Growth mechanism and interface magnetic properties of Co nanostructures on graphite

We investigated structural, electronic, and magnetic properties of Co adsorbed on highly oriented pyrolytic graphite (HOPG). Distribution and atomic sites of 3 $d$ transition-metal Co nanoislands and adatoms on HOPG were experimentally investigated by scanning tunneling microscopy with atomic resolution. In the very low thickness regime (≤0.6 Å), a strong nucleation mechanism and a preferred Co nanoisland diameter of ∼3.4 nm have been observed. Co adatoms were found to preferentially occupy β sites of the HOPG surface graphene layer and the atoms aggregated by further occupation of either α or overbond sites. This is in contrast to predictions based on density functional theory, which indicates that the hollow sites are the most energetically stable sites for Co adsorption. The presence of surface hydrocarbon contamination on graphite might be one possible cause of the observed active nucleation and stabilized nanoisland diameter of Co. The formation of Co carbide was evidenced by x-ray absorption spectroscopy. More importantly, the Co magnetic spin moment at the interface of Fe-capped ferromagnetic Co nanostructures and graphite, as determined by x-ray magnetic circular dichroism and sum-rule analysis, was found to be only 63% of the bulk value, implying a magnetically defective spin contact for carbon spintronics applications.

Figure 1

(a)–(c) STM images of Co on HOPG with submonolayer Co coverage. (d)–(f) The island diameter distributions and fitted log-normal distribution functions. (g) The island density profile and the percentage coverage of Co as a function of amount of Co deposited on HOPG at RT.

Figure 2

Atomic resolution STM image of Co adatoms adsorbed on HOPG, taken at $I$ = 1.04 nA and $V$ = 219 mV. The bright spots encircled and labeled as “Co adatoms” refer to individual Co adatoms that were adsorbed on the β site of the surface graphene layer, whereas those encircled and labeled as “Co tetramer” correspond to a Co tetramer formed by four Co adatoms. Three of these adatoms occupied the β sites and the fourth adatom attached to either the α or the overbond site. The label H indicates the hollow site.

Figure 3

C $K$ -edge XAS spectra as a function of Co deposition time. The inset shows the lower energy scale, featuring a new shoulder, as indicated by the black arrow, of which intensity increases with increasing Co deposition time.

Figure 4

Peak intensity versus Co deposition time profile for C K-edge π${}^{*}$ transition peaks and Co $L_3$-edge XAS measurements of Co/HOPG. The linearity shown by the fit of the Co XAS measurements implies the VW growth mode of Co on graphite.

Figure 5

Normalized Co $L_{2,3}$-edge XMCD intensity as a function of Co deposition times and of applied magnetic field during the measurements.

Figure 6

Normalized Co and Fe $L_3$-edge XMCD hysteresis loops of Fe-capped Co/HOPG. The inset shows the SPM Co XMCD loop of bare Co/HOPG at RT.

Figure 7

Sum-rule analysis of Co $L_{2,3}$-edge XAS and XMCD spectra of Fe-capped Co/HOPG with a deposition time of 36.5 min measured at RT. The measurements were taken in the presence of a 250-Oe magnetic field. The symbols shown in the figure are explained in the text.