. The more stable surfaces are the (001) terminated in a MnN plane and the (010), both observed experimentally in $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ thin films grown on MgO (001) crystals. While in the (001) the surface plane consists of Mn atoms ferromagnetically coupled through N, in the (010) surface rows of Mn antiferromagnetically aligned alternate along the [001] direction. Furthermore, our results suggest that, due to its relatively small surface energy, the (110) surface with a singular one-dimensional ribbonlike 8.46 \AA{} width ferromagnetic structure could also be stabilized. Despite their differences, the magnetic configuration of all the surfaces is dictated by the cleavage of the layerwise antiferromagnetic structure of bulk Mn${}_{3}$N${}_{2}$. The broken symmetry of the surface induces atomic corrugations, which however do not alter drastically either the spin configuration or the electronic structure. Mn-N bond states tend to be close to the bottom of the valence bands, and although an enhancement of the surface magnetic moments are obtained, they remain close to the bulk values. All the surfaces are metallic as bulk $\ensuremath{\eta}$-Mn${}_{3}$N${}_{2}$ crystals.
Published Version
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