Abstract
To extend current charge-based electronics by new features and functionalities, the electron spin, as a new degree of freedom, is likely to play a major role in future information technology. Devices using such spin-based electronics (spintronics), for example magnetic random-access memories, are about to enter the market. To be competitive with other information carriers, it is required to push the bandwidth of the elementary spintronic operations to the terahertz (THz) frequency range. This thesis addresses central open questions regarding ultrafast spin transport in prototypical F|N thin-film stacks. Is ultrafast spin transport mediated by magnons as universal as indicated by previous modelling? What impact does the F/N interface between F and N have on the ultrafast spin-to-charge current conversion (S2C)? How can we exploit spintronic features for new functionalities of spintronic THz emitters (STEs)? By studying spin current dynamics on their natural timescale, one may find new interesting effects or push existing concepts to THz frequencies, which might advance future spintronic applications to work at higher clock rates. To study ultrafast spin transport in F|N bilayers, we excite them with femtosecond laser pulses. Following absorption of the pulse, a spin current in F is launched and converted into a transverse charge current in N and/or F, giving rise to the emission of a THz electromagnetic pulse. Using this approach, along with an analysis based on symmetry arguments and modeling, the following insights are gained: First, depending on the conductivity of F, spin currents can be carried by either (i) conduction electrons or (ii) magnons. Remarkably, in the half-metallic ferrimagnet Fe3O4 we observe the coexistence of these two spin transport types and disentangle them based on their distinctly different ultrafast dynamics. Our results also indicate that the ultrafast SSE spin current is localized close to the F/Pt interface and its ultrafast dynamics are determined by the relaxation dynamics of the electrons in the Pt layer. Second, interfaces of metallic heterostructures are known to have a marked impact on the S2C process. We study thin metal lms of a ferromagnetic layer F and nonmagnetic layer N with strong and weak spin-orbit coupling. Varying the interface composition allows us to drastically change the amplitude and even invert the polarity of the THz charge current. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. Third, we add a functionality to the STE and modulate the polarization of broadband THz electric field pulses at tens of kHz by time-dependent external magnetic field with a contrast exceeding 99 %. In conclusion, THz emission spectroscopy is a powerful tool to explore and exploit spintronic effects in the ultrafast regime, which will lay the cornerstone for spintronics at THz frequencies.
