Final Report Summary - FURORE (FUndamental studies and innovative appROaches of REsearch on magnetism)
Our project has focussed on fundamental studies of magnetic structures and processes at the atomic level. In order to gain insight into magnetic properties and interactions of single atoms at surfaces, we were making use of innovative experimental approaches combining atomic resolution and single-spin sensitivity. Moreover, we have developed novel tools to address magnetism at ultimate length and time scales. Based on our earlier developments of Spin-Polarized Scanning Tunnelling Microscopy/ Spectroscopy (SP-STM/STS) and Magnetic Exchange Force Microscopy (MExFM), both offering spin sensitivity and spatial resolution down to the ultimate limit of single atoms, we have studied spin-dependent properties of and spin-dependent interactions between individual magnetic atoms on metal surfaces, in diluted magnetic semiconductors, and on surfaces of magnetic insulators. Besides the investigation of static and dynamic spin states and interactions between spins, we have manipulated spin states in a controlled manner in various ways, including the construction of artificial magnets and atomic-scale spintronic devices in a bottom-up approach. Moreover, we have developed an instrumental set-up for novel types of experiments combining MExFM techniques with ultra-cold quantum gases.
As a first step toward the investigation and manipulation of single spins and of artificial magnetic nanostructures obtained by single-atom manipulation techniques on non-magnetic metal substrates, we demonstrated the measurement of single atom magnetization curves with SP-STM, making use of the model-type system of Fe atoms adsorbed onto a Cu(111) surface. Utilizing SP-STM/STS, we fully characterized this distant-dependent interaction which either favorably aligns or anti-aligns pairs of Fe atoms depending on their atomic separation. Based on the knowledge of these interactions, we were able to construct a roadmap of the magnetic interactions between pairs of atoms. We developed the ability to combine SP-STM techniques with tip assisted single-atom manipulation to fabricate complex artificial magnetic nanostructures of interest. We demonstrated the assembly of Fe atoms on the Cu(111) surface, for example, to realize antiferromagnetically coupled linear chains and spin frustrated nanostructures. As a centerpiece of this work, we demonstrated that such chains and frustrated nanostructures can be combined in a particular way with ferromagnetic islands to realize an all-spin based logical gate on the atomic level. The spin state of a specified output atom reflects the bit state, namely 0 or 1, after logical operation of two specified ferromagnetic inputs. The realization of such a device represents a huge cross-disciplinary technological development as concepts of both nanotechnology, where devices are being scaled to ever smaller dimensions, and the largely energy efficient concepts of spintronics, where information is transmitted by spin transport, were combined.
After understanding the static magnetic properties, we extended our studies to address the dynamics of individual Fe atoms on Cu(111), Ag(111), and Pt(111) surfaces. By utilizing inelastic electron tunneling spectroscopy (IETS), we were able to explore the nature of how substrate electrons interact with the local spin excitations of an individual Fe moment on the surface. For Fe atoms in direct contact with a metallic surface, the conduction electron cloud of the supporting substrate plays a large role in limiting the lifetime of the excitation. This strong substrate induced interaction was seen by magnetic field-dependent spin excitations where the measured energy broadened lifetime was related to femtosecond dynamical processes. This experiment demonstrated the direct combination of two powerful techniques, namely SP-STM and IETS, to realize simultaneously ultra-high spatial, temporal, and spin resolution to address a single atom spin on a non-magnetic metallic surface. More recently, we applied the same techniques in order to reveal the current-driven spin dynamics of artificially constructed quantum magnets consisting of a few atoms only. Here, the transition regime between the dynamics of a quantum system and a quasi-classical system could be studied both experimentally and theoretically.
Based on a unique SP-STM set-up with a 3D superconducting magnet system, we were able to perform vectorial spin mapping with atomic resolution in order to reveal non-collinear spin structures in nano-scale systems, such as Néel-ordered states or spin spiral states. A special highlight was the discovery of novel types of interface-driven skyrmionic states in monolayer Fe films which attracted great attention by the magnetism community. In particular, we were able to demonstrate for the first time a controlled nucleation and deletion of individual skyrmions by local spin current injection from a SP-STM probe tip.
Furthermore, we developed two complementary techniques to readout the magnetization curve and measure the magnetic excitations of individual sub-surface dopants supported on a III-V compound semiconductor. In particular, we found that a magnetic dopant atom can serve as an atomic size magnetometer, which can be used to measure the local magnetization of the supporting two-dimensional electron system induced at the surface of the semiconductor.
The invention of Magnetic Exchange Force Microscopy (MExFM) by our group opened up the possibility of atomic scale studies with single spin sensitivity on insulating as well as on conducting surfaces. In the framework of this project, we recently succeeded in the development of a spectroscopic mode, namely Magnetic Exchange Force Spectroscopy (MExFS), which allows us to quantitatively measure the strength of the magnetic exchange interaction between single spin states of probe tip and sample across a vacuum gap. Moreover, the dissipation signal, reflecting the energy loss in single cantilever oscillation cycles due to non-conservative processes, has been employed to study spin dynamics on magnetic insulators. While we have experimentally proven the feasibility of MExFS in combination with dissipation imaging, theoretical studies helped to understand energy loss mechanisms between tip and sample.
As a first step toward the investigation and manipulation of single spins and of artificial magnetic nanostructures obtained by single-atom manipulation techniques on non-magnetic metal substrates, we demonstrated the measurement of single atom magnetization curves with SP-STM, making use of the model-type system of Fe atoms adsorbed onto a Cu(111) surface. Utilizing SP-STM/STS, we fully characterized this distant-dependent interaction which either favorably aligns or anti-aligns pairs of Fe atoms depending on their atomic separation. Based on the knowledge of these interactions, we were able to construct a roadmap of the magnetic interactions between pairs of atoms. We developed the ability to combine SP-STM techniques with tip assisted single-atom manipulation to fabricate complex artificial magnetic nanostructures of interest. We demonstrated the assembly of Fe atoms on the Cu(111) surface, for example, to realize antiferromagnetically coupled linear chains and spin frustrated nanostructures. As a centerpiece of this work, we demonstrated that such chains and frustrated nanostructures can be combined in a particular way with ferromagnetic islands to realize an all-spin based logical gate on the atomic level. The spin state of a specified output atom reflects the bit state, namely 0 or 1, after logical operation of two specified ferromagnetic inputs. The realization of such a device represents a huge cross-disciplinary technological development as concepts of both nanotechnology, where devices are being scaled to ever smaller dimensions, and the largely energy efficient concepts of spintronics, where information is transmitted by spin transport, were combined.
After understanding the static magnetic properties, we extended our studies to address the dynamics of individual Fe atoms on Cu(111), Ag(111), and Pt(111) surfaces. By utilizing inelastic electron tunneling spectroscopy (IETS), we were able to explore the nature of how substrate electrons interact with the local spin excitations of an individual Fe moment on the surface. For Fe atoms in direct contact with a metallic surface, the conduction electron cloud of the supporting substrate plays a large role in limiting the lifetime of the excitation. This strong substrate induced interaction was seen by magnetic field-dependent spin excitations where the measured energy broadened lifetime was related to femtosecond dynamical processes. This experiment demonstrated the direct combination of two powerful techniques, namely SP-STM and IETS, to realize simultaneously ultra-high spatial, temporal, and spin resolution to address a single atom spin on a non-magnetic metallic surface. More recently, we applied the same techniques in order to reveal the current-driven spin dynamics of artificially constructed quantum magnets consisting of a few atoms only. Here, the transition regime between the dynamics of a quantum system and a quasi-classical system could be studied both experimentally and theoretically.
Based on a unique SP-STM set-up with a 3D superconducting magnet system, we were able to perform vectorial spin mapping with atomic resolution in order to reveal non-collinear spin structures in nano-scale systems, such as Néel-ordered states or spin spiral states. A special highlight was the discovery of novel types of interface-driven skyrmionic states in monolayer Fe films which attracted great attention by the magnetism community. In particular, we were able to demonstrate for the first time a controlled nucleation and deletion of individual skyrmions by local spin current injection from a SP-STM probe tip.
Furthermore, we developed two complementary techniques to readout the magnetization curve and measure the magnetic excitations of individual sub-surface dopants supported on a III-V compound semiconductor. In particular, we found that a magnetic dopant atom can serve as an atomic size magnetometer, which can be used to measure the local magnetization of the supporting two-dimensional electron system induced at the surface of the semiconductor.
The invention of Magnetic Exchange Force Microscopy (MExFM) by our group opened up the possibility of atomic scale studies with single spin sensitivity on insulating as well as on conducting surfaces. In the framework of this project, we recently succeeded in the development of a spectroscopic mode, namely Magnetic Exchange Force Spectroscopy (MExFS), which allows us to quantitatively measure the strength of the magnetic exchange interaction between single spin states of probe tip and sample across a vacuum gap. Moreover, the dissipation signal, reflecting the energy loss in single cantilever oscillation cycles due to non-conservative processes, has been employed to study spin dynamics on magnetic insulators. While we have experimentally proven the feasibility of MExFS in combination with dissipation imaging, theoretical studies helped to understand energy loss mechanisms between tip and sample.