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

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we show that UDM and TST are driven by the same force: a generalized spin voltage, which is induced by the incident femtosecond laser pulse. Using broadband terahertz emission spectroscopy, we find that the rate of UDM of a single ferromagnetic film F has the same time evolution as the flux of TST from F into an adjacent normal-metal layer N. 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 Δ , which is defined for arbitrary, nonthermal electron distributions. Our findings open up unexpected synergies and new pathways toward large-amplitude terahertz spin currents and, thus, energy-efficient ultrafast spintronic devices. Figure 1 | Ultrafast demagnetization (UDM) vs terahertz spin transport (TST). (a) Side view of a single ferromagnetic metal layer (F) with magnetization = parallel to the axis with unit vector . Excitation by a femtosecond laser pulse triggers UDM. The transient magnetic dipole gives rise to the emission of a terahertz pulse with field ( ) ∝ ̇ ( ). (b) F|N stack consisting of F and an adjacent normal paramagnetic metal layer (N). Femtosecond laser excitation drives a spin current with density = from F to N. In N, is converted into a charge current with density , leading to the emission of a terahertz electromagnetic pulse with electric field ( ) ∝ ( ) directly behind the sample. Both ( ) and ( ) are linearly polarized perpendicular to and measured by electrooptic sampling. (c) Schematic of the density of states of spin-up and spin-down electrons of a Stoner-type ferromagnet such as Fe. Quasi-elastic spin-flip scattering events (white curved arrow) lead to transfer of spin angular momentum to the crystal lattice. (d) N acts as an additional sink of spin angular momentum through spinconserving electron transfer across the F|N interface (blue curved arrows). In (a) and (b), the spin transfer rate scales with the generalized spin voltage Δ (Eq. (4)), which equals ↑ − ↓ in the case of FermiDirac electron distributions. Figure 2 | Terahertz emission from F and F|N films. Typical terahertz electrooptic signals, odd with respect to magnetization , from samples consisting of F=CoFe(3 nm) and N=Pt(3 nm). (a) Terahertz emission signal | ( , 0°) from an F|N stack. When the sample is turned by 180° about , the signal | ( , 180°) is obtained. Note that the sample is optically symmetrized by a cap window that is identical to the diamond substrate. (b) Same as panel (a), but for the F sample. Note the asymmetry between ( , 0°) and ( , 180°). (c) Signals ( ) and ( ) symmetric and antisymmetric with respect to sample turning. (d) Extracted magnetization dynamics from the symmetric signal of panel (c) (red curve), along with magnetization dynamics as measured by the magnetooptic Kerr effect (MOKE, black curve). 0.1