Spin-Polarized Scanning Tunneling Spectroscopy of Nanoscale Cobalt Islands on Cu(111)

Spin-averaged and spin-polarized scanning tunneling spectroscopy at low temperature was performed on nanometer-scale triangular Co islands grown epitaxially on Cu(111) in the submonolayer coverage regime. Two structurally different island types can clearly be distinguished by their spin-averaged electronic structure. Spin-polarized measurements allow a separation of spectral contributions arising from different island stacking or from opposite magnetization states, respectively. In an applied magnetic field, both island types are found to be magnetized perpendicular to the surface, with large values of saturation field, remanence, and coercivity.

Figure 1

(a) Spin-averaged tunneling spectra, (b)–(c) $dI/dV$ maps at voltages as indicated. The curves were averaged in the colored boxes in (b), the island types are labeled $u$ (unfaulted) and $f$ (faulted). Spectra of the two island types differ in peak energetic positions and intensities. At the voltages of maps (b)–(c), the contrasts are maximal. Inset in (a): Topographic image.

Figure 2

(a) Spin-resolved tunneling spectra. Arrows $\uparrow \uparrow$ ($\downarrow \uparrow$) refer to parallel (antiparallel) magnetization alignment of sample and tip. (b) Asymmetries arising from different stacking [upper panel; grey (black) spin averaged (spin polarized)], and from opposite magnetization (lower panel). (c)–(f) $dI/dV$ maps at bias voltages as indicated. Maps (c)–(d) allow a direct comparison to the spin-averaged case in Figs. 1b, 1c. As exemplified by maps (e)–(f), the spin-dependent contribution is clearly dominant in most parts of the energy range. Stabilization parameters: $I=1\mathrm{n}\mathrm{A}$, $V=+0.6\mathrm{V}$.

Figure 3

Spin-polarized $dI/dV$ maps ($V=-0.18\mathrm{V}$) of Co islands in variable perpendicular magnetic fields. At zero field, islands of both stacking types are found bright or dark, corresponding to parallel or antiparallel alignment of sample and tip magnetization, respectively. A field of 1 T switches only a few of the smaller islands (circles). 1.5 T is not sufficient to achieve saturation. After field removal, only two small islands reverse their magnetization (circles), indicating a remanence $M_{\mathrm{r}}$ close to saturation $M_{\mathrm{s}}$.