Spin-based phenomena are the central concept in the field of spintronics, where efficient spin current generation, spin-to-charge conversion, and spin transport are the key concepts for the development of next-generation electronic devices. With the extensive availability of femtosecond lasers a new research field of ultrafast spintronics has opened up, aiming to understand spin-transport on the ultrafast time scales. One way to generate picosecond transient spin current pulses is the illumination of ferromagnet (FM)/heavy metal (HM) heterostructures by femtosecond laser pulses. Via the inverse spin-Hall effect (ISHE), these transient spin currents are converted into transient charge currents, detectable via Terahertz (THz) radiation. In this thesis, we employed the detection of THz radiation as a non-invasive means to investigate spin-current generation in a variety of thin films and hetero-structures. It was shown earlier that the spin-orbit coupling plays a decisive role in spin-to-charge conversion efficiency and we have chosen materials with high SOC in our study. We first studied Ni80Fe20/HM hetero-structures, where the HM material was either Pt or Pd. In the second part we have investigated a Heusler alloy Co2Fe0.4Mn0.6Si (CFMS), capped with a thin film of either Pt, Ta, Al or Cr. We have found the inverse spin-Hall effect to be the dominating mechanism of THz generation in these hetero-structures and confirmed the role of spin-orbit coupling as a key factor for spin-to-charge conversion efficiency. Moreover we have investigated the dependency on magnetization and found that the amplitude of THz follows the sample’s magnetization. Additionally, we have found that the oxidation of the HM layer can reduce spin-to-charge conversion efficiency, whereas interdiffusion at the interface of CFMS/Al was found to be beneficial. In a seperate study on wedged topological insulator Bi2Se3 samples the thickness dependent THz-investigation revealed a strong variation of THz amplitude with the Bi2Se3 thickness. We ascribe these variations to a change in structural properties of the Bi2Se3. We have found a strong increase in the THz transient amplitude in Bi2Se3 capped with 3nm of Al indicating that the generation mechanism is occurring at the surface of Bi2Se3. Our study contributes to the understanding of THz generation in Heusler alloys and topological insulators.
Sarah Heidtfeld (Wed,) studied this question.