Ab initio investigations of magnetic properties of ultrathin transition ...

6.3 Co atomic chain on Rh(664): 109
as on fcc(111) surface, in B-type step. These types of steps are called 〈001〉 or 〈111〉
microfaceted for A- and B-type, respectively. An example of B-type is a (p, p, p − 2)
surface which consists of terraces with a width of p full atomic wires. For example, a Co
wire on a B-type step edge can be modeled by Rh(111) surface which is cut in a way that
produces Rh terraces with 6-atomic rows, i. e. a Rh(664) surface with each Rh-edge atom
is replaced by Co, as illustrated in figure 6.5.
Figure 6.5: A Rh(664) surface (S) atoms shown as gray spheres, the step-edge atoms are
replaced by Co atoms represented as smaller red spheres. The three surface atoms that
are nearest neighbors to the step-edge Co atom are indicated: in the upper terrace Su, the
lower terrace Sl and in the direction of the bulk Sb. The angles θ and ϕ characterizing the
direction of the spin-quantization axis are indicated with respect to the terrace normal.
We performed calculations with ab initio full-potential linearized augmented plane wave
(FLAPW) method, as implemented in the FLEUR code[50]. The calculations are based
on GGA[69]. In our simulations of the Co atomic chain on Rh(664) surface, we used
symmetric film geometry (Fig. 6.5) with 45 atoms per unit cell. We used the Rh in-plane
lattice constant, 2.70 ˚A, and the six topmost interlayer distances were relaxed by force
calculations. We used about 90 basis functions/atom with a muffin-tin radius of 1.22 ˚A
for the Co chain atoms and 1.28 ˚A for the Rh atoms. The irreducible part of the surface
Brillouin zone was sampled with 42 k� points.
In table 6.1, we show the relaxation results of the Co atoms in a chain decorating the
Rh(664) step edge calculated with respect to the ideal atom spacing in bulk Rh. From
these results we notice that the Co relaxations with the upper terrace Rh atoms Co-Rhu
are smaller by factor of one half than the similar Co-Rhl and Co-Rhb relaxations towards
the lower atoms (c. f. fig.6.5). By that we expect a stronger Co hybridization with the
lower terrace and the bulk Rh atoms. If we compare Co/Rh(664) relaxation results to what