Single femtosecond laser pulse excitation of individual cobalt nanoparticles

TL;DR

This study investigates how single femtosecond laser pulses affect individual cobalt nanoparticles, revealing that scattering and substrate absorption limit energy transfer, preventing magnetization reversal and causing irreversible chemical changes at high fluences.

Contribution

It provides new insights into the effects of femtosecond laser pulses on magnetic nanoparticles, highlighting the roles of scattering, substrate effects, and chemical stability, which were not fully understood before.

Findings

No magnetization reversal observed up to high intensities

Laser excitation causes irreversible chemical changes at high fluences

Scattering and substrate absorption limit energy transfer to nanoparticles

Abstract

Laser-induced manipulation of magnetism at the nanoscale is a rapidly growing research topic with potential for applications in spintronics. In this work, we address the role of the scattering cross section, thermal effects, and laser fluence on the magnetic, structural, and chemical stability of individual magnetic nanoparticles excited by single femtosecond laser pulses. We find that the energy transfer from the fs laser pulse to the nanoparticles is limited by the Rayleigh scattering cross section, which in combination with the light absorption of the supporting substrate and protective layers determines the increase in the nanoparticle temperature. We investigate individual Co nanoparticles (8 to 20 nm in size) as a prototypical model system, using x-ray photoemission electron microscopy and scanning electron microscopy upon excitation with single femtosecond laser pulses of varying intensity and polarization. In agreement with calculations, we find no deterministic or stochastic reversal of the magnetization in the nanoparticles up to intensities where ultrafast demagnetization or all-optical switching is typically reported in thin films. Instead, at higher fluences, the laser pulse excitation leads to photo-chemical reactions of the nanoparticles with the protective layer, which results in an irreversible change in the magnetic properties. Based on our findings, we discuss the conditions required for achieving laser-induced switching in isolated nanomagnets.