Strain and Structure Driven Complex Magnetic Ordering of a CoO Overlayer on Ir(100)

We investigate the magnetic ordering in the ultrathin $c(10\times 2)$ CoO(111) film supported on Ir(100) on the basis of ab initio calculations. We find a close relationship between the local structural properties of the oxide film and the induced magnetic order, leading to alternating ferromagnetically and antiferromagnetically ordered segments. While the local magnetic order is directly related to the geometric position of the Co atoms, the mismatch between the CoO film and the Ir substrate leads to a complex long-range order of the oxide.

Figure 1

Illustration of the calculated structure for a $c(10\times 2)$ CoO(111) monolayer on Ir(100): O (red/gray), Co (blue/dark gray), Ir (white). (a) Experimentally proposed structure, (b) competing structure generated by shifting the CoO layer by half the Ir-Ir distance along the [010] direction. All directions [$z,x,y$] are given with respect to a $(1\times 1)$ iridium cell. See also the positional data shown in Fig. 3.

Figure 2

SQE model of a hexagonal $(1\times 1)$ CoO(111) monolayer on Ir(100): O (red/gray), Co (blue/dark gray), Ir (white). (a) Side view along [010] and (b) top view. (c)–(e) Different magnetic configurations, Co majority (blue/dark gray), Co minority (light blue/light gray). Configurations (c) and (d) are energetically almost degenerate (see text), while (e) is disfavored by $25\mathrm{meV}/\mathrm{Co}$ atom.

Figure 3

Structural details of a hexagonal $(1\times 1)$ CoO(111) monolayer on Ir(100). (a)–(c) Comparison between experiment and $\mathrm{PBE}+U$ ($U\mathrm{\text{−}}J=1\mathrm{eV}$) results. (d)–(f) CoO layer shifted by half of the substrate Ir-Ir spacing along $x$ [010]. Thin vertical lines show the ideal lateral positions of the substrate Ir atoms along $x$ [010], and empty (filled) diamonds show the ideal hollow (top) sites of the Ir substrate. For comparison, the “SQE” labeled dashed horizontal lines denote the atomic positions for the SQE model of Fig. 2. The $z$ [100] (height) values for CoO in (a) and (d) are given with respect to the mean Ir positions in the interface layer, while in (b)–(c) and (e)–(f) the $y$ [001] values are given with respect to the ideal hollow sites of the Ir substrate where Co atoms of a metallic Co overlayer would be found. To allow for a better comparison with experiments, the calculated lateral distances have been scaled to the experimental Ir lattice constant of 3.84 Å (theory: 3.88 Å).

Figure 4

Magnetic structure of a $c(10\times 2)$ CoO(111) monolayer on Ir(100): O (red/gray), Co majority (blue/dark gray), Co minority (light blue/light gray), Ir (white). The top panel shows the experimentally proposed $c(10\times 2)$ structure [12, 14], the bottom panel shows a competing structure with a registry-shifted $c(10\times 2)$ CoO overlayer, which is slightly favored by a $6\mathrm{meV}/\mathrm{Co}$ atom.