Charge and spin current pumping by ultrafast demagnetization dynamics

The phenomenon of ultrafast demagnetization was first observed in 1966 when a thin film of ferromagnetic nickel lost its magnetization after being irradiated by ultrafast laser pulses. However, despite nearly three decades of intense studies, the understanding of the microscopic origins of such spin currents and how efficiently they can be converted into charge currents, as the putative source of THz radiation, is still lacking. Using data obtained from experiments on an ultrafast-light-driven Ni layer and applying time-dependent nonequilibrium Green's functions, we reveal how demagnetization dynamics pump both spin and charge currents. Additionally, we show that the electric field produced in the far-field regime, calculated with the Jeffimenko equations, exhibits features in the 0.1--30 THz frequency range. These frequencies have been experimentally probed and explored for applications in spintronic THz emitters. We found that the terahertz radiation is generated not only by a charge current induced within the nonmagnetic layer but also by an additional charge current excited in the ferromagnetic layer itself due to the demagnetization process. And we also found that although the pumped currents follow "dM/dt" in some setups, this relationship becomes obscured when nonmagnetic layers are disconnected, causing pumped currents to reflect from ferromagnetic boundaries. These characteristics support the notion that demagnetization, as an effective theory, is capable of generating both spin and charge currents accompanied by terahertz radiation and reveals new pumping phenomena inside the ferromagnet itself. Furthermore, an experiment involving a ferromagnetic film sandwiched between two nonmagnetic layers could easily validate this theory.