Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization

TL;DR

This study demonstrates that laser-induced terahertz spin transport and ultrafast demagnetization are driven by the same force, linking two key phenomena in femtomagnetism and spintronics, and provides a model to enhance spin current generation.

Contribution

The paper reveals that UDM and TST share a common origin driven by a generalized spin voltage, supported by experimental measurements and an analytical model, advancing understanding in ultrafast spin dynamics.

Findings

UDM rate matches TST flux evolution in ferromagnetic films.

Both phenomena are driven by a common spin voltage, not temperature differences.

The spin-current amplitude can be increased by an order of magnitude.

Abstract

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we use broadband terahertz emission spectroscopy to reliably measure both processes in one setup. We find that the rate of UDM of a single ferromagnetic metal film F has the same time evolution as the flux of TST from F into an adjacent normal-metal layer N. This remarkable agreement shows that UDM and TST are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model consistently and quantitatively explains our observations. It reveals that both UDM in F and TST in the F|N stack arise from a generalized spin voltage, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between F and N are minor and that the spin-current amplitude can, in principle, be increased by one order of magnitude. In general, our findings allow one to apply the vast knowledge of UDM to TST, thereby opening up new pathways toward large-amplitude terahertz spin currents and, thus, energy-efficient ultrafast spintronic devices.