The influence of photon angular momentum on ultrafast spin dynamics in Nickel

TL;DR

This study investigates how photon angular momentum influences ultrafast spin dynamics in Nickel, finding that laser photon helicity does not directly affect demagnetization, suggesting phonons or impurities as angular momentum sources.

Contribution

The paper provides experimental evidence that photon angular momentum does not directly impact ultrafast demagnetization in Nickel, challenging previous microscopic models.

Findings

Demagnetization time is unaffected by pump helicity

Electron-phonon equilibration time remains unchanged

Photon angular momentum transfer is not the primary demagnetization mechanism

Abstract

The role of photon angular momentum in laser-induced demagnetization of Nickel thin films is investigated by means of pump-probe time-resolved magneto-optical Kerr effect in the polar geometry. The recorded data display a strong dependency on pump helicity during pump-probe temporal overlap, which is shown to be of non-magnetic origin. By accurately fitting the demagnetization curves we also show that demagnetization time and electron-phonon equilibration time are not affected by pump-helicity. Thereby our results do not support direct transfer of angular momentum between photons and spins to be relevant for the demagnetization process. This suggests, in agreement with the microscopic model that we recently presented, that the source of angular momentum could be phonons or impurities rather than laser photons as required in the microscopic model proposed by Zhang and Huebner.