Magnetic switching in granular FePt layers promoted by near-field laser enhancement

TL;DR

This study demonstrates laser-assisted magnetic switching in FePt nanoparticles, facilitated by near-field effects that locally enhance the laser interaction, enabling switching at lower magnetic fields and revealing inhomogeneous switching behavior.

Contribution

It uncovers the role of near-field laser enhancement in promoting magnetic switching in FePt nanoparticles, a novel insight into nanoscale light-matter interaction for magnetic control.

Findings

Magnetic switching occurs with a static magnetic field much smaller than coercivity.

Near-field effects cause inhomogeneous switching among nanoparticles.

Heat flow influences the fraction of nanoparticles that do not switch.

Abstract

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle x-ray scattering at an x-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, one order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material, with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.