Visualizing the Spin of Individual Cobalt-Phthalocyanine Molecules

Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead exchange interaction.

Figure 1

CoPc molecules adsorbed on cobalt nanoislands grown on Cu(111). (a) Structure model for CoPc. (b) Pseudo-three-dimensional STM image ($40\times 20{\mathrm{nm}}^2$, 0.1 V, 0.5 nA). The spatial oscillations on the Cu(111) surface are due to the scattering of the Shockley surface-state electrons.

Figure 2

Spin-polarized tunneling conductance through the nanoislands and CoPc. (a) Typical spin-polarized $dI/dV$ over two cobalt nanoislands of opposite magnetization (noted $\uparrow \uparrow$ and $\uparrow \downarrow$). Feedback loop opened at 0.6 V and 0.5 nA. (b) Differential conductance ($dI/dV$) over the center of single CoPc molecules adsorbed on cobalt nanoislands of opposite magnetization. Two sets of spectra acquired with distinct tips (noted 1 and 2) are presented. The spectra acquired with tip 1 are displaced upward by 6 nS for clarity. Feedback loop opened at 0.6 V and 0.5 nA. (c) Asymmetries arising from opposite magnetizations for CoPc and the Co nanoislands. The asymmetries are an average of all the recorded asymmetries obtained with different tips.

Figure 3

Spin-polarized tunneling conductance maps for CoPc. Maps (a), (c), and (e) were taken at $-0.29$, $-0.16$, and $-0.32\mathrm{V}$, respectively (image size: $40\times 20{\mathrm{nm}}^2$). Each map is normalized to span the same color palette range. Feedback loop opened at 0.6 V and 0.5 nA. (b) Spin-polarized map of Co nanoislands at $-0.29\mathrm{V}$—as in (a)—in the absence of CoPc molecules ($45\times 25{\mathrm{nm}}^2$). (d) Profiles of the differential conductance along one axis of CoPc extracted from (c). Molecules on the $\uparrow \uparrow$ island [right island in (c)] have the highest profile. (f) Map of a $\uparrow \downarrow$ molecule of (e) with model molecular structure superimposed ($2.6\times 2.6{\mathrm{nm}}^2$).

Figure 4

Electronic and magnetic structure of CoPc on the cobalt nanoislands. (a) Schematic adsorption geometry of CoPc. (b) PDOS over CoPc and the cobalt surface (top panel, majority PDOS; bottom panel, minority PDOS). (c) Isosurface plot of the magnetization density ($\uparrow$, magnetization in the “up” direction; $\downarrow$, “down” direction). The cobalt atom of CoPc and the cobalt atoms of the surface have parallel magnetizations. (d) Calculated differential conductance based on 19 (majority, $\uparrow$; minority, $\downarrow$).