Sub 1K UHV scanning tunneling microscope made of shapal and mapping of magnetic skyrmion collapse rates

This 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 $^4$He with $^3$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 $\delta_{\mathrm{z}}\leq2$ 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 $B_\parallel =$ 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