Single Atom Magnets on Thermally Stable Adsorption Sites: Dy on NaCl(100)

Abstract

We report magnetic bistability in single Dy atoms on NaCl(100) thin films. Individual Dy atoms substituting Na at the surface of the NaCl layer are thermally stable up to at least 300 K, display $4f^{9}$ occupancy, out-of-plane easy magnetization axis, and long spin relaxation time $T_1$ of about 10 s at 2.5 K; thereby they are the first single atom magnet on a thermally stable adsorption site. Dy atoms adsorbed onto the Cl and bridge sites display $4f^{10}$ occupancy. Dy on top-Cl exhibit magnetic hysteresis and a $T_1$ of 550 s at 0.3 T and 2.5 K. The observed slow magnetic relaxation of Dy on both adsorption sites introduces NaCl as an effective platform for single atom magnets.

Figures (4)

• Figure 1: a) STM images of Na-substitutional Dy atoms on 2 ML NaCl/Ag(111) (Dy deposition temperature $T_{\rm dep}= 300$ K, Dy coverage $\theta_{\rm Dy}= 0.5$ % ML), the quasi-horizontal stripes are a moiré pattern of the NaCl ($V_{\rm t}=+300$ mV, $I_{\rm t}=50$ pA, $T_{\rm STM}= 6$ K). b) Zoom into the region in the square; Cl atoms are imaged as protrusions, Dy atoms are in registry with Na sites ($V_{\rm t}=-50$ mV, $I_{\rm t}=50$ pA). c) Experimental and simulated XAS and XMCD spectra for Na-substitutional Dy atoms on 3.5 ML NaCl/Ag(111) ($T_{\rm dep}= 300$ K, $\theta_{\rm Dy}=1.5$ % ML), at the Dy $M_{4,5}$ edges ($B = 5.5$ T, $T_{\rm XAS} = 2.5$ K), vertically shifted for clarity. d,e) Magnetization curves acquired by recording the XMCD signal at 1289.3 eV with field sweep rate $|\dot B | = 2$ T /min: d) normal (NI) and grazing (GI) incidence curves, compared with simulated ones, at flux $\phi = 1.1 \cdot 10^{-2}$ ph nm$^{-2}$ s$^{-1}$ (the amplitude of the simulated curves at large fields is normalized to the amplitude of the experimental ones) and e) NI curve showing opening at a lower photon flux of $\phi = 7.5 \cdot 10^{-3}$ ph nm$^{-2}$ s$^{-1}$. Measured and simulated magnetization curves are at $T= 2.5$ K.
• Figure 2: a) Experimental and simulated XAS and XMCD spectra at the Dy $M_{4,5}$ edges of Dy adatoms on 8 ML NaCl/Cu(111) ($T_{\rm dep}= 4$ K, $\theta_{\rm {Dy}}=1.9$ % ML, $B=6.8$ T, $T_{\rm XAS}=2.5$ K). b) NI and c) GI magnetization curves acquired by recording the XMCD signal at 1287.2 eV ($|\dot B | = 2$ T /min, $\phi =1.5\cdot10^{-2}$ ph nm$^{-2}$ s$^{-1}$), and simulated equilibrium curves at $T=2.5$ K and $T_{\rm eff}=9.0$ K. d) Top-view and side-view of Dy adatoms at top-Cl and two symmetry-equivalent bridge adsorption sites; pink arrows indicate the easy magnetization directions for the different species.
• Figure 3: Magnetic level schemes at $B=0.3$ T and $B=0$ T provided by the multiplet calculations for a) Na-substitutional Dy atoms, b) top-Cl and c) bridge Dy adatoms. Insets in b) and c) show the polar dependence of the total angular momentum component in a magnetic field of 6.8 T along the respective easy axes.
• Figure 4: Difference between the self-consistent DFT charge density and the superposition of individual atomic pseudo-charges bib:kresse99 placed at the equilibrium atomic coordinates. a) Top view of the NaCl(3ML)/Ag(100) substrate, Gd (magenta) occupies a Na-substitutional site. b) Top view of the NaCl(6ML) substrate, Eu (magenta) occupies a top-Cl or a bridge site. c-e) Transversal cuts of the charge density difference $\Delta \rho$ with respect to the superposition of atomic densities. The $z=0$ origin corresponds to the average height of the third NaCl layer. The horizontal distances are taken along the directions indicated by the dashed magenta arrows, labeled 1-4 in panels a) and b).