Itinerant nature of atom-magnetization excitation by tunneling electrons

Single-Spin Sensing: A Molecule-on-Tip Approach

Magnetometry plays a pivotal role in addressing the requirements of ultradense storage technology and overcoming challenges associated with downscaled spin qubits. A promising approach for atomic-scale single-spin sensing involves utilizing a magnetic molecule as a spin sensor, although such a realization is still in its early stages. To tackle this challenge and underscore the potential of this method, we combined a nickelocene molecule with scanning tunneling microscopy to perform versatile spin-sensitive imaging of magnetic surfaces. We investigated model Co islands on Cu(111) of different thicknesses having variable magnetic properties. Our method demonstrates robustness and reproducibility, providing atomic-scale sensitivity to spin polarization and magnetization orientation, owing to a direct exchange coupling between the nickelocene-terminated tip and the Co surfaces. We showcase the accessibility of magnetic exchange maps using this technique, revealing unique signatures in magnetic corrugation, which are well described by computed spin-density maps. These advancements significantly improve our capacity to probe and visualize magnetism at the atomic level.

Electrostatic potentials of atomic nanostructures at metal surfaces quantified by scanning quantum dot microscopy

The discrete and charge-separated nature of matter - electrons and nuclei - results in local electrostatic fields that are ubiquitous in nanoscale structures and relevant in catalysis, nanoelectronics and quantum nanoscience. Surface-averaging techniques provide only limited experimental access to these potentials, which are determined by the shape, material, and environment of the nanostructure. Here, we image the potential over adatoms, chains, and clusters of Ag and Au atoms assembled on Ag(111) and quantify their surface dipole moments. By focusing on the total charge density, these data establish a benchmark for theory. Our density functional theory calculations show a very good agreement with experiment and allow a deeper analysis of the dipole formation mechanisms, their dependence on fundamental atomic properties and on the shape of the nanostructures. We formulate an intuitive picture of the basic mechanisms behind dipole formation, allowing better design choices for future nanoscale systems such as single-atom catalysts.

Manipulating the Spin Orientation of Co Atoms Using Monatomic Cu Chains

Harnessing the spin of single atoms is at the heart of quantum information nanotechnology based on magnetic concepts. By attaching single Co atoms to monatomic Cu chains, we demonstrate the ability to control the spin orientation by the atomic environment. Due to spin-orbit coupling (SOC), the spin is tilted by ≈58° from the surface normal toward the chain as evidenced by inelastic tunneling spectroscopy. These findings are reproduced by density functional theory calculations and have implications for Co atoms on pristine Cu(111), which are believed to be Kondo systems. Our quantum Monte Carlo calculations suggest that SOC suppresses the Kondo effect of Co atoms at chains and on the flat surface. Our work impacts the fundamental understanding of low-energy excitations in nanostructures on surfaces and demonstrates the ability to manipulate atomic-scale magnetic moments, which can have tremendous implications for quantum devices.

Controlling the Spin States of FeTBrPP on Au(111)

Spin-flip excitations of iron porphyrin molecules on Au(111) are investigated with a low-temperature scanning tunneling microscope. The molecules adopt two distinct adsorption configurations on the surface that exhibit different magnetic anisotropy energies. Density functional theory calculations show that the different structures and excitation energies reflect unlike occupations of the Fe 3d levels. We demonstrate that the magnetic anisotropy energy can be controlled by changing the adsorption site, the orientation, or the tip-molecule distance.

Extending the Spin Excitation Lifetime of a Magnetic Molecule on a Proximitized Superconductor

Molecular spins on surfaces potentially used in quantum information processing and data storage require long spin excitation lifetimes. Normally, coupling of the molecular spin with the conduction electrons of metallic surfaces causes fast relaxation of spin excitations. However, the presence of superconducting pairing effects in the substrate can protect the excited spin from decaying. In this work, we show that a proximity-induced superconducting gold film can sustain spin excitations of a FeTPP-Cl molecule for more than 80 ns. This long value was determined by studying inelastic spin excitations of the S = 5/2 multiplet of FeTPP-Cl on Au films over V(100) using scanning tunneling spectroscopy. The spin lifetime decreases with increasing film thickness, along with the decrease of the effective superconducting gap. Our results elucidate the use of proximitized gold electrodes for addressing quantum spins on surfaces, envisioning new routes for tuning the value of their spin lifetime.

Anomalous excitations of atomically crafted quantum magnets

High-energy resolution spectroscopic studies of quantum magnets proved extremely valuable in accessing magnetodynamics quantities, such as energy barriers, magnetic interactions, and lifetime of excited states. Here, we investigate a previously unexplored flavor of low-energy spin excitations for quantum spins coupled to an electron bath. In sharp contrast to the usual tunneling signature of two steps symmetrically centered around the Fermi level, we find a single step in the conductance. Combining time-dependent and many-body perturbation theories, magnetic field-dependent tunneling spectra are explained as the result of an interplay between weak magnetic anisotropy energy, magnetic interactions, and Stoner-like electron-hole excitations that are strongly dependent on the magnetic states of the nanostructures. The results are rationalized in terms of a noncollinear magnetic ground state and the dominance of ferro- and antiferromagnetic interactions. The atomically crafted nanomagnets offer an appealing model for the exploration of electrically pumped spin systems.

Moiré Tuning of Spin Excitations: Individual Fe Atoms on MoS_{2}/Au(111)

Magnetic adatoms on properly designed surfaces constitute exquisite systems for addressing, controlling, and manipulating single quantum spins. Here, we show that monolayers of MoS_{2} on a Au(111) surface provide a versatile platform for controllably tuning the coupling between adatom spins and substrate electrons. Even for equivalent adsorption sites with respect to the atomic MoS_{2} lattice, we observe that Fe adatoms exhibit behaviors ranging from pure spin excitations, characteristic of negligible exchange and dominant single-ion anisotropy, to a fully developed Kondo resonance, indicating strong exchange and negligible single-ion anisotropy. This tunability emerges from a moiré structure of MoS_{2} on Au(111) in conjunction with pronounced many-body renormalizations. We also find striking spectral variations in the immediate vicinity of the Fe atoms, which we explain by quantum interference reflecting the formation of Fe-S hybrid states despite the nominally inert nature of the substrate. Our work establishes monolayer MoS_{2} as a tuning layer for adjusting the quantum spin properties over an extraordinarily broad parameter range. The considerable variability can be exploited for quantum spin manipulations.

Comparative study of adsorptions, reactions and electronic properties of U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111).

Large magnetic anisotropy in an OsIr dimer anchored in defective graphene

Single-atom magnets represent the ultimate limit of magnetic data storage. The identification of substrates that anchor atom-sized magnets firmly and, thus, prevent their diffusion and large magnetic anisotropy has been at the centre of intense research efforts for a long time. Using density functional theory we show the binding of transition metal (TM) atoms in defect sites in the graphene lattice: single vacancy and double vacancy, both pristine and decorated by pyridinic nitrogen atoms, are energetically more favourable than away from the centre of defects, which could be used for engineering the position of TMs with atomic precision. Relativistic calculations revealed magnetic anisotropy energy (MAE) of ∼10 meV for Ir@NSV with an easy axis parallel to the graphene plane. MAE can be remarkably boosted to 50 meV for OsIr@NSV with the easy axis perpendicular to the graphene plane, which paves the way to the storage density of ∼490 Tb/inch²with the blocking temperature of 14 K assuming the relaxation time of 10 years. Magnetic anisotropy is discussed based on the relativistic electronic structures. The influence of an orbital-dependent on-site Coulomb repulsionUand a non-local correlation functional optB86b-vdW on MAE is also discussed.

Correlating Josephson supercurrents and Shiba states in quantum spins unconventionally coupled to superconductors

Local spins coupled to superconductors give rise to several emerging phenomena directly linked to the competition between Cooper pair formation and magnetic exchange. These effects are generally scrutinized using a spectroscopic approach which relies on detecting the in-gap bound modes arising from Cooper pair breaking, the so-called Yu-Shiba-Rusinov (YSR) states. However, the impact of local magnetic impurities on the superconducting order parameter remains largely unexplored. Here, we use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, we reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, we detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy. Our findings have far reaching implications for phenomena that rely on the interplay between quantum spins and superconductivity.

The impact of local magnetic impurities on superconducting order parameter remains largely unexplored. Here, the authors visualize the effect of different magnetic perturbations on a superconductor, unveiling a rich correlation of the interplay between quantum spins and superconductivity in different spectroscopic regimes.

A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces

Many-body phenomena are paramount in physics. In condensed matter, their hallmark is considerable on a wide range of material characteristics spanning electronic, magnetic, thermodynamic and transport properties. They potentially imprint non-trivial signatures in spectroscopic measurements, such as those assigned to Kondo, excitonic and polaronic features, whose emergence depends on the involved degrees of freedom. Here, we address systematically zero-bias anomalies detected by scanning tunneling spectroscopy on Co atoms deposited on Cu, Ag and Au(111) substrates, which remarkably are almost identical to those obtained from first-principles. These features originate from gaped spin-excitations induced by a finite magnetic anisotropy energy, in contrast to the usual widespread interpretation relating them to Kondo resonances. Resting on relativistic time-dependent density functional and many-body perturbation theories, we furthermore unveil a new many-body feature, the spinaron, resulting from the interaction of electrons and spin-excitations localizing electronic states in a well defined energy.

The Kondo Effect of a Molecular Tip As a Magnetic Sensor

A single molecule offers to tailor and control the probing capability of a scanning tunneling microscope when placed on the tip. With the help of first-principles calculations, we show that on-tip spin sensitivity is possible through the Kondo ground state of a spin S = 1/2 cobaltocene molecule. When attached to the tip apex, we observe a reproducible Kondo resonance, which splits apart upon tuning the exchange coupling of cobaltocene to an iron atom on the surface. The spin-split Kondo resonance provides quantitative information on the exchange field and on the spin polarization of the iron atom. We also demonstrate that molecular vibrations cause the emergence of Kondo side peaks, which, unlike the Kondo resonance, are sensitive to cobaltocene adsorption.

Antiferromagnetic MnNi tips for spin-polarized scanning probe microscopy

Spin-polarized scanning tunneling microscopy (SP-STM) measures magnetoresistance with atomic resolution. While various methods for achieving SP probes have been developed, each is limited with respect to fabrication, performance, and operating conditions. In this study, we present the fabrication and use of SP-STM tips made from commercially available antiferromagnetic Mn₈₈Ni₁₂ foils. The tips are intrinsically SP, which is attractive for exploring magnetic phenomena in the zero field limit. The tip material is relatively ductile, is straightforward to etch, and has a Néel temperature exceeding 300 K. We benchmark the topographic 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).

Spin-fluctuation and spin-relaxation effects of single adatoms from first principles

Single adatoms offer an exceptional playground for studying magnetism and its associated dynamics at the atomic scale. Here we review recent results on single adatoms deposited on metallic substrates, based on time-dependent density functional theory. First we analyze quantum zero-point spin-fluctuations (ZPSF) as calculated from the fluctuation-dissipation theorem, and show how they affect the magnetic stability by modifying the magnetic anisotropy energy. We also assess the impact of ZPSF in the limit of small hybridization to the substrate characteristic of semi-insulating substrates, connecting to recent experimental investigations where magnetic stability of a single adatom was achieved for the first time. Secondly, we inspect further the dynamics of single adatoms by considering the longitudinal and transverse spin-relaxation processes, whose time-scales are analyzed and related to the underlying electronic structure of both the adatom and the substrate. Thirdly, we analyze spin-fluctuation modes of paramagnetic adatoms, i.e. adatoms where the Stoner criterion for magnetism is almost fulfilled. Interestingly, such modes can develop well-defined peaks in the meV range, their main characteristics being determined by two fundamental electronic properties, namely the Stoner parameter and the density of states at the Fermi level. Furthermore, simulated inelastic scanning tunneling spectroscopy curves reveal that these spin-fluctuation modes can be triggered by tunneling electrons, opening up potential applications also for paramagnetic adatoms. Lastly, an overview of the outstanding issues and future directions is given.

Long Spin-Relaxation Times in a Transition-Metal Atom in Direct Contact to a Metal Substrate

Long spin-relaxation times are a prerequisite for the use of spins in data storage or nanospintronics technologies. An atomic-scale solid-state realization of such a system is the spin of a transition-metal atom adsorbed on a suitable substrate. For the case of a metallic substrate, which enables the direct addressing of the spin by conduction electrons, the experimentally measured lifetimes reported to date are on the order of only hundreds of femtoseconds. Here, we show that the spin states of iron atoms adsorbed directly on a conductive platinum substrate have a surprisingly long spin-relaxation time in the nanosecond regime, which is comparable to that of a transition metal atom decoupled from the substrate electrons by a thin decoupling layer. The combination of long spin-relaxation times and strong coupling to conduction electrons implies the possibility to use flexible coupling schemes to process the spin information.

A chemist's overview of surface electron spins

This review summarizes recent research progress in the measurement and tuning of the electron spins of alien atoms and molecules adsorbed on well-defined substrates. After a brief introduction to the main experimental techniques employed to study surface electron spins, some well-explored systems consisting of atomic and molecular spin-carriers at surfaces are overviewed from a chemist's viewpoint, focusing on the experimental measurements and chemical modifications of the electron spin states of the alien entities at the surfaces on the atomic/molecular level. Finally, personal perspectives have been provided, aiming at describing some of the remaining issues that need to be addressed in the future and proposing potential applications in surface chemistry.

Design and performance of an ultra-high vacuum spin-polarized scanning tunneling microscope operating at 30 mK and in a vector magnetic field

We describe the design and performance of a scanning tunneling microscope (STM) that operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).

A gateway towards non-collinear spin processing using three-atom magnets with strong substrate coupling

A cluster of a few magnetic atoms on the surface of a nonmagnetic substrate is one suitable realization of a bit for spin-based information technology. The prevalent approach to achieve magnetic stability is decoupling the cluster spin from substrate conduction electrons in order to suppress destabilizing spin-flips. However, this route entails less flexibility in tailoring the coupling between the bits needed for spin-processing. Here, we use a spin-resolved scanning tunneling microscope to write, read, and store spin information for hours in clusters of three atoms strongly coupled to a substrate featuring a cloud of non-collinearly polarized host atoms, a so-called non-collinear giant moment cluster. The giant moment cluster can be driven into a Kondo screened state by simply moving one of its atoms to a different site. Using the exceptional atomic tunability of the non-collinear substrate mediated Dzyaloshinskii–Moriya interaction, we propose a logical scheme for a four-state memory.

Information technology based on few atom magnets requires both long spin-energy relaxation times and flexible inter-bit coupling. Here, the authors show routes to manipulate information in three-atom clusters strongly coupled to substrate electrons by exploiting Dzyaloshinskii–Moriya interactions.

Tuning Paramagnetic Spin Excitations of Single Adatoms

We predict the existence of paramagnetic spin excitations (PSE) in nonmagnetic single adatoms. Our calculations demonstrate that PSE develop a well-defined structure in the meV region when the adatom's Stoner criterion for magnetism is close to the critical point. We further reveal a subtle tunability and enhancement of PSE by external magnetic fields. Finally, we show how PSE can be detected as moving steps in the dI/dV signal of inelastic scanning tunneling spectroscopy, opening a potential route for experimentally accessing electronic properties of nonmagnetic adatoms, such as the Stoner parameter.

The geometric phase of Zn- and T-symmetric nanomagnets as a classification toolkit

We derive the general form of the non-trivial geometric phase resulting from the unique combination of point group and time reversal symmetries. This phase arises e.g. when a magnetic adatom is adsorbed on a non-magnetic C_n crystal surface, where n denotes the fold of the principal axis. The energetic ordering and the relevant quantum numbers of the eigenstates are entirely determined by this quantity. Moreover, this phase allows to conveniently predict the protection mechanism of any prepared state, shedding light onto a large number of experiments and allowing a classification scheme. Owing to its robustness this geometric phase also has great relevance for a large number of applications in quantum computing, where topologically protected states bearing long relaxation times are highly desired.

Controlling electronic access to the spin excitations of a single molecule in a tunnel junction

Spintronic phenomena underpin new device paradigms for data storage and sensing. Scaling these down to the single molecule level requires controlling the properties of current-carrying molecular orbitals to enable access to spin states through phenomena such as inelastic electron tunnelling. Here we show that the spintronic properties of a tunnel junction containing a single molecule can be controlled using the local environment as a pseudo-gate. For tunnelling through iron phthalocyanine (FePc) on an insulating copper nitride (Cu₂N) monolayer above Cu(001), we find that spin transitions may be strongly excited depending on the binding site of the central Fe atom. Different interactions between the Fe and the underlying Cu or N atoms shift the Fe d orbitals with respect to the Fermi energy and control the relative strength of the spin excitations; this effect is captured in a simple co-tunnelling model. This work demonstrates the importance of the atomic-scale environment for the development of single molecule spintronic devices.

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.

Zero-Point Spin-Fluctuations of Single Adatoms

Stabilizing the magnetic signal of single adatoms is a crucial step toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. The quantum mechanical nature of these tiny objects, however, introduces intrinsic zero-point spin-fluctuations that tend to destabilize the local magnetic moment of interest by dwindling the magnetic anisotropy potential barrier even at absolute zero temperature. Here, we elucidate the origins and quantify the effect of the fundamental ingredients determining the magnitude of the fluctuations, namely, the (i) local magnetic moment, (ii) spin-orbit coupling, and (iii) electron-hole Stoner excitations. Based on a systematic first-principles study of 3d and 4d adatoms, we demonstrate that the transverse contribution of the fluctuations is comparable in size to the magnetic moment itself, leading to a remarkable ≳50% reduction of the magnetic anisotropy energy. Our analysis gives rise to a comprehensible diagram relating the fluctuation magnitude to characteristic features of adatoms, providing practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations.

Magnetic remanence in single atoms

A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

Absence of a spin-signature from a single Ho adatom as probed by spin-sensitive tunneling

Whether rare-earth materials can be used as single-atom magnetic memory is an ongoing debate in recent literature. Here we show, by inelastic and spin-resolved scanning tunnelling-based methods, that we observe a strong magnetic signal and excitation from Fe atoms adsorbed on Pt(111), but see no signatures of magnetic excitation or spin-based telegraph noise for Ho atoms. Moreover, we observe that the indirect exchange field produced by a single Ho atom is negligible, as sensed by nearby Fe atoms. We demonstrate, using ab initio methods, that this stems from a comparatively weak coupling of the Ho 4f electrons with both tunnelling electrons and substrate-derived itinerant electrons, making both magnetic coupling and detection very difficult when compared to 3d elements. We discuss these results in the context of ongoing disputes and clarify important controversies.

Magnetic stability of holmium atoms on a platinum(111) surface has recently been reported, raising prospects for atomic-scale spintronics, however contradictory results have since emerged. Here, Steinbrecher et al. find evidence for an invisibility of the holmium spin to scanning tunnelling spectroscopy techniques which challenges recent results.

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0±0.3  meV/atom. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

Spin polarization of the split Kondo state

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin excitations and two Kondo peaks positioned symmetrically around the Fermi energy. We fit the spin state of the Kondo-screened atom with a spin Hamiltonian independent of the Kondo effect and account for Zeeman splitting of the Kondo peak in the magnetic field. We find that the width and the height of the Kondo peaks scales with the Zeeman energy. Our observations are consistent with full spin polarization of the Kondo peaks, i.e., a majority spin peak below the Fermi energy and a minority spin peak above.

Resonant-enhanced spectroscopy of molecular rotations with a scanning tunneling microscope

We use rotational excitation spectroscopy with a scanning tunneling microscope to investigate the rotational properties of molecular hydrogen and its isotopes physisorbed on the surfaces of graphene and hexagonal boron nitride (h-BN), grown on Ni(111), Ru(0001), and Rh(111). The rotational excitation energies are in good agreement with ΔJ = 2 transitions of freely spinning p-H2 and o-D2 molecules. The variations of the spectral line shapes for H2 among the different surfaces can be traced back to a molecular resonance-mediated tunneling mechanism. Our data for H2/h-BN/Rh(111) suggest a local intrinsic gating on this surface due to lateral static dipoles. Spectra on a mixed monolayer of H2, HD, and D2 display all three J = 0 → 2 rotational transitions, irrespective of tip position, thus pointing to a multimolecule excitation, or molecular mobility in the physisorbed close-packed layer.

Non-collinear magnetism induced by frustration in transition-metal nanostructures deposited on surfaces

How does magnetism behave when the physical dimension is reduced to the size of nanostructures? The multiplicity of magnetic states in these systems can be very rich, in that their properties depend on the atomic species, the cluster size, shape and symmetry or choice of the substrate. Small variations of the cluster parameters may change the properties dramatically. Research in this field has gained much by the many novel experimental methods and techniques exhibiting atomic resolution. Here we review the ab-initio approach, focusing on recent calculations on magnetic frustration and occurrence of non-collinear magnetism in antiferromagnetic nanostructures deposited on surfaces.

Magnetic response of zigzag nanoribbons under electric fields

Spin excitations in zigzag graphene nanoribbons are studied when the system is subjected to an electric field in the transversal direction. The magnetic properties and the lifetime of the spin excitations are systematically investigated and compared using a tight-binding electron-electron model treated by a mean-field Hubbard model. The effects of electron-hole asymmetry introduced by next-nearest neighbor hopping are also investigated. We show that by increasing the electric field, the antiferromagnetic correlations between the edges of the nanoribbons are decreased due to a reduction of the magnetic moments. The results show that the spin wave lifetime may be controlled by the intensity of the transversal electric field, indicating that zigzag nanoribbons may be considered great candidates for future spintronic applications.

Reaching the magnetic anisotropy limit of a 3d metal atom

Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

Standing spin waves in ultrathin magnetic films: a method to test for layer-dependent exchange coupling

We introduce a method to test theoretical models for the layer dependence of exchange coupling constants in ultrathin magnetic films. The method is based on the observation of high-energy and high-momentum standing spin wave modes using high-resolution electron energy loss spectroscopy. Experimental data are presented for 5-8 layers of fcc cobalt deposited on Cu(100). The power of the method is illustrated by comparison to two theoretical studies predicting rather different results concerning the ratio of the interlayer and intralayer exchange coupling constants near the surface. Only the theory with a large interlayer coupling shows sufficient energy spreading in the layer dependence of the dispersion curves to match the experimental data. We furthermore discuss the reason for the surprising success of the simple nearest-neighbor Heisenberg model with a single exchange constant matched to experiment.

Magnetic moment and anisotropy of individual Co atoms on graphene

We report on the magnetic properties of single Co atoms on graphene on Pt(111). By means of scanning tunneling microscopy spin-excitation spectroscopy, we infer a magnetic anisotropy of K=-8.1  meV with out-of-plane hard axis and a magnetic moment of 2.2μ(B). Co adsorbs on the sixfold graphene hollow site. Upon hydrogen adsorption, three differently hydrogenated species are identified. Their magnetic properties are very different from those of clean Co. Ab initio calculations support our results and reveal that the large magnetic anisotropy stems from strong ligand field effects due to the interaction between Co and graphene orbitals.

Spin excitations of individual Fe atoms on Pt(111): impact of the site-dependent giant substrate polarization

We demonstrate using inelastic scanning tunneling spectroscopy and simulations based on density functional theory that the amplitude and sign of the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111) surface can be manipulated by modifying the adatom binding site. Since the magnitude of the measured anisotropy is remarkably small, up to an order of magnitude smaller than previously reported, electron-hole excitations are weak and thus the spin excitation exhibits long lived precessional lifetimes compared to the values found for the same adatom on noble metal surfaces.

Current-Driven Spin Dynamics of Artificially Constructed Quantum Magnets

The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.

Magnetic excitations of rare earth atoms and clusters on metallic surfaces

Magnetic anisotropy and magnetization dynamics of rare earth Gd atoms and dimers on Pt(111) and Cu(111) were investigated with inelastic tunneling spectroscopy. The spin excitation spectra reveal that giant magnetic anisotropies and lifetimes of the excited states of Gd are nearly independent of the supporting surfaces and the cluster size. In combination with theoretical calculations, we argue that the observed features are caused by strongly localized character of 4f electrons in Gd atoms and clusters.

Relaxation time of terahertz magnons excited at ferromagnetic surfaces

The temporal and spatial properties of terahertz magnons excited at ferromagnetic fcc Co(100) and bcc Fe(110) surfaces are investigated experimentally. The magnon lifetime is found to be a few tens of femtoseconds at low wave vectors, which reduces significantly as the wave vector approaches the Brillouin zone boundary. Surprisingly, the lifetime is very similar in both systems, in spite of the fact that the excitation energy in the Co(100) film is by a factor of two larger than in the Fe(110) film. The magnon wave packets propagate only a few nanometers within their lifetime. In addition to the fact that our results describe the damping mechanism in ultrafast time scales, they may provide a way to predict the ultimate time scale of magnetic switching in nanostructures.

In-plane magnetic anisotropy of Fe atoms on Bi2Se3(111)

The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on the prototypical 3D topological insulator Bi(2)Se(3) using local low-temperature scanning tunneling spectroscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi(2)Se(3) surface, in the coverage range ≈ 1% of a monolayer, are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface plane, contrary to what was assumed in recent investigations on the supposed opening of a gap. Using ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.

Graphene as a non-magnetic spin current lens

In spintronics, the ability to transport magnetic information often depends on the existence of a spin current traveling between two different magnetic objects acting as the source and probe. A large fraction of this information never reaches the probe and is lost because the spin current tends to travel omnidirectionally. We propose that a curved boundary between a gated and a non-gated region within graphene acts as an ideal lens for spin currents despite being entirely of non-magnetic nature. We show as a proof of concept that such lenses can be utilized to redirect the spin current that travels away from a source onto a focus region where a magnetic probe is located, saving a considerable fraction of the magnetic information that would be otherwise lost.