The electronic structure and morphology of ultrathin MgO films epitaxially grown on Ag(001) were investigated using low-temperature scanning tunneling spectroscopy and scanning tunneling microscopy. Layer-resolved differential conductance (dI/dU) measurements reveal that, even at a film thickness of three monolayers, a band gap of about 6 eV is formed corresponding to that of the MgO(001) single-crystal surface. This finding is confirmed by layer-resolved calculations of the local density of states based on density functional theory.

Cerium adatoms, deposited on a Ag(111) surface, are found by low-temperature scanning tunneling microscopy to self-assemble into large ordered hexagonal arrays covering macroscopically the entire surface. We show that the 32 A periodicity of the superlattice is caused by the interaction of surface-state electrons with Ce adatoms and that the large-scale formation of the superlattice is governed by a subtle balance between the sample temperature, the surface diffusion barrier, and the concentration-dependent adatom interaction potential.

metal electrode. We use nonmagnetic, insulating MgO, wellknown in inorganic spintronic applications, [ 17,18 ] which allows to control the electron tunneling rate over many orders of magnitude. [ 19 ] Moreover, we employ the TbPc 2 SMM [ 14,15,[20][21][22][23] as a model system. In the neutral molecule, the Tb(III) ion exhibits an electronic spin state of J = 6. It is sandwiched between two phthalocyanine (Pc) macrocycles (cf. schematic view in Figure 1 a) hosting an unpaired electron delocalized over the Pc ligands. The easy-axis-type magnetic anisotropy imposes an energy barrier of ≈65 meV for magnetization reversal, [ 23 ] which is largest within the whole series of lanthanide-Pc 2 SMMs. [ 14,15 ] On nonmagnetic conducting substrates, only vanishing remanence [6][7][8][9][10] and very narrow hysteresis loops [6][7][8][9] were observed, much smaller than in bulk measurements, [ 20 ] illustrating the disruptive effects of the surface. We note that the adsorption of TbPc 2 on (anti)ferromagnetic materials represents a different situation because of the magnetic exchange interaction with the substrate. [ 24,25 ] In those cases, the SMMs were not shown to exhibit slow relaxation of magnetization. Rather, the hysteresis is linked to the one of the magnetic substrates, i.e., it is not an intrinsic property of the SMMs. Overall, the detailed knowledge on TbPc 2 makes it an ideal candidate to test if a tunnel barrier can boost the magnetic properties of surface-adsorbed SMMs. In this communication we show that the magnetic remanence and hysteresis opening obtained with TbPc 2 on MgO tunnel barriers outperform the ones of any other surface-adsorbed SMM [4][5][6][7][8][9][10][11][12][13]26 ] as well as those of bulk samples of TbPc 2 . [ 20 ] The scanning tunneling microscopy (STM) images in Figure 1 b,c show that TbPc 2 self-assembles by forming perfectly ordered 2D islands on two monolayers (MLs) of MgO on Ag(100). In line with former results, the SMMs are adsorbed fl at on the surface (cf. discussion of our STM and X-ray linear dichroism (XLD) data below). [ 6,27 ] This excludes that the extraordinary magnetic properties observed in this study are due to upstanding molecules having their macrocycles perpendicular to the surface, which would lead to a reduced interaction of the Tb(III) ion with the surface. The high-resolution image in Figure 1 c reveals eight lobes per molecule, reminiscent of the staggered conformation of the two phthalocyanine ligands. [ 27 ] Islands with the identical molecular assembly are formed by TbPc 2 adsorbed directly onto Ag(100), as shown in the Supporting Information.The magnetic properties of the Tb(III) ions in the surfaceadsorbed SMMs are determined by X-ray magnetic circular dichroism (XMCD) measurements at the M 4,5 (3 d → 4 f ) edges of Tb. For sub-MLs of TbPc 2 on MgO we fi nd a strong remanence larger than 40% of the saturation magnetization sat M and Single-molecule magnets (SMMs) [ 1 ] are very promising for molecular spintronics [ 2 ] and quantum information processing, [...

No abstract

Regular arrays of single atoms with stable magnetization represent the ultimate limit of ultrahigh density storage media. Here we report a self-assembled superlattice of individual and noninteracting Dy atoms on graphene grown on Ir(111), with magnetic hysteresis up to 5.6 T and spin lifetime of 1000 s at 2.5 K. The observed magnetic stability is a consequence of the intrinsic low electron and phonon densities of graphene and the 6-fold symmetry of the adsorption site. Our array of single atom magnets has a density of 115 Tbit/inch 2 , defined by the periodicity of the graphene moirépattern. KEYWORDS: Single atom magnets, self-assembly, superlattice, rare earth atoms, graphene, XMCD T he fabrication of ordered structures at the nanoscale is a crucial step toward information storage at ultimate length scales.1,2 Realizing highly ordered and monodispersed magnetic structures stands as one of the key challenges for increasing the bit density of magnetic storage devices. The ultimate limit of a single atom per bit guarantees the highest storage density and minimal dipolar coupling among the bits. Single-ion molecular magnets, 3 as well as metal−organic networks, 4 allow the selfassembly of single magnetic atoms in ultradense arrays. The molecular cage defines the spacing between the magnetic cores and can protect them from contamination. However, the coupling with electrons and vibrational modes of the surrounding ligands limits the magnetic stability of the magnetic core presently to temperatures below 20 K in bulk 5 and 8 K for surface supported molecules. 6 The absence of the organic ligand, i.e., having individual atoms adsorbed on the surface, may result in a reduced interaction with the environment and greater magnetic stability. Intense research on the magnetism of single atoms 7−12 has culminated in the achievement of magnetic remanence in Ho atoms randomly adsorbed onto MgO, with magnetic stability up to 40 K. 13 Reading and writing of these atoms has recently been demonstrated.14 Nevertheless, so far the realization of an ensemble of single atoms combining long magnetic lifetimes with spatial order has remained elusive and stands as the next milestone. Here we exploit the selective adsorption of Dy atoms in the periodic moirépattern formed by graphene on lattice mismatched Ir(111) 15 to create a superlattice of single atom magnets with a mean distance of 2.5 nm and negligible mutual magnetic interactions.Ensembles of individual Dy atoms on graphene on Ir(111) are obtained by deposition with an e-beam evaporator ( Figure 1a, middle). The spatial arrangement of these atoms on the graphene moirépattern can be controlled by the sample temperature during deposition, T dep . Deposition below 10 K yields statistical growth with a random distribution of Dy atoms, as demonstrated by scanning tunneling microscopy (STM) in Figure 1a, left. At 40 K, surface diffusion of Dy is activated and, therefore, the atoms can reach the most favorable adsorption site in the moiréunit cell, namely, where the C-...