# Helicity-Changing Brillouin Light Scattering by Magnons in a Ferromagnetic Crystal

## TL;DR

This paper reports the experimental discovery of helicity-changing Brillouin light scattering involving magnons in a ferromagnetic crystal, revealing new angular momentum transfer mechanisms and conservation law considerations.

## Contribution

It demonstrates for the first time helicity-changing two-magnon and one-magnon Brillouin light scattering processes in ferromagnetic crystals, highlighting the role of crystal angular momentum.

## Key findings

- Observation of helicity-changing two-magnon scattering.
- Detection of unconventional one-magnon scattering violating angular momentum conservation.
- Crystal angular momentum compensates for missing angular momentum.

## Abstract

Brillouin light scattering in ferromagnetic materials usually involves one magnon and two photons and their total angular momentum is conserved. Here, we experimentally demonstrate the presence of a helicity-changing two-magnon Brillouin light scattering in a ferromagetic crystal, which can be viewed as a four-wave mixing process involving two magnons and two photons. Moreover, we observe an unconventional helicity-changing one-magnon Brillouin light scattering, which apparently infringes the conservation law of the angular momentum. We show that the crystal angular momentum intervenes to compensate the missing angular momentum in the latter scattering process.