Inhaltszusammenfassung

Spin-orbit torque (SOT) at surfaces of ferromagnetic materials, in particular the spin-Hall effect and the inverse spin-galvanic effect, has recently become a vivid research field because of its potential application in efficient magnetic storage devices. The microscopic origin of SOT is the spin-orbit induced hybridization of electronic states at so-called hot spots in reciprocal space in combination with the interface induced inversion asymmetry. Besides the SOT the all-optical magnetizatio...Spin-orbit torque (SOT) at surfaces of ferromagnetic materials, in particular the spin-Hall effect and the inverse spin-galvanic effect, has recently become a vivid research field because of its potential application in efficient magnetic storage devices. The microscopic origin of SOT is the spin-orbit induced hybridization of electronic states at so-called hot spots in reciprocal space in combination with the interface induced inversion asymmetry. Besides the SOT the all-optical magnetization switching using circularly polarized light, relying on the spin-orbit hybridization of electronic states, too, enables ultrafast magnetization processes. Recently, the discovery of threshold photoemission magnetic circular dichroism (TPMCD) has offered a new laboratory based investigation method of magnetic thin films. In the present work, we investigate the ferromagnetic domain structure of perpendicularly magnetized fcc Co/Pt(111) films (4.5 monolayers) using photoemission electron microscopy (PEEM) and utilizing TPMCD as a contrast mechanism in order to combine a lateral resolution of better than 100 nm with a large magnetic contrast. The perpendicularly magnetized Co/Pt interfaces, investigated here exemplarily, are the most widely used device components. This work presents a comprehensive study of electronic states near the Fermi level using photoemission spectroscopy. The momentum, energy, and spin-resolved data have led to a deeper understanding of these phenomena and are thereby an important contribution for further development of materials. We have used spin-resolved time-of-flight momentum microscopy as a novel and very efficient method to analyze energy, momentum and spin of photo-excited electrons in an ultimately parallelized detection scheme. This setup enables a complete mapping of the 3D spectral function I(E_B,k_x,k_y), in which E_B is the binding energy, and k_x and k_y are the components of the momentum parallel to the surface. An Ir(001) imaging spin-filter crystal provides mapping of the spin polarization distribution P(E_B,k_x,k_y). Electronic states are excited by femtosecond circularly polarized laser pulses (hν=1.5 eV and 3.1 eV) enabling multi-photon photoemission processes. As a main result we are able to discriminate direct from indirect transitions in the photoemission process, which is important information for the interpretation of earlier total electron yield data. Moreover, we find a significant change of the spectral density function induced by a structural phase transition occurring in ultrathin Co films with increasing thickness. Indirect transitions involving the interaction with lattice vibrations substantially contribute to the photoemission intensity. These indirect transitions possess a magnetic circular dichroism as well as a considerable spin polarization, both properties being caused by the large spin-orbit interaction at the Co interface. Even more, by reducing the work function of the Co/Pt surface by cesiation, we reveal an essential effect that has not been considered in previous work: The existence of an additional resonant two-photon photoemission (2PPE) process. As the origin, we identify an image potential state that possesses a parabolic k-dispersion. The image potential state dominates the 3D spectral function and selects particular initial states for the 2PPE process, depending on the work function.» weiterlesen» einklappen

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DDC Sachgruppe:
Physik