Spin transport-induced damping of coherent THz spin dynamics in iron

TL;DR

This study investigates the damping mechanisms of high-frequency spin waves in ultrathin iron films, revealing that spin transport effects significantly limit the observable frequencies at large wave vectors, with a new model involving transverse spin mean free path.

Contribution

It introduces a novel model incorporating the transverse spin mean free path to better describe damping at large wave vectors in ultrathin ferromagnetic films.

Findings

Damping dominated by spin transport effects scaling with k^4.

Transverse spin mean free path estimated to be ~0.5 nm.

Limitations on observable PSSW frequencies due to spin transport.

Abstract

We study the damping of perpendicular standing spin-waves (PSSWs) in ultrathin Fe films at frequencies up to 2.4 THz. The PSSWs are excited by optically generated ultrashort spin current pulses, and probed optically in the time domain. Analyzing the wavenumber and thickness dependence of the damping, we demonstrate that at sufficiently large wave vectors $k$ the damping is dominated by spin transport effects scaling with k^4 and limiting the frequency range of observable PSSWs. Although this contribution is known to originate in the spin diffusion, we argue that at moderate and large k a more general description is necessary and develop a model where the 'transverse spin mean free path' is the a key parameter, and estimate it to be ~0.5 nm.