Coherent THz Transient Spin Currents by Spin Pumping

Abstract

The generation of short spin current pulses is the basis for fast spintronic devices. In thin bilayer systems consisting of a nonmagnetic metal and a ferromagnet, a pure spin current is induced by a precessing magnetization into the nonmagnetic layer by spin pumping. This effect has been experimentally demonstrated at ferromagnetic resonance at GHz frequencies. Here, it is theoretically shown that transient magnetization dynamics efficiently generates short spin current pulses that exhibit two transient contributions. An effective coherent spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small in this regime. The results provide a concept for coherent THz spin current generation.

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

“…They may prove useful for magnon-based transport of information, for exerting ultrafast torques on remote magnetic layers 61 and for spectroscopy of spin waves with nanometer wavelength 62 . Our results also strongly suggest that coherent spin pumping should be feasible at THz frequencies 63 .…”

Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy

“…Actually two model pictures are actually admissible based on different physical picture: i) a modelling of electron dynamics in the sub picosecond scale, by which the local heating leads to transient precessing of electrons around a local change of the anisotropy magnetic field 65,66 . This constitutes a direct extension to the GHz precession regime, that we do not consider henceforth in that section, or ii) a two temperature model by which an electronic absorption gives rise to electronic heating with characteristic excitation energy over the Fermi level (temperature T e followed by inelastic relaxation due to electron-electron interactions and phonon bath characterized by a second temperature T ph 67 .…”

Nonlinear Magnetization Dynamics Driven by Strong Terahertz Fields

“…solving the Landau-Lifshitz-Gilbert equation with a non-constant magnitude of the magnetization vector to incorporate THz-induced demagnetization effect, are able to describe all the details of the experimental results to a good accuracy. Existing models of terahertz spin dynamics and spin pumping would need to be extended to include the evidence presented here [41,42]. Starting from simulations describing experimental data for THz-induced demagnetization, we extrapolate that THz fields one order of magnitude larger drive the magnetization into a nonlinear regime.…”