1,721,218 research outputs found
Scanning tunneling spectroscopy of subsurface Ag and Ge impurities in copper
No full textAbstract We investigate single Ge and Ag impurities buried below a Cu(100) surface using low temperature scanning tunneling microscopy. The interference patterns in the local density of states are surface scattering signatures of the bulk impurities, which result from 3D Friedel oscillations and the electron focusing effect. Comparing the isoelectronic d scatterer Ag and the sp scatterer Ge allows to distinguish contributions from impurity scattering and the host. Energy-independent effective scattering phase shifts are extracted using a plane wave tight-binding model and reveal similar values for both species. A comparison with ab initio calculations suggests incoherent sp scattering processes at the Ge impurity. As both scatterers are spectrally homogeneous, scanning tunneling spectroscopy of the interference patterns yields real-space signatures of the bulk electronic structure. We find a kink around zero bias for both species that we assign to a renormalization of the band structure due to many-body effects, which can be described with a Debye self-energy and a surprisingly high electron–phonon coupling parameter λ . We propose that this might originate from bulk propagation in the vicinity of the surface.Abstract We investigate single Ge and Ag impurities buried below a Cu(100) surface using low temperature scanning tunneling microscopy. The interference patterns in the local density of states are surface scattering signatures of the bulk impurities, which result from 3D Friedel oscillations and the electron focusing effect. Comparing the isoelectronic d scatterer Ag and the sp scatterer Ge allows to distinguish contributions from impurity scattering and the host. Energy-independent effective scattering phase shifts are extracted using a plane wave tight-binding model and reveal similar values for both species. A comparison with ab initio calculations suggests incoherent sp scattering processes at the Ge impurity. As both scatterers are spectrally homogeneous, scanning tunneling spectroscopy of the interference patterns yields real-space signatures of the bulk electronic structure. We find a kink around zero bias for both species that we assign to a renormalization of the band structure due to many-body effects, which can be described with a Debye self-energy and a surprisingly high electron–phonon coupling parameter λ . We propose that this might originate from bulk propagation in the vicinity of the surface.Open-Access-Publikationsfonds 202
Interplay between the Kondo effect and the Ruderman–Kittel–Kasuya–Yosida interaction
No full textThe interplay between the Ruderman–Kittel–Kasuya–Yosida interaction and the Kondo effect is expected to provide the driving force for the emergence of many phenomena in strongly correlated electron materials. Two magnetic impurities in a metal are the smallest possible system containing all these ingredients and define a bottom-up approach towards a long-term understanding of concentrated/dense systems. Here we report on the experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of low-temperature scanning tunnelling spectroscopy combined with density functional theory and numerical renormalization group calculations. The Kondo effect, in particular the width of the Abrikosov–Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. It oscillates as a function of dimer separation revealing that it is related to indirect exchange interactions mediated by the conduction electrons
High frequency STM and spin polarized STM on magnetic vortices
No full textThis thesis describes the design and performance of a newly developed high frequency scanning tunneling microscope (HF-STM), suitable for measurements in ultra high vacuum (UHV) at a base temperature of 6K and with magnetic fields up to 7T. The key concept of HF-STM is to employ a high frequency voltage signal on the order of a GHz as tunneling voltage. This time dependent tunneling voltage can excite sample dynamics, e.g., by altering the local chemical potential or by driving a time dependent tunneling current. The dynamic sample response can be measured as a change in the direct tunnel current. The STM is developed in three steps: The first version of the STM is specifically optimized towards application of high frequency voltage signals to the tunneling tip. The tip is connected to a high frequency plug, with the counterpart rigidly mounted to the STM body. Thus, the tip can be exchanged without breaking the UHV. The performance of this STM is tested on a table-top setup and shows a time resolution of 120ps together with atomic spatial resolution. The time resolution is measured by a pulse autocorrelation technique, where a time averaged tunnel current is measured with respect to the time delay between two tunnel voltage pulses. The autocorrelation shows an additional oscillation, which can be attributed to a resonance of the electric field in the microscope cavity. In the next step, an x/y-coarse positioning stage is integrated into the tip module of the microscope. Its field of view of 2x2mm requires the use of a flexible coaxial cable for the tunneling voltage signal. This microscope is integrated into the UHV-system with helium bath cryostat, using a semi rigid coaxial cable for HF wiring of the microscope insert. The HF performance of this configuration is tested at 77K with liquid nitrogen as cooling agent and shows a slightly degraded time resolution of 160ps. At the final base temperature of 6K, the coarse positioning stage was not working due to the increased stiffness of the flexible coaxial wire. This required a third development step, where the outer insulator of the coaxial cable is stripped on a length of 1cm to increase the cable flexibility. The final performance test reveals, that the cable stripping reduced the time resolution to approximately 320ps. The autocorrelation shows distinct side peaks, which can be attributed to multiple resonances between STM tip and the stripped part of the flexible coaxial cable. Measurement of the real part of the transfer function between signal generator and tunnel junction reveals a bandwidth of only 200MHz.In the second part of this thesis, the interaction of a magnetic texture with atomic size defects is investigated. For this, iron nano-islands with a diameter of approx. 300nm and a height of approx. 10nm are prepared by electron beam evaporation of Fe onto a clean tungsten (110) substrate in UHV. These islands exhibit a magnetic vortex, which is characterized by a curled magnetization on the outskirt of the island and an out-of-plane magnetized core (diameter of 12nm) in the center. The core is basically a zero-dimensional domain wall, that can be positioned laterally in both directions by application of a proportional in-plane magnetic field. Furthermore, it can be compressed in diameter by a perpendicular field to increase the exchange energy density and, thus, the interaction strength with the defects. Spin polarized STM is used to image not only the atomic size adsorbates on the surface, but also to map the magnetization of the island. This way, the vortex core position and the defect position can be correlated with atomic precision. The interaction between the vortex core and oxygen adsorbates on the island surface is investigated by forcing the vortex core by the in-plane magnetic field along a well defined path and recording, how the core position is affected by the adsorbates. It is observed, that the displacement rate dr/dB of the core is reduced, when the core is pinned by an adsorbate. The pinning force increases, when the core is reduced in diameter to a diameter of 3.8nm by a perpendicular field of -1.5T, which increases its exchange energy density. By comparison with micromagnetic simulations, the pinning energy of the squeezed core with respect to a single oxygen adatom is determined to be 221meV. Ab initio based calculations show, that the adatoms induce an anisotropic exchange interaction in the neighboring substrate atoms, which explains the observed eccentric pinning position of the core
First-principles study of collective spin excitations in noncollinear magnets
No full textThe pace of the current data revolution depends on the world's technological capability to store and process information. A great share of that is done by manipulating magnetic materials with astonishing speed and precision, which involves several dynamical processes. Among the latter are the collective spin excitations known as spin waves. Just like the strings of a guitar, spin waves are the natural "tunes" of a material's magnetization, and knowing their properties allows to predict, design and control technological devices. In this thesis, we study the properties of spin waves in complex magnets focusing on systems of low-dimensionality. The manifestation of spin waves in collinear magnets, such as ferromagnets, has been extensively investigated. However, spin waves in noncollinear magnets are not fully understood yet. For instance, no experimental data is available concerning large-wavevector spin waves in thin films and surfaces. Nevertheless, novel noncollinear spin textures, such as the topologically nontrivial skyrmions, are at the heart of many recent proposals of information nanotechnologies for the future. Therefore, we develop in this thesis an atomistic description of the spin waves in noncollinear magnets applicable to real materials. We achieve that by combining the density functional theory, as implemented within the Korringa-Kohn-Rostoker method, with the spin-wave adiabatic approximation. Effectively, we parametrize from first-principles a generalized quantum Heisenberg Hamiltonian accounting for relativistic effects of the spin-orbit coupling. Thus, besides calculating the magnetic exchange interaction, we also have access to the Dzyaloshinskii-Moriya interaction(DMI) and the magneto crystalline anisotropy. To further relate our results with experimental works, we calculate the inelastic-electron-scattering spectrum using timedependent perturbation theory. This led us to propose spin-resolved electron-energy-loss spectroscopy (SREELS) as an experimental tool to probe large-wavevector spin waves in noncollinear magnets. [...
Spin-orbitronics at the nanoscale: From analytical models to real materials
No full textThis thesis provides a theoretical description of magnetic nanostructures in inversion-asymmetric environments with strong spin-orbit interaction (SOI). The theoretical concepts introduced here can be applied in the field of spin-orbitronics, which consists ofexploiting the SOI to manipulate the electron spin without external magnetic fields. The investigated systems display a plethora of interesting phenomena ranging from chiral magnetic interactions to gapped magnetic excitations. In practice, we adopt two different approaches: First, a model-based one relying on the Rashba Hamiltonian, which is employed to demystify and understand magnetic and transport properties of magnetic nanostructures embedded in a Rashba electron gas. Second, we use a first-principles approach within the framework of the Korringa-Kohn-Rostoker (KKR) Green function method to investigate the ground state properties of magnetic impurities in topologically insulating hosts. This method is suitable to simulate nanostructures in real space. Then, we employed our newly developed code based on time-dependent density functional theory to compute the spin excitation spectra of these magnetic nanostructures embedded in topological insulators. Moreover, the KKR Green function method was used to simulate the electronic structure and ground state properties of large magnetic nanostructures, namely magnetic Skyrmions. In the first part, the analytical Rashba Green function and the scattering matrices modeling the magnetic impurities in the s-wave approximation are employed for the computation of the magnetic interaction tensor which contains: isotropic exchange, Dzyaloshinskii-Moriya (DM) and pseudo-dipolar interactions. The competition between these interactions leads to a rich phase diagram depending on the distance between the magnetic impurities. Next, we consider an external perturbing electric field and investigate the transport properties by computing the residual resistivity tensor within linear response theory. The contribution of SOI is explored. The investigation of arbitrary orientations of the impurity magnetic moment allowed a detailed analysis of contributions from the anisotropic magnetoresistance and planar Hall effect. Moreover, we calculate the impurity induced bound currents in the Rashba electron gas, which are used to compute the induced orbital magnetization. For a trimer of impurities with a non-vanishing spin chirality (SC) a finite orbital magnetization is observed when SOI is turned off. Since it emerges from the SC, it was named chiral orbital magnetization. [...
Sub 1K UHV scanning tunneling microscope made of shapal and mapping of magnetic skyrmion collapse rates
No full textThis thesis is separated into two main parts. The first part describes the design and performance of an ultra high vacuum (UHV) scanning tunneling microscope (STM) system proven to be operating at a continuous base temperature of 1.1 K in magnetic fields up to 3 T. Via exchanging the He with He in the Joule-Thomson closed cycle it is anticipated to reach 0.5 K in the future. The concept of a versatile multi-chamber system hosting the ability for preparation of STM tips and samples of different sizes aims to give access to a wider group of possible users. For in-situ preparation the system is equipped with a commercially available ion source, home built in-situ exchangeable electron beam evaporators and different heating stages for STM tips or samples up to 2700 K.Surface analysis can be carried out via an implemented LEED/Auger unit and a quadrupole mass spectrometer. The design and implementation into the cryostat of the ceramic-based STM is discussed in detail. It features a capacitive position readout and the ability to guide high frequency voltage pulses up to 10 GHz down to the tunneling junction. A time resolution of 420 ps and a noise level between tip and sample of pm at a bandwidth of 1 KHz is determined. Evaluating the superconducting gap of an evaporated Pd layer on a W(110) crystal verifies an energy resolution of 0.4 meV at 1.2 K. The suppression of external vibrations and of acoustic as well as electrical noise is realized by an advanced insulation concept. This features a 70 ton concrete room which is decoupled from the building and is hosting a specially designed frame for the instrument employing further active and passive damping stages. The instrument could not be finalized in this thesis due to a not functioning magnet delivered by the company, such that further measurements on magnetic skyrmions had to be performed at a different system. Hence, part of the characterization of the instrument is not final. In the second part of this thesis, the influence of an in-plane magnetic field on the collapse and creation of magnetic skyrmions in the palladium/ iron (Pd/Fe) bilayer on a clean Iridium (111) surface is analysed. Under magnetic fields parallel to the surface normal this system hosts magnetic skyrmions, characterized by a non-collinear spin arrangement which is topologically protected by a local integer winding number. On a discrete atomic lattice the protection is incomplete but still manifests as a large energy barrier between the metastable skyrmion state and the ferromagnetic uniform ordering. By the application of an in-plane magnetic field up to 3 T i could tune the collapse rate of the skyrmions by up to four orders of magnitude, while the creation rate was barely affected. The comparison of these results with theoretically obtained rates assuming a quasi equilibrium Arrhenius behaviour exhibits a good agreement with our findings. This implies a thermal model of the impact of a single tunneling electron that just shortly heats part of the skyrmion enabling the collapse. By taking advantage of the high lateral resolution of the STM i could unravel the mechanism of the skyrmion collapse, via accessing the commonly elusive transition state. As the collapse develops on timescales of femto-seconds the transition state is not directly accessible by STM. However the electron induced switching rates of individual skyrmions are evaluated in dependency on the STM tip position featuring a radial symmetric pattern without in-plane magnetic field an an asymmetric pattern with in-plane magnetic field. The latter could be related by quantitative comparison with theory to the recently proposed chimera type collapse mechanism proceeding via a topological dipole at the transition
Ab-initio investigation of the interplay between the hyperfine interaction and complex magnetism at the nanoscale
No full textGroundbreaking advances in quantum technologies have recently been achieved through the use of innovative scanning tunneling microscopy techniques that demonstrate nuclear magnetometry of single magnetic adatoms. The weak hyperfine interaction between the nuclear and electron spins is atomically resolved, representing a significant step towards realizing quantum devices based on well-shielded individual nuclear spins that are impervious to environmental disturbances. Such nuclear spins could represent an ideal realization of qubits constructed atom-by-atom on surfaces. Notably, these experimental works have so far only yielded successful measurements on the hyperfine interaction for a selection of few chemical species adsorbed on two-layer thick MgO deposited on a Ag surface. This represents a rather unexplored topic of interest to the broad quantum computational and experimental community aimed at exploring hyperfine interactions and nuclear spins to encode quantum information. To broaden the scope of this emergent topic, we present an extensive first-principles computational study of the hyperfine interaction of the complete series of transition-metal adatoms deposited on diverse thicknesses of insulating thin films of experimental interest, including MgO, NaF, NaCl, h--BN, and CuN films. The investigation identifies the atoms and substrates that trigger the most efficient hyperfine interactions and uncovers the relevant trends. Physical mechanisms are meticulously analyzed, and a valuable map of the hyperfine interactions that will guide corresponding experimental and theoretical communities is summarized. - Furthermore, we explore the correlation between the hyperfine interaction and the magnetic state of a multi-atomic nanostructure. We choose Fe dimers and investigate both cases: free-standing and deposited dimers on a bilayer of MgO(001), and compare them to the case of Fe single adatoms. Fe-adatom is a prototypical atom that carries a large hyperfine interaction with a minimal nuclear spin, offering several advantages over the rest of potential 3 transition metal atoms. Our findings indicate that the magnitude of the hyperfine interaction can be controlled by switching the magnetic state of the dimers. The antiferromagnetic state enhances the hyperfine interaction with respect to that of the ferromagnetic state for short Fe-Fe distances. A transition towards the opposite behavior is observed by increasing the distance between the magnetic atoms. Furthermore, we demonstrate the ability to substantially modify the hyperfine interaction by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine Hamiltonian, and therefore, we propose an extension based on multiple-scattering theory. - Designing systems with large magnetic anisotropy energy (MAE) is crucial for nanoscale magnetic devices since it defines the energy barrier that would protect a magnetic bit to flip its orientation. However, the MAE 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. Experimentally, the maximum MAE for a 3d transition metal atom was recently achieved by coordinating a single Co atom to the O site of a MgO(100) surface. Theoretically, conventional density functional theory (DFT) calculations do not recover the large MAE of this system. Here, including a Hubbard- correction and spin-orbit coupling, we reproduce the large MAE of an individual Co adatom on a MgO (001) surface and unveil the underlying mechanism. More importantly, we take one step further by investigating the possibility of enhancing the MAE of 3d transition metal adatoms by considering various structural geometries of 3d--O molecules deposited on MgO. In one of the structures, where the molecules are perpendicular to the surface, the MAE can be enhanced while reducing the interaction with the substrate, which should minimize spin fluctuations and enhance the magnetic stability. Additionally, we provide evidence of the ability to substantially modify the MAE by atomic control of the 3d--O molecules location on the substrate and inspect the underlying hyperfine interactions
First-principles investigation of inelastic magnetic excitations in nanostructures deposited on surfaces
Full text linkThis thesis provides a theoretical description of inelastic scanning tunneling spectroscopy(ISTS), using a newly developed first-principles approach, by combining time-dependentdensity functional theory and many-body perturbation theory. The Korringa-Kohn-Rostoker Green function method is utilized, since it affords a real-space description of nanostructures, well-suited to the ISTS context. The central quantity is the electron self-energy, containing the interactions between the tunneling electrons and the spin excitations of the nanostructure. This self-energy leads to a renormalized electronic structure in the vacuum region above the adsorbate, which can be directly compared with the experimental ISTS signal, in the spirit of the Tersoff-Hamann approximation. As a first application, the developed method is applied to individual 3 transition-metal adatoms (Cr, Mn, Fe, and Co) deposited on metallic surfaces (Cu(111) and Pt(111)). The obtained magnetic excitation spectra for the regarded structures show differences in the excitation lifetime and the shift, which can be attributed to the electronic structure of both, the adsorbate and the substrate. The calculated theoretical inelastic spectra reveal different non-trivial shapes of the excitation signatures, that vary with distance to the adsorbate. Observed asymmetries in these spectra could explain asymmetries in experimental findings. Furthermore, some spectra show additional bound states (satellites) that are not predictable by use of a simple Heisenberg model. For Fe and Co adatoms on Pt(111) the impact of hydrogen contamination on the excitation spectrum is investigated. In agreement to experimental findings, the presence or absence of hydrogen has a significant impact on the shape of the excitation spectrum. In addition to the above analysis, we also consider clusters of two or more 3 transition-metal adatoms deposited on the Cu(111) surface, investigating the resulting magnetic excitation spectra. The magnetic moments are coupled by the exchange interaction which results in different excitation modes of acoustic and optical character. The obtained excitation spectra depend on the regarded adatom species, the interatomic distance, the alignment of the magnetic moments, the number of involved atoms, as well as the arrangement on the surface. A comparison of a ring and a chain structure reveals the impact of geometrical topology on magnetic excitations. The semiclassical Landau-Lifshitz-Gilbert model is used to provide an insightful interpretation of the first-principles spin-excitation modes
Complex magnetism of nanostructures on surfaces : from orbital magnetism to spin excitations
No full textMagnetic nanostructures on surfaces are promising building blocks of future spintronics devices, as they represent the ultimate limit in miniaturization. In this thesis, a combination of density functional theory and model-based studies is used to investigate magnetic nanostructures on surfaces with respect to fundamental theoretical properties and in relation to scanning tunneling microscopy experiments. Novel properties are unveiled in this class of systems by several methodological developments, from a new perspective on the orbital magnetism to the static and dynamic properties of complex non-collinear magnetic states. Firstly, we shed light on the orbital magnetic moment in magnetic nanostructures on surfaces and find a new component -- the inter-atomic orbital moment. A systematic analysis uncovers its distinct physical origin, its non-negligible strength, and its particular long range in realistic systems like adatoms deposited on the Pt(111) surface. Our results show unambiguously the importance and the potential of this new contribution to the orbital magnetism. Secondly, we investigate magnetic exchange interactions in magnetic nanostructures going beyond the common bilinear exchange interactions. Special focus is given to higher-order interactions whose microscopic origin is clarified using a model-based study. Using the prototypical test systems of magnetic dimers we find a new chiral pair interaction, the chiral biquadratic interaction, which is the biquadratic equivalent to the well-known Dzyaloshinskii-Moriya interaction, and investigate its properties and its implications not only for finite nanostructures but also for extended systems. Thirdly, we focus on the spin dynamics and the damping in non-collinear magnetic structures by investigating the dependencies of the Gilbert damping tensor on the non-collinearity in an atomistic form using a combination of a model-based study and first-principles calculations. We show how isotropic and chiral dependencies evolve from an Anderson-like model and in realistic systems like magnetic dimers on the Au(111) surface. These results have the potential to drive the field of atomistic spin dynamics to a more sophisticated description of the damping mechanisms. Fourthly, we investigate the magnetic stability of nanostructures, which is one of the key ingredients on the road towards future data storage devices. The impact of magnetic exchange interactions between nanostructures on the magnetic stability as probed in telegraph noise scanning tunneling microscopy experiments is analyzed by using the example of a magnetic trimer and a magnetic adatom. We find three regimes each driven by a distinct magnetic exchange interaction and show how this knowledge can be used to engineer the magnetic stability. Lastly, we analyze the complex interplay of magnetism, spin-orbit coupling and superconductivity in magnetic chains on a superconducting substrate with a special focus on the emergence of boundary states. We shed light on the puzzling magnetic ground state of Fe chains on the Re(0001) substrate and show how boundary effects can be minimized by termination with non-magnetic Co chains. Our results provide vital clues on the nature of the boundary states found in Fe chains on Re(0001), and support their identification as Majorana states
- …
