Substrate‐Independent Magnetic Bistability in Monolayers of the Single‐Molecule Magnet Dy2ScN@C80 on Metals and Insulators

Abstract Magnetic hysteresis is demonstrated for monolayers of the single‐molecule magnet (SMM) Dy2ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2ScN@C80 adsorbed on Au(111) were studied by STM. X‐ray magnetic CD studies show that the Dy2ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2ScN 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.


Introduction
Single-molecule magnets (SMMs) are molecular materials exhibiting magnetic bistability and slow relaxation of magnetization. [1] Since these properties should be preserved even for single molecules,t he prospects of SMMs in information storage and spintronics [2] have been pushing the field toward developing new molecules with better SMM performance and higher operating temperatures. [3] So far,t he vast majority of the studies of SMMs have been performed for their powders and crystals.A tt he same time,r ealization of the true advantages of SMMs over bulk magnetic materials require the scaling down to 2D,1D, and eventually asingle-molecule level. Investigation of the magnetic properties of monolayers is thus the next logical step after the bulk SMM behavior is established. [4] However, formation of monolayers requires certain chemical and/or thermal stability,which many SMMs do not have,w hereas surface techniques for the study of the sample morphology or magnetism are rather complicated. As ar esult, the SMMs,w hose magnetic properties have been studied on surfaces,are very rare in comparison to hundreds of known SMM compounds.T he scaling down to monolayer level also raises aq uestion of the substrate influence on the SMM properties of adsorbed molecules.I np articular,i nteraction with conducting electrons is believed to be deteriorating for the SMM behavior. So far, magnetic hysteresis on conducting substrates was demonstrated only for three types of SMMs:LnPc 2 (Ln = Tb,Dy), [5] Fe 4 , [6] and metallofullerenes Dy 1(2) Sc 2(1) N@C 80 . [7] Endohedral metallofullerenes [8] form as pecial class of SMMs in which lanthanides are encapsulated within the carbon cage. [9] This allows stabilization of unusual lanthanide clusters with strong single-ion magnetic anisotropy [10] or giant exchange coupling. [11] Furthermore,h igh thermal stability of fullerenes enables formation of monolayers by sublimation. [12] Thef irst study of the surface magnetism of the fullerene-SMM monolayer was performed by X-ray magnetic circular dichroism (XMCD) technique for Dy 2 ScN@C 80 on Rh(111) and indeed revealed am agnetic bistability. [7a] However, magnetic hysteresis of the monolayer was considerably narrower than for am ultilayer sample.Abroader hysteresis than on Rh(111) was observed for Dy 2 ScN@C 80 on h-BN j Rh(111). [7b] Here we demonstrate that the SMM properties of Dy 2 ScN@C 80 monolayers on Au(111) and Ag(100), as well on at hin film of insulator MgO grown on Ag(100), do not depend on the substrate,w hich highlights the robustness of fullerene SMMs and the protective function of the carbon cage.

Results and Discussion
Themolecule of Dy 2 ScN@C 80 is built from an icosahedral carbon cage C 80 -I h ,which encapsulates aplanar metal nitride cluster Dy 2 ScN (Figure 1a). Then itride ion N 3À is located in the center of at riangle formed by metal ions in their formal oxidation state of 3 + .D y ÀNb ond lengths in Dy 2 ScN@C 80 are shorter than 2.1 , [10b] which results in as trong uniaxial ligand field imposed by the nitride onto Dy 3+ ions. [10b,13] The magnetic ground state of each Dy 3+ ion is aKramers doublet with m J =AE 15/2 and aq uantization axis aligned along the corresponding Dy À Nb ond. Very large ligand-field splitting ensures that single-ion magnetic moments are locked to the DyÀNb onds up to high temperature.T he intramolecular interactions between magnetic moments of Dy ions are predominantly ferromagnetic (Figure 1a), and the state with anti-parallel alignment of magnetic moment is higher in energy by 0.8 meV (10 K). [10b, 14] Dy 2 ScN@C 80 is as inglemolecule magnet with ab locking temperature of magnetization near 8Kand a1 00 sb locking temperature of 5K. [10b] At 2K,t he powder sample of Dy 2 ScN@C 80 shows magnetic hysteresis with ac oercive field of 0.7-0.8 T (Figure 1b).
To ensure the suitability of the evaporated thin films of Dy 2 ScN@C 80 for the studies of magnetic properties by XMCD technique,d eposition of the fullerene onto Au(111) substrate and characterization of the monolayer properties by scanning tunneling microscopy (STM) were first performed. Them olecules were deposited under ultra-high-vacuum conditions (p 10 À9 mbar) via organic molecular beam epitaxy using ah ome build evaporator.T ypical evaporation conditions were 20 minutes at 450-460 8 8C. Thes ubstrate was kept at room temperature during deposition, and STM studies were also performed at room temperature using Omicron VT-STM/AFM microscope. STM measurements revealed that at these evaporation conditions the surface of the substrate is covered with ca. 0.5 ML of fullerene molecules (Supporting Information, Figure S1). Thee ndohedral fullerenes diffuse across the terraces of Au(111) and self-assemble into closed monolayer islands of several 10 10 nm 2 anchored to the edges of the substrate step ( Figure 2a). As already observed for other M 3 N@C 80 fullerenes on Au(111), [12a-c] Dy 2 ScN@C 80 molecules are organized in ah exagonal close-packed (hcp) structure with al attice parameter of 1.15 AE 0.05 nm (Figure 2b). Fourier transformation of the topographic image gives Bragg peak positions proving at hree-fold symmetric periodic structure (Figure 2b). On the bare part of the Au(111) substrate,t he herringbone reconstruction typical for ac lean Au(111) surface is observed (Figure 1a). Ther econstruction double stripes also appear on and across the monolayer islands ( Figure 2a,b), but the inter-stripe distance and their course varies from that of unperturbed Au(111): Thed ensity of reconstruction stripes under the fullerene layer is reduced, whereas the Au(111) fcc-like terminated areas in between the double stripes become wider in comparison to the pristine herringbone pattern. Thus,t he formation of quasi-epitaxial 4 4s uperstructure of fullerenes on Au(111) fcc sites is enhanced and consequently the interface energy is reduced. [12a,c, 15] Thefacts that the fullerenes show asufficiently high mobility at RT to form monolayer islands and that the herringbone reconstruction is not fully lifted at the interface imply ac omparably weak interaction of Dy 2 ScN@C 80 molecules with the Au(111) surface.
Thee lectronic structure of Dy 2 ScN@C 80 on Au(111) was studied by scanning tunneling spectroscopy (STS). The spectra acquired in the field of view presented in Figure 2b showed four somewhat different patterns ( Figure 2c)o ccurring with almost equal abundance.P resumably,t hey are caused by different tip position over fullerenes and by different adsorption geometries of the Dy 2 ScN@C 80 molecules on the Au(111) surface. [16] Then oticeable difference between the splitting patterns demonstrates that the fullerene-fullerene and fullerene-substrate interactions are not negligible. [17] Each spectrum type exhibits awell-defined gap and distinct peaks corresponding to the occupied and unoccupied states of the fullerene molecule.DFT calculations show (see discussion below and in Supporting Information), that the density of fullerene states is rather high, and each peak in STS spectra corresponds to several fullerene orbitals. TheH OMO-derived state overlaps with other lower-energy occupied states and form ap eak at À(0.9-1) V. Fort he fullerene LUMO,D FT calculations predict as tand-alone peak at the energy near + 0.6 Vabove Fermi level, whereas other unoccupied orbitals are densely packed in the energy range above + 1V .Calculations also show that depending on the orientation of the endohedral cluster inside the fullerene versus the substrate,the orbital energies may vary within the range of 0.2-0.3 eV.Based on these predictions,wetentatively suggest that the LUMO-derived states may corresponds to the features marked by asterisks in Figure 2c.Inthe average spectrum of the whole studied area (Figure 2d), these fine features are smeared out, leading to an effective energy gap of   Figure 2e.T his effect can be also seen as the difference of the HOMO-derived peak heights in the STS spectra averaged over corresponding areas ( Figure 2d). Obviously,t he adsorption sites of Dy 2 ScN@C 80 molecules on Au(111) vary following the altered reconstruction from quasi-epitaxial regions (in between the double stripes) to incommensurate double stripe areas,which affects the local electronic structure of the fullerene j metal interface. [18] Note that the four types of STS spectra shown in Figure 2c are found for both dark blue and bright red regions in Figure 2e (Supporting Information, Figure S2).
Themagnetic properties of Dy 2 ScN@C 80 monolayers were studied by Dy-M 4,5 XMCD at the X-Treme beam line at the Swiss Light Source,P aul Scherrer Institut. [19] Them agnetic field was kept parallel to the X-ray beam in all measurements. Thethin film of MgO (10-11 ML) on Ag(100) was grown by sublimation of Mg in O 2 atmosphere (10 À6 mbar) while keeping the substrate at 645 K. Dy 2 ScN@C 80 evaporation conditions onto the Au(111) substrate were adopted from the ex situ studies described above.I nsitu characterization by STM confirmed formation of similar monolayer islands (Supporting Information, Figure S3). Thes ame evaporation conditions were then used for the growth on Ag(100) and MgO j Ag(100), for which in situ STM characterization was not possible during the beamtime.T he coverage of the Ag(100) and MgO j Ag(100) substrates by Dy 2 ScN@C 80 was lower than for Au(111) (as estimated from XAS intensity), which ensures that all XMCD measurements were performed in the submonolayer regime.A tt he base temperature of the cryostat the temperature at the sample was near 2Kfor the Au(111) crystal as estimated from separate measurements of Er(trensal) [20] powder (Supporting Information, Figure S4). ForA g(100) and MgO j Ag(100) substrates the temperature may be slightly higher (up to ca 2.5 K).
Dy-M 5 XAS and XMCD spectra of Dy 2 ScN@C 80 submonolayers measured at T % 2Kin the magnetic field of 6.5 T are shown in Figure 3. Them easurements at ad ifferent incidence of X-rays and magnetic field show an oticeable difference between the substrates.O nA u(111), the XMCD signal of Dy 2 ScN@C 80 is slightly stronger in the grazing (308 8) than in the normal (908 8)i ncidence.O nA g(100), the difference between the two orientations is enhanced, with much stronger XMCD response at the grazing incidence.Finally,no difference can be seen between the spectra measured at 308 8 and 908 8 on the MgO j Ag(100) substrate.T hese results are further corroborated by the angular dependence of XMCD asymmetry (Figure 4), which is almost isotropic for Au(111) with as light increase at smaller angles,i sotropic for MgO j Ag(100) within the experimental uncertainty,b ut strongly anisotropic for the Ag(100) substrate.
As magnetic moments of Dy ions are strongly anisotropic and are locked to the direction of the DyÀNb onds (Figure 1a), the angular dependence of XMCD essentially provides information on the orientation of the Dy 2 ScN clusters in the monolayers.T he isotropic behavior of the XMCD signal on Au(111) and MgO j Ag(100) shows that the Dy 2 ScN cluster is randomly oriented in the Dy 2 ScN@C 80 monolayers on these substrates.O nt he other hand, the anisotropy of the XMCD in the Dy 2 ScN@C 80 monolayer on Ag(100) indicates that the endohedral cluster preferentially adopts ap arallel orientation with respect to the surface. Parallel alignment of Dy 2 ScN was also observed for the Dy 2 ScN@C 80 monolayer on Rh(111). [7a] Themagnetic moment of Dy 3+ ions in Dy 2 ScN@C 80 can be estimated using XMCD sum rules. [21] ForD y 2 ScN@C 80 on Au(111), the sum rule analysis gives the moment of 5.4 AE 0.5 m B per Dy 3+ ion at 908 8 and 6.1 AE 0.5 m B at 308 8.OnAg(100), the moment increases from 2.9 AE 0.5 m B at 908 8 to 5.7 AE 0.5 m B at 308 8.T hesevalues are significantly smaller than 10 m B ,t he ground-state magnetic moment of Dy 3+ ion in the axial ligand field. However,when magnetic moments are anisotropic,the measured moment corresponds to the projection of the magnetic field onto the easy axis.A sar esult, in powder  samples of Dy 2 ScN@C 80 with disordered Dy 2 ScN clusters,the apparent magnetic moment of Dy 3+ ions is reduced to 5 m B . [14] Them agnetic moments of Dy 3+ ions in Dy 2 ScN@C 80 on the Au(111) substrate are thus close to the value expected for the disordered cluster,w hich agrees with the lack of the angular dependence of the XMCD.F or Dy 2 ScN@C 80 on the Ag(100) substrates,t he small magnetization perpendicular to the surface agrees well with the preferential in-plane alignment of the endohedral cluster.
TheX MCD signal at the Dy-M 5 edge was used to study magnetization curves of Dy 2 ScN@C 80 monolayers.M agnetic hysteresis with ac oercive field of ca 0.4 Ti so bserved for Dy 2 ScN@C 80 monolayers on all three substrates near 2K (Figure 2b). When temperature is increased to about 6K,the hysteresis is not observed any more (Supporting Information, Figure S7). Theasymmetric orientation of the Dy 2 ScN cluster is also reflected in magnetization curves measured at 308 8 and 908 8 on Au(111) and Ag(100) substrates (Figure 3b). This width of the 2K hysteresis is twice smaller than for the powder sample measured by SQUID magnetometry at ac omparable sweep rate (Figure 1b). Ap ossible reason is the X-ray-induced demagnetization. [22] Besides,c hanging polarity of the magnet during the field ramp takes 30 seconds, which also reduces the observed coercive field. Taking this into account, we conclude that there is no dramatic deterioration of the hysteretic behavior of Dy 2 ScN@C 80 monolayers when compared to the bulk samples.A pparently,t he fullerene cage provides sufficient protection for the endohedral magnetic cluster from the demagnetizing influence of metallic substrates.E arlier it was found that at hin layer of MgO dramatically increases the hysteresis of aT bPc 2 monolayer [5c, 23] and boosts the temperature of the magnetic bistability of Ho atoms up to 30 K. [24] Recently,s ome of us showed that the improved SMM properties on MgO substrate are caused by its low phonon density of states,which leads to the dramatic reduction of the relaxation of magnetization via the Raman mechanism. [23] Apparently,t his is not the relaxation rate-limiting factor for Dy 2 ScN@C 80 ,and the use of the MgO layer does not lead to anoticeable improvement of the surface SMM behavior in comparison to metallic substrates. Thus,wec onclude that the substrate plays an important role in the structural ordering of endohedral cluster,b ut has no strong influence on the SMM properties of adsorbed metallofullerenes.T herefore,t he SMM performance of Dy 2 ScN@C 80 monolayers is not limited by the interactions with the substrate.
Foradeeper insight into the fullerene-substrate interactions,D FT calculations of the Dy 2 ScN@C 80 molecule placed on Au(111), Ag(100), and MgO surfaces were performed at the PBE-D level with PAW 4f-in-core potentials using the VA SP 5.0 code. [25] As the Dy 2 ScN cluster may adopt different orientations inside the carbon cage,itisimportant to have acomprehensive sampling of possible structural configurations.U sing recently proposed Fibonacci sphere sampling, [26] 120 initial configurations with different orientations of the Dy 2 ScN cluster were generated for afullerene molecule on each substrate,and their structures were optimized. Foran isolated Dy 2 ScN@C 80 molecule this approach gave only three unique conformers with the relative energies of 0, 41, and 47 meV.B ut as ubstantially different situation is found for Dy 2 ScN@C 80 molecule on as ubstrate (Figure 5a,b). Calculations did not reveal any particularly stable conformation but rather predicted am ultitude of conformers,w hich are likely to coexist under experimental conditions within ac ertain energy cut-off.T he energies of the on-surface optimized conformers are spread in the range of 300 meV for Au(111) and Ag(100) substrates and 170 meV for the MgO substrate. It should be emphasized that these conformer distributions are predicted for as ingle Dy 2 ScN@C 80 molecule in the absence of other fullerene neighbors.D ue to the incommensurate lattice parameters of the substrates and the fullerene layer, calculations of the Dy 2 ScN@C 80 monolayer are not feasible at this time.I tc an be anticipated that the intermolecular interactions should also affect the energetics of the cluster orientations and electronic properties, [26] but the influence of the metallic substrate is expected to be considerably stronger.
ForD y 2 ScN@C 80 on Au(111) and Ag(100), the lowestenergy conformers are grouped at q = 5-308 8,w hich corresponds to nearly parallel orientations of the cluster to the substrate.These results agree well with the observation of the in-plane ordering of the Dy 2 ScN cluster on the Ag(100) surface,b ut do not fully capture the experimental finding of the weaker ordering effect on the Au(111) substrate.P resumably,t he Au(111) surface reconstructions may decrease the ordering of the cluster in adsorbed fullerenes,b ut computational description of such effects is not feasible at this moment. Fort he MgO substrate,c alculations show the grouping of the most stable conformers near the angles of 35-408 8 and 75-858 8,b ut the overall energy spread of the conformers is smaller than on metals.Although XMCD measurements are performed 2K,i ti sr easonable to expect that the distribution of the conformers in the experimental samples should be different from the equilibrium for 2K.W hen the sample is cooled down from room temperature,a tc ertain point above the base temperature the angular distribution of the conformers will be frozen. Effectively,t his situation can be modelled by considering conformers within ac ertain energy cut-off.W es imulated XAS and XMCD spectra of different conformers with MULTIX code [27] (Supporting Information, Figure S8) and found that when an arbitrary energy cut-off of 50 meV is used, the simulations reproduce the experimentally found lack of the ordering on MgO j Ag-(100) and preferential in-plane alignment on Ag(100).
TheP BE-D binding energy of the fullerene molecule to the surface is 2.91 eV for Au(111), 2.53 eV for Ag(100), and 1.43 eV for MgO (Table 1). Themain contributions are from the dispersion interactions (E disp ). [25e] Deformation energy E def necessary to distort the structures of the isolated Dy 2 ScN@C 80 molecule and the substrate to those they adopt in the interacting system, is found to be 366 meV for Au(111), 157 meV for Ag(100), and 39 meV for MgO,t ow hich fullerene contributions are 132, 46, and 12 meV,respectively. Ther emaining terms of 0.64 eV for Au(111), 0.47 eV for Ag(100), and À0.13 eV for MgO are due to the electronic contribution E Coul/cov ,w hich includes both Coulomb and covalent terms.T hough 4-5 times smaller than dispersion, these interactions are responsible for the changes in the electronic structure at the interface and hence are considered further in more details.N ote also that the variation of the relative energy for different cluster orientations (Figure 5b)is mainly caused by the changes in E Coul/cov ,whereas E disp remains almost identical for the whole conformer set, and variations in E def are considerably smaller (Supporting Information, Table S1).
Interaction of af ullerene molecule with the substrate leads to ac hange in the electronic distribution of both. The net effect of this redistribution can be evaluated via the charge of the fullerene molecule (Q mol )a ccumulated on the surface.Calculations of atomic charges with the Bader code [28] showed that Dy 2 ScN@C 80 transfers 0.2-0.3 e to the Au(111) substrate,b ut acquires an egative charge of À(0.2-0.3) e on Ag(100) and À(0.09-0.14) e on MgO.Importantly,despite the considerable variation of the net fullerene charge in dependence on the substrate and the cluster orientation (Figure 5b), the charge of the Dy 2 ScN cluster in adsorbed molecules is  Ac loser look into the interfacial charge transfer is provided by the difference electron density D1 obtained by subtraction of the electron density of separately computed Dy 2 ScN@C 80 molecule and as ubstrate from the electron density of the whole system. Visualization of D1 in Figure 5c shows that the charge redistribution is restricted to the interfacial region where the fullerene molecule contacts the substrate.T he largest part of the carbon cage as well as the endohedral cluster are only weakly affected. When Dy 2 ScN@C 80 is adsorbed onto the Au(111) surface,d ensity depletion and accumulation regions are formed near the fullerene and near/at the upper gold metal atoms,respectively.For the fullerene on the Ag(100) surface,the regions of the density accumulation and depletion are intertwined in acomplex manner.T he spatial extension of the density affects at least two layers of Au and Ag metal atoms and small changes can be seen down to the fourth layer. Forthe fullerene on the MgO,t he difference density has the least extended profile, and changes in the electron density are visible only in the upper layer of the substrate.
Thes patial extension of D1 correlates with the angular dependence of the relative energy of the Dy 2 ScN@C 80 conformers.A pparently,t he Dy 2 ScN cluster tends to avoid the parts of the fullerene p-system interacting with the metallic surface.T his can be best achieved in the parallel configuration of the Dy 2 ScN cluster,inwhich all three metal atoms do not interact with the surface-perturbed parts of the fullerene cage.F or the MgO substrate,t he situation is different because the ionic substrate has strongly inhomogeneous charge distribution, and here the electrostatic interactions between the substrate and the carbon cage play an important role.A st he fragments of the fullerene cage coordinated by the endohedral metals have pronounced variation of the electrostatic potential distribution, [26] the orientation of the fullerene molecule towards the substrate by such fragments may become energetically favorable.
Interactions with the substrate may also affect the electronic structure of the adsorbed fullerene.C omparison to the density of states (DOS) of the isolated Dy 2 ScN@C 80 molecule (Supporting Information, Figure S12) shows that interaction with the MgO has only am inor effect on the fullerene-projected DOS.T he highest-occupied states are dominated by the carbon cage,w hereas the lowest-energy unoccupied states have noticeable contributions from the endohedral cluster.The LUMO of the fullerene has adistinct peak in the DOS at 0.85 eV above the Fermi level. TheDFTpredicted gaps for the isolated Dy 2 ScN@C 80 molecule and the molecule on the MgO are 1.65 and 1.63 eV,respectively.T he interaction with the Au(111) surface results in more pronounced changes of the fullerene DOS (Figure 5d). The HOMO-and LUMO-derived peaks can be seen in the DOS at À1.09 eV and 0.59 eV,r espectively.T he gap between the LUMO peak and higher-energy unoccupied states is reduced substantially.F urthermore,n ew states appear in the gap between the HOMO and LUMO.T hey originate from the hybridization with the metal bands and can be identified as surface states.T hus,t he weak features inside the gap in the experimental STS spectra of Dy 2 ScN@C 80 on Au(111) (Figure 2c)may have its origin here.Finally,the most pronounced changes of the DOS are found for the fullerene on Ag(100). Here the LUMO-derived feature is not asingle peak anymore but is split into several components at the Fermi energy.Thus, the LUMO of Dy 2 ScN@C 80 is contributing strongly to the surface states and becomes partially occupied.

Conclusion
Realization of magnetic bistability in monolayers of SMMs in contact with an electrode is an ecessary step towards making use of the single-molecule nature of their magnetism in spintronic devices.Inthis work we showed that Dy 2 ScN@C 80 fulfills this criterion. XMCD studies of the Dy 2 ScN@C 80 submonolayers on Au(111), Ag(100), and MgO j Ag(100) revealed ad istinctive influence of the substrates on the ordering of the endohedral cluster.O nA g(100), the Dy 2 ScN units are preferentially aligned parallel to the surface, on Au(111) there is only as light preference of the parallel alignment, whereas on MgO j Ag(100) no ordering is found at all. However,d espite the strong influence of the surface on the structural ordering, the magnetic behavior of Dy 2 ScN@C 80 molecules does not show an oticeable dependence on the substrate.Magnetic hysteresis with the coercivity of ca 0.4 Tisfound near 2Kin submonolayers of Dy 2 ScN@C 80 on all three substrates.D FT calculations showed that the charge redistribution at the metal-fullerene interface is confined within the contact region. Thee lectron transfer affects only the carbon cage,w hereas the charge state of the endohedral cluster remains intact. Thus,the fullerene acts as aF araday cage protecting the electronic and magnetic properties of the endohedral species on conducting substrates.