Structure and magnetism of the $(2\times 2)$-FeO(111) surface

Based on ab initio calculations, we determine the features and relative stability of different models proposed to describe the $(2\times 2)$-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values.

Figure 1

Side view of the surface models considered, represented under their most stable structural and magnetic configurations, and indicating the stoichiometry of the squared region B1 (see text for details). The gray lines in the back are a guide to delimit the size of the $(2\times 2)$ unit cell.

Figure 2

Schematic representation of the different types of magnetic coupling considered for Fe atoms at B1 and B2.

Figure 3

Surface energy ($\sigma$) of the models in Fig. 1 as a function of the O chemical potential (${\mu }_O$). The RS (bulk truncation) and WZ terminations under AF-II order are also shown. The inset is a zoom of the lower right corner of the graph.