Cobalt nanoclusters on metal-supported Xe monolayers: Influence of the substrate on cluster formation kinetics and magnetism

The growth dynamics of submonolayer coverages of cobalt during buffer-layer assisted growth on Ag(111) and Pt(111) substrates is investigated by variable temperature scanning tunneling microscopy in the temperature range between 80 and 150 K. It is found that attractive cluster-substrate interactions can govern the cluster formation on the Xe buffer layer if the Xe layer is sufficiently thin. The interpretation of the microscopy results is supported by x-ray magnetic circular dichroism data which monitor the effect of cluster-substrate interactions on the formation of magnetic moments and magnetic anisotropy of Co nanocluster during the different stages of growth. Ab initio calculations show that the cluster magnetism is controlled by the interface anisotropy, leading to perpendicular magnetization for Co on Pt(111). Limits of and new potential for nanocluster fabrication by buffer-layer-assisted growth are discussed.

Figure 1

(Color online) STM topographies for thick xenon buffer layers: Co/thick Xe layer/Ag(111) deposited at 30 K and then annealed to progressively higher temperatures $T_a$ as indicated in the caption; (d) zoom-in of image (e) together with a line scan over a Co cluster and a cartoon showing the Co immersed in Xe.

Figure 2

(Color online) Magnetic characterization of 0.05 ML Co/thick Xe layer/Ag(111) [(a) and (b)] before and [(c) and (d)] after annealing at 100 K; [(a) and (b)] hysteresis curves at 25 and 8 K taken right after Co deposition on the Xe buffer layer; [(c) and (d)] hysteresis curves at 35 and 8 K taken after annealing the sample to 100 K with consequent desorption of the bulk Xe, cluster growth and contact with the substrate. The inset in (c) shows quasiremanence measurements at $B=5\text{mT}$; the full lines in the hysteresis plot correspond to a Langevin fit according to Eq. 5 in Ref. 14. The magnetic moment used in the fits are (a) the Co bulk value and (b) the calculated values displayed in Table . In the latter case, $n_{\text{h}}=2.49$ (Co bulk value) and ${\mu }_T$ is derived from the ab initio calculations (Table ). The slight asymmetry in the hysteresis curve in (b) is not a real effect but within the larger error margins for TEY experiments on the thick (semi-insulating) Xe layer.

Figure 3

(Color online) STM topographies for Co/thin Xe layer/Ag(111) (left column) and Co/thin Xe layer/Pt(111) (right column) taken after annealing at $T_a$ indicated in the figure caption. The scanning temperature was 20 K for (a)–(d) and 300 K for (e) and (f). The inset in (e) and (f) shows the line scan across a Co island supported on the two different substrates.

Figure 4

(Color online) Magnetic characterization of the systems in Fig. 3: 0.05 ML Co/thin Xe layer/Ag(111) (left column) and 0.05 ML Co/thin Xe layer/Pt(111) (right column): [(a) and (b)] before and [(c)–(f)] after annealing at 100 K, measured at [(c) and (d)] 25 K and [(e) and (f)] 8 K. The full lines in the hysteresis plot correspond to a Langevin fit according to Eq. 5 in Ref. 14. The magnetic moment used in the fits are the calculated values displayed in Table with $n_{\text{h}}=2.49$ (Co bulk value) and ${\mu }_T$ derived from the ab initio calculations (Table ).

Figure 5

(Color online) Detailed scheme of the cluster formation dynamics for the three different systems: Co/Xe buffer layer/Ag(111)(left), Co/Xe single layer/Ag(111)(center), and Co/Xe single layer/Pt(111)(right). The arrow on the left indicates the raising temperature $T$ from top to bottom.

Figure 6

(Color online) XAS signal for the three samples described in the text, showing the sample composition and dichroism right after deposition of Xe and Co at 25 K: $\text{Xe}{M}_{3,2}$ edges for (a) 12 ML Xe/Ag(111), (c) 3–4 ML Xe/Ag(111), and (e) 3–4 ML Xe/Pt(111); the substrate backgrounds (dotted line) are also plotted; $\text{Co}{L}_{3,2}$ edges for (b) 0.05 ML Co/12 ML Xe/Ag(111), (d) 0.08 ML Co/3–4 ML Xe/Ag(111), and (f) 0.06 ML Co/3–4 ML Xe/Pt(111); dashed lines indicate the Xe/substrate system backgrounds. The inset in (c) shows the XAS taken after annealing at 100 K for Xe/Ag(111) (pink colored line): a small amount of Xe (submonolayer coverage) is still present at this temperature on the Ag(111) surface.