Controlling the Spin of Co Atoms on Pt(111) by Hydrogen Adsorption

We investigate the effect of H adsorption on the magnetic properties of individual Co atoms on Pt(111) with scanning tunneling microscopy. For pristine Co atoms, we detect no inelastic features in the tunnel spectra. Conversely, CoH and ${\mathrm{CoH}}_2$ show a number of low-energy vibrational features in their differential conductance identified by isotope substitution. Only the fcc-adsorbed species present conductance steps of magnetic origin, with a field splitting identifying their effective spin as $S_{\text{eff}}=2$ for CoH and $3/2$ for ${\mathrm{CoH}}_2$. The exposure to ${\mathrm{H}}_2$ and desorption through tunnel electrons allow the reversible control of the spin in half-integer steps. Because of the presence of the surface, the hydrogen-induced spin increase is opposite to the spin sequence of ${\mathrm{CoH}}_n$ molecules in the gas phase.

Figure 1

(a) STM image of Co adatoms on Pt(111) ($V_{\mathrm{t}}=-50\mathrm{mV}$, $I_{\mathrm{t}}=100\mathrm{pA}$, $T=4.4\mathrm{K}$, $\mathrm{\Theta }=3.5\times {10}^{-3}\mathrm{ML}$, shallow depressions are subsurface C atoms, $\mathrm{\Theta }=2.5\times {10}^{-2}\mathrm{ML}$, pronounced depressions are surface C atoms, $\mathrm{\Theta }=2.3\times {10}^{-3}\mathrm{ML}$). (b) Apparent height profile along the line marked in (a). (c) Differential conductance ($dI/dV$) spectra of fcc- and hcp-adsorbed Co adatoms ($V_t=-50\mathrm{mV}$, $I_t=1\mathrm{nA}$, $V_{\text{mod}}=2.8\mathrm{mV}$, $T=4.4\mathrm{K}$). (d) $dI/dV$ spectra of CoH complexes on both adsorption sites ($V_t=-50\mathrm{mV}$, $I_t=1\mathrm{nA}$, $V_{\text{mod}}=0.3\mathrm{mV}$, $T=0.4\mathrm{K}$). (e) $dI/dV$ spectra of ${\mathrm{CoH}}_2$ complexes ($V_t=-45\mathrm{mV}$, $I_t=0.75\mathrm{nA}$, $V_{\text{mod}}=1\mathrm{mV}$, $T=0.4\mathrm{K}$). Dashed lines mark the inelastic conductance steps.

Figure 2

(a) Magnetic field dependence of $dI/dV$ spectra of fcc-adsorbed CoH. A parabolic background was subtracted from each spectrum ($V_t=-5\mathrm{mV}$, $I_t=0.25\mathrm{nA}$, $V_{\text{mod}}=500\mu \mathrm{V}$, $T=0.4\mathrm{K}$). Dashed orange lines show simulated spin-excitation spectra. Inset: schematics of the corresponding quantum levels labeled by $m$. The effects of uniaxial and transverse anisotropies are represented by red and blue dashed lines, respectively. (b) Experimental (dots) and calculated (solid line) transition amplitudes $A$ and threshold energies $E_{\text{th}}$ between the lowest two states of a $S_{\text{eff}}=2$ multiplet. The best fit obtained with $S_{\text{eff}}=1$ is shown as dashed line for comparison ($g=2.1$, $D=-9\mathrm{meV}$, $E=0.175\mathrm{meV}$).

Figure 3

(a) Isotope shifts in conductance steps of fcc-adsorbed ${\mathrm{CoH}}_2$, CoHD, and ${\mathrm{CoD}}_2$. The energies at half the step height are marked with vertical lines. A parabolic background was subtracted from each spectrum ($V_t=-50\mathrm{mV}$, $I_t=0.75\mathrm{nA}$, $V_{\text{mod}}=1\mathrm{mV}$, $T=4.4\mathrm{K}$). (b) Temperature dependence of the zero-bias peak and of its width (inset) of ${\mathrm{CoH}}_2$. Orange lines: simulated $dI/dV$ spectra, see text. (c) Field splitting of zero-bias peak of ${\mathrm{CoH}}_2$ ($V_t=-15\mathrm{mV}$, $I_t=0.1\mathrm{nA}$, $V_{\text{mod}}=500\mu \mathrm{V}$, $T=0.4\mathrm{K}$). Orange lines: simulated $dI/dV$ spectra, see Supplemental Material [25]. Inset: quantum levels of an $S_{\text{eff}}=3/2$ system with $D>0$. (d) Field splitting of Kondo peak and spin-excitation step. Symbols are experiment and correspond to different ${\mathrm{CoH}}_2$ complexes. Lines show the field splitting expected from Eq. (1) for the indicated spins and anisotropies, $g=2$, $|D|=3\mathrm{meV}$.