Slow Magnetic Relaxation of Dy Adatoms with In-Plane Magnetic Anisotropy on a Two-Dimensional Electron Gas.

We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO3. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO2-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO3 crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3dxy character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields.

Correlation between Electronic Configuration and Magnetic Stability in Dysprosium Single Atom Magnets.

Single atom magnets offer the possibility of magnetic information storage in the most fundamental unit of matter. Identifying the parameters that control the stability of their magnetic states is crucial to design novel quantum magnets with tailored properties. Here, we use X-ray absorption spectroscopy to show that the electronic configuration of dysprosium atoms on MgO(100) thin films can be tuned by the proximity of the metal Ag(100) substrate onto which the MgO films are grown. Increasing the MgO thickness from 2.5 to 9 monolayers induces a change in the dysprosium electronic configuration from 4f9 to 4f10. Hysteresis loops indicate long magnetic lifetimes for both configurations, however, with a different field-dependent magnetic stability. Combining these measurements with scanning tunneling microscopy, density functional theory, and multiplet calculations unveils the role of the adsorption site and charge transfer to the substrate in determining the stability of quantum states in dysprosium single atom magnets.

Mapping Orbital-Resolved Magnetism in Single Lanthanide Atoms.

Single lanthanide atoms and molecules are promising candidates for atomic data storage and quantum logic due to the long lifetime of their magnetic quantum states. Accessing and controlling these states through electrical transport requires precise knowledge of their electronic configuration at the level of individual atomic orbitals, especially of the outer shells involved in transport. However, no experimental techniques have so far shown the required sensitivity to probe single atoms with orbital selectivity. Here we resolve the magnetism of individual orbitals in Gd and Ho single atoms on MgO/Ag(100) by combining X-ray magnetic circular dichroism with multiplet calculations and density functional theory. In contrast to the usual assumption of bulk-like occupation of the different electronic shells, we establish a charge transfer mechanism leading to an unconventional singly ionized configuration. Our work identifies the role of the valence electrons in determining the quantum level structure and spin-dependent transport properties of lanthanide-based nanomagnets.

Magnetic properties of on-surface synthesized single-ion molecular magnets.

We perform on-surface synthesis of single-ion molecular magnets on an Ag(111) surface and characterize their morphology, chemistry, and magnetism. The first molecule we synthesize is TbPc2 to enable comparison with chemically synthesized and subsequently surface adsorbed species. We demonstrate the formation of TbPc2 with a yield close to 100% and show that on-surface synthesis leads to identical magnetic and morphological properties compared to the previously studied chemically synthesized species. Moreover, exposure of the surface adsorbed TbPc2 molecules to air does not modify their magnetic and morphological properties. To demonstrate the versatility of our approach, we synthesize novel Tb double deckers using tert-butyl-substituted phthalocyanine (tbu-2H-Pc). The Tb(tbu-Pc)2 molecules exhibit magnetic hysteresis and therefore are the first purely on-surface synthesized single ion magnet.

Two-Orbital Kondo Screening in a Self-Assembled Metal-Organic Complex.

Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl dicarbonitrile molecules exhibit strongly spatial anisotropic Kondo features with directionally dependent Kondo temperatures and line shapes, as evidenced by scanning tunneling spectroscopy. First-principles calculations find nearly full polarization for the half-filled Fe 3dxz and 3dyz orbitals, which therefore can give rise to Kondo screening with the experimentally observed directional dependence and distinct Kondo temperatures. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements confirm that the spin in both channels is effectively Kondo-screened. At ideal Fe coverage, these two-orbital Kondo impurities are arranged in a self-assembled honeycomb superlattice.

Superlattice of Single Atom Magnets on Graphene.

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/inch2, defined by the periodicity of the graphene moiré pattern.

Single-Molecule Magnets: Giant Hysteresis of Single-Molecule Magnets Adsorbed on a Nonmagnetic Insulator (Adv. Mater. 26/2016).

In Tb(Pc)2 single-molecule magnets, where Pc is phthalocyanine, adsorbed on magnesium oxide, the fluctuations of the terbium magnetic moment are strongly suppressed in contrast to the adsorption on silver. On page 5195, J. Dreiser and co-workers investigate that the molecules are perfectly organized by self-assembly, as seen in the scanning tunnelling microscopy image (top part of the design). The molecules are probed by circularly polarized X-rays depicted as green spirals.

Magnetic Hysteresis in Er Trimers on Cu(111).

We report magnetic hysteresis in Er clusters on Cu(111) starting from the size of three atoms. Combining X-ray magnetic circular dichroism, scanning tunneling microscopy, and mean-field nucleation theory, we determine the size-dependent magnetic properties of the Er clusters. Er atoms and dimers are paramagnetic, and their easy magnetization axes are oriented in-plane. In contrast, trimers and bigger clusters exhibit magnetic hysteresis at 2.5 K with a relaxation time of 2 min at 0.1 T and out-of-plane easy axis. This appearance of magnetic stability for trimers coincides with their enhanced structural stability.

Giant Hysteresis of Single-Molecule Magnets Adsorbed on a Nonmagnetic Insulator.

TbPc2 single-molecule magnets adsorbed on a magnesium oxide tunnel barrier exhibit record magnetic remanence, record hysteresis opening, perfect out-of-plane alignment of the magnetic easy axes, and self-assembly into a well-ordered layer.

Out-of-Plane Alignment of Er(trensal) Easy Magnetization Axes Using Graphene.

We have studied Er(trensal) single-ion magnets adsorbed on graphene/Ru(0001), on graphene/Ir(111), and on bare Ru(0001) by scanning tunneling microscopy and X-ray absorption spectroscopy. On graphene, the molecules self-assemble into dense and well-ordered islands with their magnetic easy axes perpendicular to the surface. In contrast, on bare Ru(0001), the molecules are disordered, exhibiting only weak directional preference of the easy magnetization axis. The perfect out-of-plane alignment of the easy axes on graphene results from the molecule-molecule interaction, which dominates over the weak adsorption on the graphene surface. Our results demonstrate that the net magnetic properties of a molecular submonolayer can be tuned using a graphene spacer layer, which is attractive for hybrid molecule-inorganic spintronic devices.

Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene.

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.

Temperature-dependent self-assembly of NC-Ph5-CN molecules on Cu(111).

We present the results of temperature-dependent self-assembly of dicarbonitrile-pentaphenyl molecules (NC-Ph5-CN) on Cu(111). Our low-temperature scanning tunneling microscopy study reveals the formation of metal-organic and purely organic structures, depending on the substrate temperature during deposition (160-300 K), which determines the availability of Cu adatoms at the surface. We use tip functionalization with CO to obtain submolecular resolution and image the coordination atoms, enabling unequivocal identification of metal-coordinated nodes and purely organic ones. Moreover, we discuss the somewhat surprising structure obtained for deposition and measurement at 300 K.

Formation of fe cluster superlattice in a metal-organic quantum-box network.

We report on the self-assembly of Fe adatoms on a Cu(111) surface that is patterned by a metal-organic honeycomb network, formed by coordination of dicarbonitrile pentaphenyl molecules with Cu adatoms. Fe atoms landing on the metal surface are mobile and steered by the quantum confinement of the surface state electrons towards the center of the network hexagonal cavities. In cavities hosting more than one Fe, preferential interatomic distances are observed. The adatoms in each hexagon aggregate into a single cluster upon gentle annealing. These clusters are again centered in the cavities and their size is discerned by their distinct apparent heights.

Coverage-dependent self-assembly of rubrene molecules on noble metal surfaces observed by scanning tunneling microscopy.

Coverage-dependent self-assembly of rubrene molecules on different noble metal surfaces, Au(111) and Au(100), Ag(111) and Ag(100), is presented. On Au(111), the homochiral supramolecular assemblies evolve with increasing rubrene coverage from very small structures composed of a few molecules, to honeycomb islets, and to one-dimensional chains of supramolecular pentamers. At higher coverage, the racemic mixture of molecules forms close-packed islands. On Au(100), chains of pentamers and two different types of densely packed islands are formed. On the Ag surfaces, exclusively close-packed islands are created, independently of the rubrene coverage. Moreover, the role of the chiral nature of the molecules in the self-assembly process is discussed, as well as the existence of different molecular conformers depending on the supramolecular assembled phase. The observed differences and similarities reflect the influence of the electronic properties and the geometric structure of the various substrates on molecular self-assembly.

Three-dimensional chirality transfer in rubrene multilayer islands on Au(111).

The growth of rubrene (C(42)H(28), 5,6,11,12-tetraphenylnaphthacene) multilayer islands up to a thickness of six layers on a Au(111) surface has been investigated by scanning tunneling microscopy. The molecules self-organize in parallel twin rows, forming mirror domains of defined local structural chirality. Each layer is composed of twin-row domains of the same structural handedness rotated by 120 degrees with respect to each other. Moreover, this structural chirality is transferred to all successive layers in the island, resulting in the formation of three-dimensional objects having a defined structural chirality. The centered rectangular surface unit cell differs from the one characteristic for the single-crystal orthorhombic phase.

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling.

Tunneling electrons from a scanning tunneling microscope (STM) induce luminescence from C(60) and C(70) molecules forming fullerene nanocrystals grown on ultrathin NaCl films on Au(111). Intramolecular fluorescence and phosphorescence associated with the transitions between the lowest electronic excited state and ground state of C(70) molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover we demonstrate that the molecular luminescence is selectively enhanced by localized surface plasmons in the STM tip-sample gap.

Diatomic molecular switches to enable the observation of very-low-energy vibrations.

Using low-temperature scanning tunneling microscopy and spectroscopy, we found that the coadsorption of atomic hydrogen to single transition-metal and rare-earth-metal atoms on a Ag(100) surface gives rise to surprising phenomena, a bias dependent switching from a large to a small apparent size of the diatomic molecules and a concomitant appearance of very low-energy vibrational features of 3 to 7 meV in the differential conductance spectra. These phenomena, which have until now escaped observation, may be of general relevance for low-temperature adsorption.

Fluorescence and phosphorescence from individual molecules excited by local electron tunneling.

Using the highly localized current of electrons tunneling through a double barrier scanning tunneling microscope junction, we excite luminescence from a selected C60 molecule in the surface layer of fullerene nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed fluorescence and phosphorescence spectra, pure electronic as well as vibronically induced transitions of an individual C60 molecule are identified, leading to unambiguous chemical recognition on the single-molecular scale.