Electronic and magnetic effects of a stacking fault in cobalt nanoscale islands on the Ag(111) surface

By utilizing spin-polarized scanning tunneling microscopy (STM) and spectroscopy, we observe the coexistence of perpendicularly and in-plane magnetized cobalt nanoscale islands on an Ag(111) surface. The magnetization direction has the relationship with the observed moiré-corrugation amplitude on the islands; the islands with stronger moiré corrugation show perpendicular magnetization, and the ones with weaker moiré corrugation exhibit in-plane magnetization. We calculate the magnetic anisotropy energy for various stackings of a Co nanostructure based on density functional theory, and we reveal that perpendicular magnetic anisotropy is reduced drastically with increasing fcc stacking faults in the Co layer. Simulated STM images reproduce the moiré-corrugation difference observed experimentally when the stacking difference between perpendicularly and in-plane magnetized islands is considered. These theoretical analyses strongly suggest that the electronic and magnetic differences between the two types of islands are caused by the presence of fcc stacking faults in the intrinsic hcp stacking of Co.

Figure 1

(a) Overview image of Co-deposited Ag(111) surface (sample bias voltage $V_{\mathrm{S}}=-0.2$ V, tunneling current $I_{\mathrm{T}}=1$ nA). Co islands with thicknesses ranging from 5 to 7 MLs are observed. (b) Constant-current image of the 6-ML Co islands on Ag(111) $(V_{\mathrm{S}}=-0.2$ V, $I_{\mathrm{T}}=0.5$ nA). The island types are labeled with S (strong moiré) and W (weak moiré). (c) Cross-sectional profile as indicated in panel (b), along the two islands that has the same thickness. S and W indicate the island types.

Figure 2

Spin-averaged $dI/dV$ spectra obtained at three different sites $(\alpha ,\beta ,\gamma )$ in the moiré pattern on the (a) strong- and (b) weak-moiré-patterned islands. The tip-sample separation was stabilized with the setting of $I_{\mathrm{T}}=1.0$ nA and $V_{\mathrm{S}}=-0.7$ V. Inset of panel (a) is the corresponding STM image $(3.1\text{nm}\times 3.1\text{nm},I_{\mathrm{T}}=1.0$ nA and $V_{\mathrm{S}}=-0.2$ V) showing the exact positions of $\alpha$ (black circle), $\beta$ (red triangle), and $\gamma$ (blue triangle) sites.

Figure 3

Topography (leftmost panel) and spin-resolved $dI/dV$ colorized topographies at magnetic fields of $-0.5,-0.1$, and $-0.26$ T (in the downward field sweep) $(V_{\mathrm{S}}=-0.4$ V, $I_{\mathrm{T}}=1$ nA). A more detailed field dependence of the $dI/dV$ mapping is available as a movie [28]. The island types are labeled S (strong moiré) and W (weak moiré) in the topography. The thicknesses of the islands are A: 7 MLs, B: 8 MLs, C: 6 MLs, D: 6 MLs, E: 6 MLs.

Figure 4

Magnetic-field dependencies of the $dI/dV$ values corresponding to islands A to E in the spin-resolved $dI/dV$ images shown in Fig. 3 $(V_{\mathrm{S}}=-0.4$ V, $I_{\mathrm{T}}=1$ nA). The magnetic field is applied perpendicular to the surface. A butterfly shaped curve was obtained on strong moiré islands A to D but not on the weak-moiré island E. The inset of the top panel is the hysteretic $dI/dV$ curve of island A estimated from the butterfly shaped curve given the flippings of the tip magnetization direction at $-0.28$ and 0.25 T.

Figure 5

(a) Spin-resolved $dI/dV$ spectra taken on three different sites of a strong-moiré island and their averages. Arrows $\uparrow \uparrow$ $\left(\uparrow \downarrow \right)$ refer to parallel (antiparallel) configurations of the tip and sample magnetizations. The spectra with the parallel and antiparallel configurations were obtained at 1 and 0.15 T (in the downward field sweep), respectively. The tip-sample separation is stabilized with $I_{\mathrm{T}}=1.0$ nA and $V_{\mathrm{S}}=-0.7$ V. (b) Magnetic asymmetries arising from opposite magnetization configurations $[A=(\uparrow \uparrow -\uparrow \downarrow )/(\uparrow \uparrow +\uparrow \downarrow \left)\right]$ at the three different sites.

Figure 6

(a) Variation in the differential conductance $dI/dV$ at $-0.2$ V as a function of magnetic field as acquired for the weak-moiré island (6 MLs) shown in the STM image (inset, $19\text{nm}\times 22\text{nm},V_{\mathrm{S}}=-0.2$ V, $I_{\mathrm{T}}=1$ nA). The magnetic field was applied perpendicular to the surface plane. (b) Schematic showing the decomposition of the shape of the curve in panel (a). The experimental data are qualitatively explained as a combination of the magnetization curves of the in-plane-magnetized Co island and the perpendicularly magnetized tip in perpendicular magnetic fields.

Figure 7

Local density of states integrated between $-0.2$ eV and $E_{\mathrm{F}}$ in a plane parallel to the surface, which is $\sim 0.27$ nm above the surface, for (a) hcp with ABABA stacking, (b) pseudo-hcp with $\mathrm{AB}\left|\mathrm{CB}\right|\mathrm{A}$ stacking, and (c) fcc with $\mathrm{AB}\left|\mathrm{C}\right|\mathrm{A}|\mathrm{B}$ stacking. $a_0$ represents the Bohr radius. The $2\times 2$ supercells of the moiré pattern are shown.