Structure and Spin-Polarized States of Cobalt Nanoislands

Magnetic nanoislands at surfaces are considered key ingredients for various applications ranging from magneto-electronics, data storage, optoelectronics, to catalysis. Their unique magnetic and electronic properties rely on the quantization of the electron energy spectrum, which in turn accounts for the nanoisland’s size, shape, structure, and hybridization with the local environment. For instance, a fascinating fundamental issue is to manipulate the island magnetization by tuning the electron occupancy of the exchange-split d-bands. In this context, we investigated the underlying factors that allow controlling the d-band energy position in a model system of Co nanoislands on metal surfaces [1-3]. We established for the first time a direct correlation between nanoisland/surface lattice mismatch and upward shifts of minority d-bands [2]. The effect was also found to depend on the size of the nanoislands as well as on the position over the nanoislands [1]. The driving force is the distribution of the atomic-bond lengths in the nanoisland. In this way, the notion of mesoscopic atomic-relaxations in nanostructures was demonstrated experimentally for the first time. In our effort to tailor the spin-polarization, we also studied the impact of adsorbents on nanoislands [3]. For example, single-metal atoms were found to reverse the surface spin-polarization near the Fermi level [3].

The measurements were performed at 4.6 K by Scanning Tunneling Spectroscopy (STS) in an ultrahigh vacuum STM. The main experimental findings are summarized in the figure. For Co/Au(111), the spectral peaks acquired on both, unfaulted and faulted nanoislands, cross the Fermi energy, being thus only partly “occupied”. For Co/Cu(111) the peaks are 0.2 eV lower in energy and consequently they are fully located in the occupied states. An additional downward shift is measured when spectra is acquired on nanoislands of decreasing size (see lower panel). A quantitative evaluation of the size-dependent shift reveals a monotonic displacement over 0.09 V for unfaulted islands, whereas the faulted ones (not shown here) show a steeper increase of the shift with an asymptotic behavior appearing above islands larger than 13 nm. To understand the origin of these spectral features, spectral density maps (imaginary part of momentum-resolved energy-dependent Green’s function) were calculated in collaboration with the Max-Planck Institute. We unveiled that all peaks are located in the first half of the Brillouin zone and originate from the minority d3z2-r2 band split at the intersection with the Cu(111) bulk bands. We rationalized the shifts in the framework of a tight-binding model, as first established by J. Friedel. For a Co nanostructure the shift of a band as a function of Co-Co atomic-bond length is given by, where is positive. For occupied states the exchange integral is negative. The bands exhibit thus a negative shift, which varies linearly with the Co-Co bond length [1]. A downward shift was also observed when performing spectroscopy close to the nanoisland’s edges and corners, indicating inwards atomic-scale relaxations [1].

In collaboration with P.A. Ignatiev, V.S. Stepanyuk, P. Bruno, Max-Planck Institut, Halle, Germany.

M.V. Rastei, B. Heinrich, L. Limot, P. A. Ignatiev, V. S. Stepanyuk, P. Bruno, J. P. Bucher, Phys. Rev. Lett. 99, 246102 (2007).

M. V. Rastei, J. P. Bucher, P. A. Ignatiev, V. S. Stepanyuk, P. Bruno, Phys. Rev. B 75, 045436 2007.

B. W. Heinrich, C. Iacovita, M. V. Rastei, L. Limot, J. P. Bucher, P. A. Ignatiev, V. S. Stepanyuk, P. Bruno, Phys. Rev. B 79, 113401 2009.