Stefan Krause

University of Hamburg | UHH · Department of Physics

The electronic properties of the Be(0001) surface are investigated on the atomic scale at low temperature. Utilizing laterally resolved scanning tunneling spectroscopy (STS) a pronounced surface state is identified, additionally manifested by the observation of bias-dependent standing-wave patterns. Fourier analysis reveals a parabolic dispersive b...

We study how impurities influence the current-induced dynamics of magnetic skyrmions moving in a racetrack geometry. For this, we solve numerically the generalized Landau-Lifshitz-Gilbert equation extended by the current-induced spin transfer torque. In particular, we investigate two classes of impurities, nonconducting and magnetic impurities. The...

The thermal stability of atomic-scale skyrmions is of high relevance for potential spintronics applications and validation of theoretical models. We investigated Pd/Fe nanoislands on an Ir(111) substrate as a function of temperature and magnetic field. Utilizing noncollinear magnetoresistance contrast in scanning tunneling microscopy, the thermomag...

Spin-polarized scanning tunneling microscopy (SP-STM) experiments on ultrathin films with non-collinear spin textures demonstrate that resonant tunneling allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm. Spin-polarized resonance states evolving between the foremost atom of a magnetic probe tip and t...

Spin-polarized scanning tunneling microscopy (SP-STM) experiments on ultrathin films with non-collinear spin textures demonstrate that resonant tunneling allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm. Spin-polarized resonance states evolving between the foremost atom of a magnetic probe tip and t...

A scanning tunneling microscope for spin-resolved studies of dynamic systems is presented. The cryogenic setup allows the scanning tunneling microscope to achieve a cutoff frequency beyond 26 GHz at the tunnel junction and to be operable at temperatures of 1.1 K–100 K in a magnetic field of up to 3 T. For this purpose, the microscope and its wiring...

The reflection of electrons at noncollinear magnetic surfaces is investigated by spin-polarized scanning tunneling microscopy and spectroscopy on unoccupied resonance states located in vacuo. Even for energies up to 20 eV above the Fermi level, the resonance states are found to be spin split, exhibiting the same local spin quantization axis as the...

Step edges represent a local break of lateral symmetry in ultrathin magnetic films. In our experiments, we investigate the spin coupling across atomic step edges on Fe/W(110) by means of spin-polarized scanning tunneling microscopy and spectroscopy. Local modifications of the spin texture toward step edges separating double from single layer areas...

Mapping out the magneto-Seebeck coefficient In the Seebeck effect, a temperature difference across a device generates voltage. If the thermal gradient is imposed across a magnetic tunnel junction—with two magnetized layers separated by an insulating tunnel barrier—the magnitude of the generated voltage depends on the relative orientation of the mag...

Spin-polarized scanning tunneling microscopy is used to study the magnetization reversal of individual quasiclassical and quantum atomic-scale magnets. Modifications of the dynamics arising from the presence of the biased magnetic probe tip are identified in terms of spin-transfer torque, Joule heating, Oersted field, magneto-electric coupling and...

The tunneling Seebeck effect in a metal-vacuum-metal junction is studied experimentally in a scanning tunneling microscopy setup. Selective heating of the tip with a laser generates a well-defined temperature difference at the tunnel junction. The thermovoltage between the tip and the sample is measured with atomic-scale lateral resolution and rela...

The observation of magnetic skyrmions at room temperature that can be driven by short current pulses at speeds exceeding 100 m/s raises great expectations for skyrmion-based racetrack memories.

The magnetization dynamics of individual nanomagnets is studied by spin-polarized scanning tunneling microscopy, combining real-time telegraphic noise analysis with pump-probe schemes. A transition between two Arrhenius regimes is observed as a function of temperature. The switching rates at high temperature are found to be orders of magnitude lowe...

Growth, electronic properties, and magnetic properties of an Fe monolayer (ML) on an Ir/YSZ/Si(111) multilayer system have been studied using spin-polarized scanning tunneling microscopy. Our experiments reveal a magnetic nano-skyrmion lattice, which is fully equivalent to the magnetic ground state that has previously been observed for the Fe ML on...

The thermal stability of the magnetic nano-skyrmion lattice in the monolayer Fe/Ir(111) is investigated using temperature dependent spin-polarized scanning tunneling microscopy. Our experiments show that the skyrmion lattice disappears at a temperature of Tc=27.8 K, indicating a loss of long-range magnetic order. At second-layer iron islands the la...

Magnetoelectric coupling is studied using the electric field between the tip of a spin-polarized scanning tunneling microscope and a nanomagnet. Our experiments show that a negative (positive) electric field stabilizes (destabilizes) in-plane magnetization against thermal agitation, whereas it destabilizes (stabilizes) out-of-plane magnetization. W...

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 a...

We resonantly inject spin-polarized field-emitted electrons in thermally switching nanomagnets. A detailed lifetime analysis as a function of the spin-polarized emission current reveals that considerable Joule heating is generated, and spin-transfer torque results in a directed switching. A trend of higher switching efficiency per electron is obser...

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of...

Understanding of the relationship between stress and magnetic properties in nanostructures is of both fundamental and practical interest. In the present paper, we illustrate this statement with some recent research results. First, we will see how the magnetoelastic interaction in Dy films controls the magnetic structure at the nanoscale due to the...

A full magnetic characterization of bulk Cr tips has been achieved using spin-polarized scanning tunneling microscopy at low temperature. A detailed bias-dependent study of the spatial magnetic sensitivity on the system of 1.5 monolayers of Fe/W(110) reveals that all magnetic directions in space are sensed over a wide bias range, thereby indicating...

Current-induced magnetization switching of thermally quasistable magnetic nanoislands is demonstrated using a spin-polarized scanning tunneling microscope. The magnetization of an individual Fe nanoisland consisting of about 40 atoms on a W(110) surface is reversibly switched between two quasistable states by the application of spin-polarized tunne...

We report on studies of spin structures that appear around screw and edge dislocations in Dy (0001) films grown on W (110) substrates. By means of spin-polarized scanning-tunneling microscopy, we have observed the formation of vortexlike and lobe-shaped magnetic structures. We have studied the effect of fundamental dislocations in micromagnetic sim...

The thermal switching behavior of individual in-plane magnetized Fe/W(110) nanoislands is investigated by a combined study of variable-temperature spin-polarized scanning tunneling microscopy and Monte Carlo simulations. Even for islands consisting of less than 100 atoms the magnetization reversal takes place via nucleation and propagation. The Arr...

In present data storage applications magnetic nanostructures are switched by external magnetic fields. Due to their non-local character, however, cross-talk between adjacent nanomagnets may occur. An elegant method to circumvent this problem is magnetization switching by spin-polarized currents, as observed in GMR,1] as well as in TMR,2] studies. H...

Switching the magnetization of a magnetic bit by injection of a spin-polarized current offers the possibility for the development of innovative high-density data storage technologies. We show how individual superparamagnetic iron nanoislands with typical sizes of 100 atoms can be addressed and locally switched using a magnetic scanning probe tip, t...

Spin-polarized scanning tunneling microscopy and spectroscopy (SP-STS) are employed to investigate the contrast mechanism and the magnetic structure of Dy films epitaxially grown on W(110). While relatively complicated tip preparation procedures were necessary in the past, we now obtain magnetic contrast by bringing the tip in gentle contact with a...

We have studied the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1 1 0) surfaces. For this purpose the surface morphology and electronic properties are measured by means of scanning tunneling microscopy (STM) and spectroscopy (STS), respectively. It is found that the surfaces with impurity atom densities as lo...

The narrow domain wall width w of high-anisotropy materials induces significant pinning of magnetic domains at line defects which—due to spatial resolution limitations—could not be studied directly in the past. By means of spin-polarized scanning tunneling microscopy we have directly correlated the morphology and domain structure of ferromagneti...

Spin–polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) has been performed on clean and sulfur-covered three-dimensional Fe islands on W(1 1 0). Upon dosing with H2S the island surface is covered with 1/3 ML S leading to a c(3 × 1) reconstruction. The characteristic magnetic vortex structure is observable before and after dosing,...

The temperature-dependent surface electronic structure of Cr001 is investigated by means of scanning tunneling spectroscopy STS in the temperature range between 22 and 350 K. Consistent with earlier obser-vations, a sharp peak is found in the tunneling spectra close to the Fermi level E F . While the binding energy remains unchanged, the peak broad...

The spin-density wave SDW on Cr001 has been investigated at temperatures between 20– 300 K by means of spin-polarized scanning tunneling microscopy SP-STM. Although neutron-scattering data measured on the same crystal clearly show a spin-flip transition from a transversal T-SDW to a longitudinal L-SDW at the expected spin-flip SF temperature T SF =...