Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2 ScN@C80 on Metals and Insulators

Abstract

Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy₂ ScN@C₈₀ deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy₂ ScN@C₈₀ adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy₂ ScN@C₈₀ monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy₂ ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics.

Keywords: XMCD; endohedral metallofullerenes; monolayers; scanning probe microscopy; single-molecule magnets.

Figure 1

a) Molecular structure of Dy₂ScN@C₈₀ (Dy green, N blue, Sc magenta; magnetic moments of Dy ions are visualized as red arrows). b) Magnetic hysteresis measured for the powder sample of Dy₂ScN@C₈₀ by SQUID magnetometry at T=2 K with magnetic field sweep rates of 0.17 and 1 T min⁻¹.

Figure 2

a) Constant current topography image of an hcp monolayer island of Dy₂ScN@C₈₀ on Au(111) surface anchored to the Au(111) step edge (V _Bias=2 V; I _Set=200 pA); the arrow indicates the fullerenes closed packed direction coinciding with the Au‐[110] direction; the features of the reconstruction are altered at the interface. b) The field of view of the constant current topography image of the Dy₂ScN@C₈₀ monolayer on Au(111) (V _Bias=1.5 V; I _Set=600 pA) in which the electronic structure was investigated; the left upper inset shows the hcp arrangement of the single fullerenes and in the lower right inset the FFT of the image is presented. c) Four types of STS spectra measured for Dy₂ScN@C₈₀ monolayer on Au(111); asterisks mark features tentatively assigned to the fullerene LUMO. d) The average STS spectrum over the area shown in (b), revealing an effective gap of Δ_eff=1.7±0.1 eV (gray curve) and the spectra averaged over two areas in (e) (red and blue curves). e) The dln(I)/dln(V)‐map measured at the HOMO level energy of E=−1.035 eV showing the spatial variations of the electronic structure attributed to the altered herringbone reconstruction at the interface to the fullerene monolayer. Average spectra of the regions in between the stripes (dark blue in (e)) and on the stripes (yellow/red in (e)) are presented in (d).

Figure 3

a) XAS and XMCD spectra of Dy₂ScN@C₈₀ sub‐monolayers on Au(111), Ag(100), and MgO|Ag(100) measured at 30° and 90° orientation of the X‐ray and magnetic field versus the surface; T≈2 K, H=6.5 T, only the Dy‐M ₅ edge is shown (see the Supporting Information for the whole Dy‐M _5,4 range). X‐ray polarizations are denoted at I ⁺ and I ⁻, non‐polarized XAS is a sum of I ⁺ and I ⁻, and XMCD is their difference normalized to the XAS maximum. b) Magnetic hysteresis of Dy₂ScN@C₈₀ on Au(111), Ag(100), and MgO|Ag(100) measured by XMCD technique at T≈2 K, sweep rate 2 T min⁻¹; dots are experimental values, and lines are added to guide the eye. For Au(111) and Ag(100), hysteresis measurements are shown for two angles of the X‐ray and magnetic field versus the surface.

Figure 4

Angular dependence of XMCD asymmetry for the Dy₂ScN@C₈₀ submonolayer on Au(111), Ag(100), and MgO|Ag(100) measured at 1290 eV at T≈2 K. θ is the angle between X‐ray beam/magnetic field and the surface. Dots are experimental values, lines are fits with the function C ₁ cos²(θ)+C ₂.

Figure 5

a) Definition of the cluster tilting angle θ as the angle between the axis z normal to the surface and the vector n radiating from the nitrogen atom perpendicular to the cluster plane; θ=0° and θ=90° correspond to the parallel and perpendicular alignment of the cluster versus the substrate, respectively. b) Relative energies of Dy₂ScN@C₈₀ conformers on Au(111), Ag(100), and MgO surfaces plotted versus θ; gray dashed lines mark the relative energies of the conformers of the free Dy₂ScN@C₈₀ molecule; color of the dots codes the net charge of Dy₂ScN@C₈₀ molecules (Q _mol) on a surface. c) Isosurfaces of the difference electron density for Dy₂ScN@C₈₀ molecule on different substrates (red color marks regions with the increased electron density, whereas cyan corresponds to the depletion of the density; all three systems are plotted at the same isovalue). d) DFT‐computed density of states (DOS) near the Fermi level projected onto Dy₂ScN@C₈₀ molecule and Dy₂ScN cluster states (dark red and green, respectively; the axis is denoted as DOS_mol) and the substrate‐projected DOS (DOS_substr, semi‐transparent gray). Note that the DOS_mol scale is the same, whereas the DOS_substr scale varies in each part of the figure.