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 <27 fs 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 js arises on the same ~100 fs 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 js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.
Highlights
Understanding the transfer of spin angular momentum is essential in modern magnetism research
Simulations consistently reveal that the rise of js(t) mirrors the thermalization process during which the photoexcited electrons approach a Fermi–Dirac distribution. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal–insulator interface
Any spin current js(t) arising in the metal is expected to be converted into a charge current jc(t) by the inverse spin Hall effect (ISHE) with a bandwidth extending into the THz range[17]
Summary
Understanding the transfer of spin angular momentum is essential in modern magnetism research. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs 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. Note that Eq (1) presumes a static temperature difference and a frequency-independent SSE coefficient K It is still an open question how the SSE current js evolves for fast temperature variations and in the presence of nonthermal states. To search for the SSE speed limit, even finer time resolution is required, reaching the 10 fs scale, which resolves the fastest spin dynamics in magnetic materials[24]
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