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

TL;DR

This study uses terahertz spectroscopy to observe femtosecond-scale dynamics of the spin Seebeck effect in magnetic insulator-metal bilayers, revealing rapid spin transfer mechanisms driven by conduction electrons.

Contribution

It provides the first direct femtosecond-resolution observation of the initial spin Seebeck current generation and elucidates the ultrafast electron-driven spin transfer process.

Findings

Spin Seebeck current arises within ~100fs after laser excitation.

Electron spin correlation time is approximately 4fs, enabling rapid spin transfer.

Efficient spin transfer depends on conduction electron scattering at interfaces.

Abstract

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto $j_\textrm{s}$ because their spins have a correlation time of only ~4fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.