The three essential pillars of magnetic data storage devices are readability, writeability, and s... more The three essential pillars of magnetic data storage devices are readability, writeability, and stability. However, these requirements compete as magnetic domain sizes reach the fundamental limit of single atoms and molecules. The proven magnetic bistability of individual holmium atoms on magnesium oxide appeared to operate within this magnetic trilemma, sacrificing writeability for unprecedented stability. Using the magnetic stray field created by the tip of a spin-polarized scanning tunneling microscope (SP-STM), we controllably move the Ho state into the quantum regime, allowing us to write its state via the quantum tunneling of magnetization (QTM). We find that the hyperfine interaction causes both the excellent magnetic bistability, even at zero applied magnetic field, and the avoided level crossings which we use to control the magnetic state via QTM. We explore how to use such a system to realize a high-fidelity single atom NOT gate (inverter). Our approach reveals the prospec...
Spin-polarized scanning tunneling microscopy (SP-STM) measures tunnel magnetoresistance (TMR) wit... more Spin-polarized scanning tunneling microscopy (SP-STM) measures tunnel magnetoresistance (TMR) with atomic resolution. While various methods for achieving SP probes have been developed, each is limited with respect to fabrication, performance, and allowed operating conditions. In this study, we present the fabrication and use of SP-STM tips made from commercially available antiferromagnetic Mn_88Ni_12 foil. The tips are intrinsically SP, which is attractive for exploring magnetic phenomena in the zero field limit. The tip material is relatively ductile and straightforward to etch. We benchmark the conventional STM and spectroscopic performance of our tips and demonstrate their spin sensitivity by measuring the two-state switching of holmium single atom magnets on MgO/Ag(100).
Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy r... more Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (B_z = 8.0 T, B_x = 0.8 T) to an ESR-STM. The system is capable of delivering RF power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 ± 0.001) μ_B. Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of...
The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robu... more The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robust when exposed to high magnetic fields and temperatures up to 35 K. Here we address the stability of Ho at small magnetic fields, where the hyperfine interaction creates several avoided level crossings. Using spin-polarized scanning tunneling microscopy, we demonstrate quantum state control via Landau-Zener tunneling and stable magnetization at zero field. Our observations indicate a total spin ground state of J z = ±8. Combined quantum and classical control render Ho a promising qubit candidate.
We present spin-polarized scanning tunneling microscopy measurements demonstrating spin-polarizat... more We present spin-polarized scanning tunneling microscopy measurements demonstrating spin-polarizations of up to 80 % for Co islands on a Pt(111) surface and a tunnel magneto resistance of 850 % between the islands and an anti-ferromagnetic Cr-coated W-tip. These values are stable up to ±0.7 V bias. We report on the magnetic moments and anisotropy energies of two-dimensional Co islands on Pt(111) comprising only a few atoms. Our results show the correlation between orbital moments and magneto-crystalline anisotropies and reveal that both properties strongly depend on the lateral atomic coordination. The anisotropy of single adatoms is found to be 200 times the Co hcp bulk value. We also present well ordered superlattices of Co islands self-assembled on Au(788). The particles have uniaxial out-of-plane magnetization and no dipolar interactions. They present a model system for ultra-high density storage media since they have the most uniform anisotropy energies and the highest density of non-interacting particles so far realized.
We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magn... more We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with 4f n−1 occupancy, and in a divalent state with 4f n occupancy for all other studied RE atoms (n corresponds to the 4f occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.
We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assem... more We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assembled arrays of FeH single molecule magnets on graphene/Ir(111). The energetic positions of molecular states are probed by means of scanning tunneling spectroscopy, showing pronounced long- and short-ranged spatial modulations, indicating the presence of both locally varying intermolecular as well as adsorption-site dependent molecule-substrate interactions. From the magnetic field dependence of the X-ray magnetic circular dichroism signal, we infer that the magnetic easy axis of each FeH molecule is oriented perpendicular to the sample surface and that after the deposition the value of the uniaxial anisotropy is identical to the one in bulk. Our findings therefore suggest that the observed interaction of the molecules with their surrounding does not modify the molecular magnetism, resulting in a two-dimensional array of molecular magnets that retain their bulk magnetic properties.
Stable magnets from single atoms An important goal in molecular magnetism is to create a permanen... more Stable magnets from single atoms An important goal in molecular magnetism is to create a permanent magnet from a single atom. Metal atoms adsorbed on surfaces can develop strong magnetization in an applied field (paramagnetism). Donati et al. show that single holmium atoms adsorbed on a magnesium oxide film grown on a silver substrate show residual magnetism for temperatures up to 30 K and bistabilty that lasts for 1500 s at 10 K (see the Perspective by Khajetoorians and Heinrich). The atom avoids spin relaxation by a combination of quantum-state symmetry and by the oxide film preventing the spin from interacting with the underlying metal via tunneling. Science , this issue p. 318 ; see also p. 296
High-density arrays (5ϫ10 6 cm Ϫ1) of parallel nanowires have been grown using the vicinal Pt͑997... more High-density arrays (5ϫ10 6 cm Ϫ1) of parallel nanowires have been grown using the vicinal Pt͑997͒ surface as a template. Single monatomic rows of Ag and Cu can be deposited with subrow precision. We demonstrate real-time monitoring and characterization of the growth of the atomic chains as a function of temperature by thermal energy helium atom scattering. Scanning tunneling microscopy provides further insight into the structure of the metal rows. Growth mode and alloying with the Pt substrate are discussed as a function of temperature. Our results provide the basis for the creation of surfaces with a uniform distribution of wires having the same average width for the investigation of the electronic, magnetic, and chemical properties of one-dimensional and quasi-one-dimensional metal structures.
Ligand-protected Au clusters are non-bleaching fluorescence markers in bio-and medical applicatio... more Ligand-protected Au clusters are non-bleaching fluorescence markers in bio-and medical applications. Here we show that their fluorescence can be an intrinsic property of the Au cluster itself. We find a very intense and sharp fluorescence peak located at λ = 739.2 nm (1.68 eV) for Au 20 clusters in a Ne matrix held at 6 K. The fluorescence reflects the Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) diabatic bandgap of the cluster. Au 20 shows a very rich absorption fine structure reminiscent of well defined molecule-like quantum levels. These levels are resolved since Au 20 has only one stable isomer (tetrahedral); therefore our sample is mono-disperse in cluster size and conformation. Density-functional theory (DFT) and time-dependent DFT calculations clarify the nature of optical absorption and predict both main absorption peaks and intrinsic fluorescence in fair agreement with experiment.
Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl-dicarbonitrile molecules exh... more 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.
We report on the surface-guided synthesis of an unprecedented dinuclear organocobalt complex, its... more We report on the surface-guided synthesis of an unprecedented dinuclear organocobalt complex, its self-assembly into a complex nanoarchitecture, and a single-molecule level investigation of its switching behavior. Initially, an organic layer is prepared by depositing hexakis((trimethylsilyl)ethynyl)-benzene under ultra-high vacuum conditions onto Ag(111). After Co dosage at 200 Kelvin, low-temperature scanning tunneling microscopy (STM) reveals an epitaxy-mediated organization mechanism of molecules and on-surface formed host-guest complexes. Dinuclear complexes containing two bis(η2-alkynyl) π-tweezer motifs, each stabilizing a single Co atom, express two enantiomers due to a conformation twist. The chirality is transferred to the two-dimensional architecture, whereby its Co adatoms are located at the corners of a 3.4.6.4 rhombitrihexagonal tessellation due to the systematic arrangement and anchoring of the complexes. Extensive density functional theory simulations support our inte...
We have studied Er(trensal) single-ion magnets adsorbed on graphene/Ru(0001), on graphene/Ir(111)... more 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.
We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(1... more 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.
We discuss the structure and size of metal islands formed by atomic-beam deposition onto single c... more We discuss the structure and size of metal islands formed by atomic-beam deposition onto single crystal metal surfaces. Most of the procedures described can straightforwardly be transferred to oxide and semiconductor substrates. We start with the preparation and stability of the smallest islands, namely single atoms. Isolated and immobile adatoms are formed by deposition at substrate temperatures where thermal diffusion is inhibited. In the ideal case this leads to statistical growth, however one often observes slight deviations caused by neighbor driven mobility. We discuss how ordered superlattices of adatoms can be stabilized by using short-range repulsive together with long-range oscillatory interactions between adsorbates. In some examples these interactions add up and become anisotropic, leading to the formation of atomic chains in-midst of atomic terraces. We discuss coarsening by Ostwald ripening or by diffusion of entire islands. Both give rise to more narrow size distributions than nucleation and growth. We show examples for efficient stress relief at steps in monolayer high islands. Finally methods are presented to create bimetallic core-shell islands. These islands are ideally suited to compare physical and chemical properties of step atoms with the ones of higher coordinated surface atoms, or to investigate the role of one-dimensional interfaces between two metals.
Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule ... more Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule magnet DySc 2 N@C 80 at the Dy M 4,5 edges reveal a shrinking of the opening of the observed hysteresis with increasing x-ray flux. Time-dependent measurements show that the exposure of the molecules to x-rays resonant with the Dy M 5 edge accelerates the relaxation of magnetization more than off-resonant x-rays. The results cannot be explained by a homogeneous temperature rise due to x-ray absorption. Moreover, the observed large demagnetization cross sections indicate that the resonant absorption of one x-ray photon induces the demagnetization of many molecules.
This chapter gives an introduction to the epitaxial growth of thin films on solid substrates. The... more This chapter gives an introduction to the epitaxial growth of thin films on solid substrates. The term epitaxy refers to the growth of a crystalline layer on (epi) the surface of a crystalline substrate, where the crystallographic orientation of the substrate surface imposes a crystalline order (taxis) onto the thin film. This implies that film elements can be grown, up to a certain thickness, in crystal structures differing from their bulk. If film and substrate have the same crystal lattices, but different lattice constants, the film will be under strain, that is, it will have a slightly different lattice constant than in its own bulk. Both effects, together with the electronic hybridization at the interface, lead to novel properties. One distinguishes homo-and heteroexpitaxy, where the former refers to the growth on one element on a crystal surface of its own and the latter refers to the more general case, where film and substrate materials are different. Note that the first distinguishes itself from crystal growth, as we will see in more detail later. We start this chapter by giving examples from technology, illustrating where thin epitaxial films are used and outlining potential applications that become reality once we are able to grow the respective thin film sequences. We then contrast thin film and crystal growth, respectively, with kinetics and thermodynamics of growth. We introduce the deposition techniques used in epitaxial thin film growth and then discuss the classical thermodynamic approach, which led to the definition of the growth modes. These modes refer to the morphology taken on by a system grown close to thermodynamic equilibrium. Often films are grown far away from equilibrium and their morphology is determined by kinetics, that is, it is the result of the microscopic path taken by the system during growth. This path is determined by the hierarchy of rates of the single atom, cluster or molecular precursor displacements as compared to the deposition rate. Owing to the importance of kinetics, we focus in the rest of the chapter on the kinetic description of growth. In order to simplify the topic, we start with coverages below one atomic layer that is referred to as a monolayer. The first submonolayer part will be on nucleation, followed by a discussion of island shapes that, very much like snowflakes, tell us about the elementary processes that took place during their formation. We then discuss island coarsening, either by evaporation of atoms from their edges, referred to as the Ostwald ripening, or by the diffusion and subsequent coalescence of entire islands, referred to as the
The three essential pillars of magnetic data storage devices are readability, writeability, and s... more The three essential pillars of magnetic data storage devices are readability, writeability, and stability. However, these requirements compete as magnetic domain sizes reach the fundamental limit of single atoms and molecules. The proven magnetic bistability of individual holmium atoms on magnesium oxide appeared to operate within this magnetic trilemma, sacrificing writeability for unprecedented stability. Using the magnetic stray field created by the tip of a spin-polarized scanning tunneling microscope (SP-STM), we controllably move the Ho state into the quantum regime, allowing us to write its state via the quantum tunneling of magnetization (QTM). We find that the hyperfine interaction causes both the excellent magnetic bistability, even at zero applied magnetic field, and the avoided level crossings which we use to control the magnetic state via QTM. We explore how to use such a system to realize a high-fidelity single atom NOT gate (inverter). Our approach reveals the prospec...
Spin-polarized scanning tunneling microscopy (SP-STM) measures tunnel magnetoresistance (TMR) wit... more Spin-polarized scanning tunneling microscopy (SP-STM) measures tunnel magnetoresistance (TMR) with atomic resolution. While various methods for achieving SP probes have been developed, each is limited with respect to fabrication, performance, and allowed operating conditions. In this study, we present the fabrication and use of SP-STM tips made from commercially available antiferromagnetic Mn_88Ni_12 foil. The tips are intrinsically SP, which is attractive for exploring magnetic phenomena in the zero field limit. The tip material is relatively ductile and straightforward to etch. We benchmark the conventional STM and spectroscopic performance of our tips and demonstrate their spin sensitivity by measuring the two-state switching of holmium single atom magnets on MgO/Ag(100).
Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy r... more Electron spin resonance with a scanning tunneling microscope (ESR-STM) combines the high energy resolution of spin resonance spectroscopy with the atomic scale control and spatial resolution of STM. Here we describe the upgrade of a helium-3 STM with a 2D vector-field magnet (B_z = 8.0 T, B_x = 0.8 T) to an ESR-STM. The system is capable of delivering RF power to the tunnel junction at frequencies up to 30 GHz. We demonstrate magnetic field sweep ESR for the model system TiH/MgO/Ag(100) and find a magnetic moment of (1.004 ± 0.001) μ_B. Our upgrade enables to toggle between a DC mode, where the STM is operated with the regular control electronics, and an ultrafast-pulsed mode that uses an arbitrary waveform generator for pump-probe spectroscopy or reading of spin-states. Both modes allow for simultaneous radiofrequency excitation, which we add via a resistive pick-off tee to the bias voltage path. The RF cabling from room temperature to the 350 mK stage has an average attenuation of...
The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robu... more The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robust when exposed to high magnetic fields and temperatures up to 35 K. Here we address the stability of Ho at small magnetic fields, where the hyperfine interaction creates several avoided level crossings. Using spin-polarized scanning tunneling microscopy, we demonstrate quantum state control via Landau-Zener tunneling and stable magnetization at zero field. Our observations indicate a total spin ground state of J z = ±8. Combined quantum and classical control render Ho a promising qubit candidate.
We present spin-polarized scanning tunneling microscopy measurements demonstrating spin-polarizat... more We present spin-polarized scanning tunneling microscopy measurements demonstrating spin-polarizations of up to 80 % for Co islands on a Pt(111) surface and a tunnel magneto resistance of 850 % between the islands and an anti-ferromagnetic Cr-coated W-tip. These values are stable up to ±0.7 V bias. We report on the magnetic moments and anisotropy energies of two-dimensional Co islands on Pt(111) comprising only a few atoms. Our results show the correlation between orbital moments and magneto-crystalline anisotropies and reveal that both properties strongly depend on the lateral atomic coordination. The anisotropy of single adatoms is found to be 200 times the Co hcp bulk value. We also present well ordered superlattices of Co islands self-assembled on Au(788). The particles have uniaxial out-of-plane magnetization and no dipolar interactions. They present a model system for ultra-high density storage media since they have the most uniform anisotropy energies and the highest density of non-interacting particles so far realized.
We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magn... more We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with 4f n−1 occupancy, and in a divalent state with 4f n occupancy for all other studied RE atoms (n corresponds to the 4f occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.
We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assem... more We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assembled arrays of FeH single molecule magnets on graphene/Ir(111). The energetic positions of molecular states are probed by means of scanning tunneling spectroscopy, showing pronounced long- and short-ranged spatial modulations, indicating the presence of both locally varying intermolecular as well as adsorption-site dependent molecule-substrate interactions. From the magnetic field dependence of the X-ray magnetic circular dichroism signal, we infer that the magnetic easy axis of each FeH molecule is oriented perpendicular to the sample surface and that after the deposition the value of the uniaxial anisotropy is identical to the one in bulk. Our findings therefore suggest that the observed interaction of the molecules with their surrounding does not modify the molecular magnetism, resulting in a two-dimensional array of molecular magnets that retain their bulk magnetic properties.
Stable magnets from single atoms An important goal in molecular magnetism is to create a permanen... more Stable magnets from single atoms An important goal in molecular magnetism is to create a permanent magnet from a single atom. Metal atoms adsorbed on surfaces can develop strong magnetization in an applied field (paramagnetism). Donati et al. show that single holmium atoms adsorbed on a magnesium oxide film grown on a silver substrate show residual magnetism for temperatures up to 30 K and bistabilty that lasts for 1500 s at 10 K (see the Perspective by Khajetoorians and Heinrich). The atom avoids spin relaxation by a combination of quantum-state symmetry and by the oxide film preventing the spin from interacting with the underlying metal via tunneling. Science , this issue p. 318 ; see also p. 296
High-density arrays (5ϫ10 6 cm Ϫ1) of parallel nanowires have been grown using the vicinal Pt͑997... more High-density arrays (5ϫ10 6 cm Ϫ1) of parallel nanowires have been grown using the vicinal Pt͑997͒ surface as a template. Single monatomic rows of Ag and Cu can be deposited with subrow precision. We demonstrate real-time monitoring and characterization of the growth of the atomic chains as a function of temperature by thermal energy helium atom scattering. Scanning tunneling microscopy provides further insight into the structure of the metal rows. Growth mode and alloying with the Pt substrate are discussed as a function of temperature. Our results provide the basis for the creation of surfaces with a uniform distribution of wires having the same average width for the investigation of the electronic, magnetic, and chemical properties of one-dimensional and quasi-one-dimensional metal structures.
Ligand-protected Au clusters are non-bleaching fluorescence markers in bio-and medical applicatio... more Ligand-protected Au clusters are non-bleaching fluorescence markers in bio-and medical applications. Here we show that their fluorescence can be an intrinsic property of the Au cluster itself. We find a very intense and sharp fluorescence peak located at λ = 739.2 nm (1.68 eV) for Au 20 clusters in a Ne matrix held at 6 K. The fluorescence reflects the Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) diabatic bandgap of the cluster. Au 20 shows a very rich absorption fine structure reminiscent of well defined molecule-like quantum levels. These levels are resolved since Au 20 has only one stable isomer (tetrahedral); therefore our sample is mono-disperse in cluster size and conformation. Density-functional theory (DFT) and time-dependent DFT calculations clarify the nature of optical absorption and predict both main absorption peaks and intrinsic fluorescence in fair agreement with experiment.
Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl-dicarbonitrile molecules exh... more 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.
We report on the surface-guided synthesis of an unprecedented dinuclear organocobalt complex, its... more We report on the surface-guided synthesis of an unprecedented dinuclear organocobalt complex, its self-assembly into a complex nanoarchitecture, and a single-molecule level investigation of its switching behavior. Initially, an organic layer is prepared by depositing hexakis((trimethylsilyl)ethynyl)-benzene under ultra-high vacuum conditions onto Ag(111). After Co dosage at 200 Kelvin, low-temperature scanning tunneling microscopy (STM) reveals an epitaxy-mediated organization mechanism of molecules and on-surface formed host-guest complexes. Dinuclear complexes containing two bis(η2-alkynyl) π-tweezer motifs, each stabilizing a single Co atom, express two enantiomers due to a conformation twist. The chirality is transferred to the two-dimensional architecture, whereby its Co adatoms are located at the corners of a 3.4.6.4 rhombitrihexagonal tessellation due to the systematic arrangement and anchoring of the complexes. Extensive density functional theory simulations support our inte...
We have studied Er(trensal) single-ion magnets adsorbed on graphene/Ru(0001), on graphene/Ir(111)... more 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.
We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(1... more 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.
We discuss the structure and size of metal islands formed by atomic-beam deposition onto single c... more We discuss the structure and size of metal islands formed by atomic-beam deposition onto single crystal metal surfaces. Most of the procedures described can straightforwardly be transferred to oxide and semiconductor substrates. We start with the preparation and stability of the smallest islands, namely single atoms. Isolated and immobile adatoms are formed by deposition at substrate temperatures where thermal diffusion is inhibited. In the ideal case this leads to statistical growth, however one often observes slight deviations caused by neighbor driven mobility. We discuss how ordered superlattices of adatoms can be stabilized by using short-range repulsive together with long-range oscillatory interactions between adsorbates. In some examples these interactions add up and become anisotropic, leading to the formation of atomic chains in-midst of atomic terraces. We discuss coarsening by Ostwald ripening or by diffusion of entire islands. Both give rise to more narrow size distributions than nucleation and growth. We show examples for efficient stress relief at steps in monolayer high islands. Finally methods are presented to create bimetallic core-shell islands. These islands are ideally suited to compare physical and chemical properties of step atoms with the ones of higher coordinated surface atoms, or to investigate the role of one-dimensional interfaces between two metals.
Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule ... more Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule magnet DySc 2 N@C 80 at the Dy M 4,5 edges reveal a shrinking of the opening of the observed hysteresis with increasing x-ray flux. Time-dependent measurements show that the exposure of the molecules to x-rays resonant with the Dy M 5 edge accelerates the relaxation of magnetization more than off-resonant x-rays. The results cannot be explained by a homogeneous temperature rise due to x-ray absorption. Moreover, the observed large demagnetization cross sections indicate that the resonant absorption of one x-ray photon induces the demagnetization of many molecules.
This chapter gives an introduction to the epitaxial growth of thin films on solid substrates. The... more This chapter gives an introduction to the epitaxial growth of thin films on solid substrates. The term epitaxy refers to the growth of a crystalline layer on (epi) the surface of a crystalline substrate, where the crystallographic orientation of the substrate surface imposes a crystalline order (taxis) onto the thin film. This implies that film elements can be grown, up to a certain thickness, in crystal structures differing from their bulk. If film and substrate have the same crystal lattices, but different lattice constants, the film will be under strain, that is, it will have a slightly different lattice constant than in its own bulk. Both effects, together with the electronic hybridization at the interface, lead to novel properties. One distinguishes homo-and heteroexpitaxy, where the former refers to the growth on one element on a crystal surface of its own and the latter refers to the more general case, where film and substrate materials are different. Note that the first distinguishes itself from crystal growth, as we will see in more detail later. We start this chapter by giving examples from technology, illustrating where thin epitaxial films are used and outlining potential applications that become reality once we are able to grow the respective thin film sequences. We then contrast thin film and crystal growth, respectively, with kinetics and thermodynamics of growth. We introduce the deposition techniques used in epitaxial thin film growth and then discuss the classical thermodynamic approach, which led to the definition of the growth modes. These modes refer to the morphology taken on by a system grown close to thermodynamic equilibrium. Often films are grown far away from equilibrium and their morphology is determined by kinetics, that is, it is the result of the microscopic path taken by the system during growth. This path is determined by the hierarchy of rates of the single atom, cluster or molecular precursor displacements as compared to the deposition rate. Owing to the importance of kinetics, we focus in the rest of the chapter on the kinetic description of growth. In order to simplify the topic, we start with coverages below one atomic layer that is referred to as a monolayer. The first submonolayer part will be on nucleation, followed by a discussion of island shapes that, very much like snowflakes, tell us about the elementary processes that took place during their formation. We then discuss island coarsening, either by evaporation of atoms from their edges, referred to as the Ostwald ripening, or by the diffusion and subsequent coalescence of entire islands, referred to as the
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Papers by Harald Brune