Magnetic Moment and Anisotropy of Individual Co Atoms on Graphene

We report on the magnetic properties of single Co atoms on graphene on Pt(111). By means of scanning tunneling microscopy spin-excitation spectroscopy, we infer a magnetic anisotropy of $K=-8.1\mathrm{meV}$ with out-of-plane hard axis and a magnetic moment of $2.2{\mu }_B$. Co adsorbs on the sixfold graphene hollow site. Upon hydrogen adsorption, three differently hydrogenated species are identified. Their magnetic properties are very different from those of clean Co. Ab initio calculations support our results and reveal that the large magnetic anisotropy stems from strong ligand field effects due to the interaction between Co and graphene orbitals.

Figure 1

(a) STM image of $\mathrm{graphene}/\mathrm{Pt}(111)$ showing atomic contrast together with moiré pattern ($V_t=-1\mathrm{mV}$, $I_t=8\mathrm{nA}$, $T=4.5\mathrm{K}$). (b) STM image of three Co adatoms ($V_t=-20\mathrm{mV}$, $I_t=1\mathrm{nA}$, $T=1.7\mathrm{K}$). (c) Hollow adsorption site inferred from adsorbate apex with respect to atomically resolved C lattice sketched in black ($V_t=-50\mathrm{mV}$, $I_t=100\mathrm{pA}$, $T=4.5\mathrm{K}$). Inset: Simulated STM image. (d) Line profile of Co atom on $\mathrm{graphene}/\mathrm{Pt}(111)$ (blue dots, $\mathrm{FWHM}=9.0\AA$) and on Pt(111) (black dots, $\mathrm{FWHM}=8.3\AA$) taken with the same tip ($V_t=-100\mathrm{mV}$, $I_t=100\mathrm{pA}$, $T=4.5\mathrm{K}$).

Figure 2

(a),(b) Distinct apparent heights for clean Co ($D$), CoH ($C$), ${\mathrm{CoH}}_2$ ($B$), and ${\mathrm{CoH}}_3$ ($A$) ($V_t=-25\mathrm{mV}$, $I_t=50\mathrm{pA}$, $T=4.5\mathrm{K}$). (c) Characteristic inelastic conductance steps of the four species (feedback gated at $V_t=-25\mathrm{mV}$ and $I_t=250\mathrm{pA}$, $V_{\mathrm{mod}}=1\mathrm{mV}$, $f=611\mathrm{Hz}$, $T=4.5\mathrm{K}$). $A$, $B$, and $D$ spectra are offset for clarity. (d) Three ${\mathrm{CoH}}_3$ complexes imaged as triangles. (e),(f) Successive dehydrogenation of the upper one by voltage pulses ($V_t=-100\mathrm{mV}$, $I_t=50\mathrm{pA}$, $T=0.4\mathrm{K}$). Insets: Simulated STM images of the corresponding ${\mathrm{CoH}}_n$ complexes.

Figure 3

(a) Field-dependent $dI/dV$ curves of a clean Co atom proving the magnetic origin of conductance steps at $\pm 8.1\mathrm{mV}$ (dots: raw data; lines: smoothened; $V_t=-15\mathrm{mV}$, $I_t=250\mathrm{pA}$, $V_{\mathrm{mod}}=200\mu \mathrm{V}$, $f=287\mathrm{Hz}$, $T=0.4\mathrm{K}$). Spectra at 4 and 8 T are offset for clarity. (b) $d^{2}I/d{V}^2$ of left-hand conductance step in (a) showing its Zeeman splitting. (c) Field-dependent spin-excitation energies averaged over 26 adatoms (error bars show standard deviation). (d) An external out-of-plane field lifts the excited state degeneracy leading to the observed Zeeman splitting of the spin-excitation energies. (e) DFT-calculated spin-dependent energies and filling of Co $3d$ and $4s$ levels in the ligand field of graphene.

Figure 4

(a) Field-induced splitting of the conductance steps of a ${\mathrm{CoH}}_3$ complex ($V_t=-20\mathrm{mV}$, $I_t=100\mathrm{pA}$, $V_{\mathrm{mod}}=200\mu \mathrm{V}$, $f=611\mathrm{Hz}$, $T=0.4\mathrm{K}$). Spectra at 4 and 8 T are offset for clarity. (b) Numerical derivative of the negative bias part of (a) showing each step as a minimum. Zero-field width $\Delta E=700\pm 100\mu \mathrm{eV}$. (c) Zeeman splitting of spin-excitation energies averaged over 15 ${\mathrm{CoH}}_3$ (error bars show standard deviation).