High frequency STM and spin polarized STM on magnetic vortices

This 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