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The research field of spintronics emerges as a promising solution to address the core challenges posed by charge-based electronic information processing, aiming to reduce power dissipation while delivering lasting endurance and robust read and write capabilities. Spintronic circuits have already made their mark, finding practical applications in commercially accessible magnetic random-access memories. However, to effectively compete with future complementary metal–oxide–semiconductor (CMOS) and photonic technologies, fundamental spin operations speed should ideally extend to terahertz (THz) frequencies. In this respect, an exciting discovery is a new class of highly efficient and broadband THz emitters based on magnetic heterostructures, harnessing spintronic effects at terahertz frequencies. This work is dedicated to addressing a series of fundamental questions, for example: What is the primary driving force for spin currents in spintronic THz emitters (STEs)? How do these spin currents relate to other processes such as ultrafast demagnetization dynamics? How do spin currents propagate through different material systems? What are the maximum speeds of spin current propagation? How can the spin conductance of various materials be quantified? Finally, how can the amplitude of the THz radiation from STEs be maximized to values exceeding 1 MV/cm? First, we compare two central phenomena in femtomagnetism: ultrafast demagnetization and ultrafast spin currents in magnetic heterostructures. Strikingly, our findings unveil that both phenomena are driven by the same force, a generalized spin voltage, i.e., the excess of magnetization relative to equilibrium. We conclude that the spin voltage is genuinely ultrafast, and the decay of the spin voltage is predominantly due to spin-flip processes inside the ferromagnet, with only a minor fraction of spins contributing to the transport. Subsequently, we explore spin current propagation in copper and MgO tunnel junctions. Our results reveal that spin currents in copper propagate at high speed, reaching the Fermi velocity vF = 1.1 nm/fs with velocity-relaxation time of of τ = 4 ± 2 fs, and we separate ballistic and diffusive modes of spin transport. Furthermore, we introduce the new concept of THz spin-conductance spectroscopy. We apply this method to measure the spin conductance of an MgO tunnel junction, allowing to separate different spin-transport contributions, including coherent tunneling and incoherent resonant spin tunneling mediated through MgO defects. Finally, based on these findings, we significantly improve the STE performance by optimizing the heat management and maximizing the THz outcoupling. Our proposed Si-based STE design outperforms previous glass-based STEs by a factor of six, achieving a peak electric field of 1.7 MV/cm. Eventually, Si-STEs prove to be highly effective in inducing nonlinear effects such as the THz Kerr effect in diamond or Zeeman torque in magnets. In conclusion, this work demonstrates the significant potential of terahertz spin transport that can shape the future of ultrafast circuits.
