Scanning tunneling spectroscopy on Co(0001): Spectroscopic signature of stacking faults and dislocation lines

The growth morphology and electronic structure of Co(0001) grown on W(110) are studied using scanning tunneling microscopy and scanning tunneling spectroscopy (STS) at $T=6\mathrm{K}$. Depending on growth conditions, continuous Co films or Co islands on top of a wetting layer are formed. Within the continuous films, dislocation lines appear and increase in density after annealing. Co islands and films exhibit $dI∕dV$ curves with a pronounced peak at $-0.3\text{eV}$ below the Fermi energy. The intensity of this peak is changing in different areas of the surface. Using monolayer high islands with a different shape deposited on the same Co layer we attribute the different intensity to a different stacking of the Co surface. The change in intensity is reproduced by first-principles electronic structure calculations, which reveal that the peak is caused by a $d_{3z^2-r^2}$-like surface resonance of a minority-spin character more strongly coupled to the bulk states in the case of hcp ($ABA$) stacking than in the case of fcc ($ABC$) stacking. An increased STS intensity of the surface resonance was also found above dislocation lines located at the Co/W interface.

Figure 1

Diagram showing the unit cells of the W and the Co surface oriented according to the Nishiyama-Wassermann orientation. $a_i$ and $b_i$ denote the diagonal lengths of the unit cells of W (solid lines) and Co (dashed lines), respectively, were $i=x$ points in the $\left[1\overline{1}0\right]$ direction and $i=y$ points in the $\left[001\right]$ direction of the W(110) substrate.

Figure 2

Growth of Co on W(110) at different annealing temperatures as indicated in the images. (a)–(c) Constant-current images of $3\text{ML}$ Co on a W(110) crystal with an average terrace width of $10\text{nm}$ [(a) and (b) $I=0.3\text{nA}$, $V=-0.3\mathrm{V}$; (c) $I=0.2\text{nA}$, $V=-0.8\mathrm{V}$; $T=300\mathrm{K})$]. (d) Line profiles along the lines $A$ and $B$ in (a) and (c), respectively. (e)–(g) Constant-current images of $10\text{ML}$ Co on a W(110) crystal with an average terrace width of $200\text{nm}$. The step edges of the W substrate run along the white dashed lines [(e) $I=0.6\text{nA}$, $V=-0.33\mathrm{V}$; (f) $I=0.7\text{nA}$, $V=-0.33\mathrm{V}$; (g) $I=0.3\text{nA}$, $V=1\mathrm{V}$; $T=6\mathrm{K}$]. (h) Line profile along the line $C$ in (e). The assigned numbers of MLs have been deduced as described in the text. Note that the contrast of the dislocation lines in (e) is reduced with respect to (f) and (g) because of a smaller contrast per ML.

Figure 3

(a) Constant-current image of the dislocation network across a W step edge on the $10\text{ML}$ Co film annealed at $400\mathrm{K}$ $(I=0.7\text{nA}$, $V=-0.33\mathrm{V}$; $T=6\mathrm{K})$. The white arrows indicate the underlying W step edge. (b) Line profile along the white line in (a). There are two types of dislocation lines, one appearing as a higher and one as a shallower protrusion. At the underlying W step edge the two types of dislocation lines are interchanged. (c) and (e) Constant-current images with atomic resolution on different terraces of the $10\text{ML}$ Co film annealed at $350\mathrm{K}$ [(c) $I=5\text{nA}$, $V=10\text{mV}$; (e) $I=0.9\text{nA}$, $V=6\text{mV}$; $T=6\mathrm{K}$]. (d) Line profile along the white line in (c).

Figure 4

Spatially averaged $dI∕dV$ curve on a $10\text{ML}$ Co film annealed at $450\mathrm{K}$ (black curve; $I_{\mathit{stab}}=1\text{nA}$, $V_{\mathit{stab}}=1.5\mathrm{V}$, $V_{\mathit{mod}}=10\text{mV})$ compared to $dI∕dV$ curves taken on Co monolayer islands (gray) and on the W(110) substrate (light gray) $(I_{\mathit{stab}}=0.7\text{nA}$, $V_{\mathit{stab}}=-1.5\mathrm{V}$, $V_{\mathit{mod}}=10\text{mV})$. Inset: constant-current image of Co monolayer islands prepared by deposition of $0.5\text{ML}$ Co on W(110) at room temperature $(I=0.4\text{nA}$, $V=-0.13\mathrm{V})$. Measurement temperature $T=6\mathrm{K}$.

Figure 5

(a) Spatially averaged $dI∕dV$ curves taken on Co layers with increasing thickness as they appear on the island as indicated in (b) $(I_{\mathit{stab}}=0.8\text{nA}$, $V_{\mathit{stab}}=1.5\mathrm{V}$, $V_{\mathit{mod}}=4\text{mV})$. (b) Wedge-shaped Co island of a $3\text{ML}$ Co film annealed at $500\mathrm{K}$. The island is $3\text{ML}$ thick at the right rim and $11\text{ML}$ thick at the left rim, as indicated. The step edges of the underlying W substrate are visible as dark stripes running under an angle of ${30}^{°}$ to the $\left[1\overline{1}0\right]$ direction $(I=0.5\text{nA}$, $V=-0.3\mathrm{V})$. (c) Line profile along the black line in (b). Measurement temperature $T=6\mathrm{K}$.

Figure 6

(a) Spatially averaged $dI∕dV$ curves taken before (light gray) and after the exposure of the Co film from Fig. 5b to $2.2\mathrm{L}{\mathrm{O}}_2$ $(I_{\mathit{stab}}=0.8\text{nA}$, $V_{\mathit{stab}}=1.5\mathrm{V}$, $V_{\mathit{mod}}=20\text{mV})$. The $dI∕dV$ curves after dosage are taken on the uncovered Co areas (gray triangles) and on the ${\mathrm{O}}_2$ covered areas (black upside-down triangles), as indicated in (b). (b) Constant-current image of the Co-island surface after ${\mathrm{O}}_2$ dosage $(I=0.4\text{nA}$, $V=-0.3\mathrm{V})$. (c) Line profile along the black line in (b). Measurement temperature $T=6\mathrm{K}$.

Figure 7

Constant-current images (left panels) and corresponding $dI∕dV$ maps (right panels) of the same sample area of different Co films. Terrace width of the W(110) substrate and annealing temperature are indicated in the upper left corners. (a) and (b) $3\text{ML}$ Co film $(I_{\mathit{stab}}=0.3\text{nA}$, $V_{\mathit{stab}}=-0.3\mathrm{V}$, $V_{\mathit{mod}}=30\text{mV}$, $T=300\mathrm{K})$. Inset: histogram of $dI∕dV$ values from (b). (c) and (d) $2\text{ML}$ Co film. The island is $4\text{ML}$ thick at the right and $12\text{ML}$ thick at the left rim and shows a $dI∕dV$ contrast on its atomically flat surface $(I_{\mathit{stab}}=0.4\text{nA}$, $V_{\mathit{stab}}=-0.3\mathrm{V}$, $V_{\mathit{mod}}=30\text{mV}$, $T=300\mathrm{K})$. (e) and (f) $10\text{ML}$ Co film. The numbers mark the local Co thickness in ML. (g) and (h) $10\text{ML}$ Co film $(I_{\mathit{stab}}=0.7\text{nA}$, $V_{\mathit{stab}}=-0.33\mathrm{V}$, $V_{\mathit{mod}}=20\text{mV}$, $T=6\mathrm{K})$. The dashed white lines in (e) and (g) mark the step edges of the underlying W substrate.

Figure 8

(a) Spatially resolved $dI∕dV$ curves taken on the bright and the dark Co islands and on the dislocation lines from a $10\text{ML}$ Co film annealed at $350\mathrm{K}$ $(I_{\mathit{stab}}=0.7\text{nA}$, $V_{\mathit{stab}}=1.5\mathrm{V}$, $V_{\mathit{mod}}=10\text{mV})$. The origin of the curves is marked in (e). (b)–(e) Constant-current images (left panels) and corresponding $dI∕dV$ maps of the same sample area (right panels) of two regions of the Co film $(I_{\mathit{stab}}=0.7\text{nA}$, $V_{\mathit{stab}}=-0.33\mathrm{V}$, $V_{\mathit{mod}}=20\text{mV})$. (b) and (c) shows two adjacent monolayer high fcc- and hcp-stacked islands on the same Co terrace. White arrows in (d) and (e) mark a contrast change in the same atomic layer. Black arrows in (d) mark two shallow dislocation lines. (f) Line profiles drawn along the black and white lines in (d) and (e), respectively, at the same position. Dislocation lines are marked. Measurement temperature $T=6\mathrm{K}$.

Figure 9

The calculated vacuum DOS of the unfaulted structure (black) and of the faulted structure (gray) of a $12\text{ML}$ Co film at a distance of $3\mathrm{\AA }$ from the surface layer. Maj, min, and maj $+$ min indicate the majority DOS, the minority DOS, and the sum of both, respectively. All curves are shown on the same scale.

Figure 10

The minority-spin band structure of the $12\text{ML}$ Co film in the unfaulted (left panel) and in the faulted (right panel) structure are plotted along the high symmetry directions in the neighborhood of $\overline{\Gamma }$. Black dots mark states that are localized to more than $10\%$ in vacuum. The larger circles mark the band minimum region (see text). The state used for the contour plots in Fig. 11 is indicated with an arrow.

Figure 11

Contour plots of the spin density of the minority electrons for the state marked with an arrow in Fig. 10 for the $12\text{ML}$ Co film in the unfaulted (left panel) and in the faulted (right panel) structure. The numbers marking the contour lines give the corresponding charge density in $\text{electrons}∕{\left(\text{a.u.}\right)}^3$. Two successive lines differ by a factor of $4$. Crystallographic directions of the Co(0001) lattice are indicated. The black dots mark the positions of the closest atoms projected onto the plane. The numbers beside the contour plots give the contribution of the state to the LDOS in percent for this layer or vacuum, normalized to six layers. They are obtained by integrating over the muffin tins in each layer and over the vacuum, respectively.