# Investigating optically-excited THz standing spin waves using noncollinear magnetic bilayers

## TL;DR

This paper explores the excitation and damping of THz standing spin waves in noncollinear magnetic bilayers using femtosecond laser pulses, revealing new damping behaviors and enabling dispersion analysis of spin wave stiffness.

## Contribution

It demonstrates a novel method to excite and analyze THz spin waves in noncollinear bilayers, including a new damping model accounting for thickness-dependent effects.

## Key findings

- Strong increase in damping with decreasing absorption layer thickness
- Observation of a sudden damping decrease in the thinnest films
- Effective excitation of THz spin waves using noncollinear bilayers

## Abstract

We investigate optically excited THz standing spin waves in noncollinear magnetic bilayers. Using femtosecond laser-pulse excitation, a spin current is generated in the first ferromagnetic (FM) layer, and flows through a conductive spacer layer to be injected into the second (transverse) FM layer, where it exerts a spin-transfer torque on the magnetization and excites higher-order standing spin waves. We show that the noncollinear magnetic bilayer is a convenient tool that allows easy excitation of THz spin waves, and can be used to investigate the dispersion and thereby the spin wave stiffness parameter in the thin-film regime. This is experimentally demonstrated using wedge-shaped Co and CoB (absorption) layers. Furthermore, the damping of these THz spin waves is investigated, showing a strong increase of the damping with decreasing absorption layer thickness, much stronger than expected from interface spin pumping effects. Additionally, a previously unseen sudden decrease in the damping for the thinnest films is observed. A model for the additional damping contribution incorporating both these observations is proposed.