Giant spin and orbital moment anisotropies of a Cu-phthalocyanine monolayer

The magnetism of a Cu-phthalocyanine (CuPc) monolayer on Ag(100) was investigated using x-ray magnetic circular dichroism (XMCD) and ligand-field multiplet calculations. Contrary to other metal-Pc adsorbed on metals, we show that the local CuPc moment survives the interaction with the electronic states of the substrate and presents enhanced susceptibility with respect to bulk powder samples. Our measurements reveal extraordinary orbital moment anisotropy (500%) and an anisotropic spin dipole moment up to twice the isotropic spin in a metal-organic layer. A complete description of the orbital, spin, and spin-orbit operators is provided based on the XMCD sum rules.

Figure 1

(Color online) (a) Ball and stick model of CuPc. (b) STM image of 1 ML CuPc on Ag(100), image size $170\times 170{\text{Å}}^2$, bias voltage 1.4 V, tunnelling current 100 pA. The square indicates the CuPc unit cell. (c) Experimental geometry. STM image acquired at bias voltage $-5\text{mV}$, tunnelling current 54 pA. (d) Diagram of the XMCD setup.

Figure 2

(Color online) Circularly polarized $L_{2,3}$ XAS and XMCD of (a) bulk CuPc powder and (b) 1 ML CuPc/Ag(100) at normal incidence. $\mathbf{B}=5\text{T}$ and $T=6\text{K}$. (c) Linearly polarized XAS of 1 ML CuPc/Ag(100) recorded with $\mathbf{E}$ in-plane $\left(I_{70°}^{\parallel }\right)$ and $70°$ out-of-plane $\left(I_{70°}^{\perp }\right)$. Inset: crystal-field diagram. The spectra in (b) and (c) are shown after Ag background subtraction. The inset in (b) reports the raw $I^{\pm }$ data. Solid and dashed lines are simulated spectra (see text).

Figure 3

(Color online) (a) Angular dependence of the $L_3$ XAS spectra at $\mathbf{B}=5\text{T}$ and $T=6\text{K}$. (b) Sum of $L_3$ and $L_2$ peak XAS values as a function of $\theta$. (c) Angular dependence of the $L_3$ XMCD spectra. (d) $L_3$ XMCD peak values as a function of $\theta$. The intensity scale is the same as (a) and (c). The dashed lines in (c) and (d) represent the function $f=\left({\text{cos}}^2\theta +1\right)/2$ normalized to the first data point of the series. The full symbols in (b) show the isotropic XAS intensity obtained as $I^{ISO}=\left[I_{+}\left(\theta \right)+I_{-}\left(\theta \right)\right]/f\left(\theta \right)$ (see text).

Figure 4

(Color online) (a) Angular dependence of experimental (symbols) and calculated (solid line) expectation values of the orbital magnetic moment $\left(L\right)$. The calculated values are shown after scaling by a factor $k=0.9$, see text. Crosses and dashed line refer to the right scale, reporting experimental and theoretical $L/S_{eff}$ ratio, respectively. (b) Angular dependence of the effective spin moment ($S_{eff}=2S+7T$, symbols) and calculated $S_{eff}$, $2S$, and $7T$ values at 6 K, $B=5\text{T}$.