Light-matter interaction at the nanoscale in magnetic materials is a topic ofintense research in view of potential applications in next-generationhigh-density magnetic recording. Laser-assisted switching provides a pathwayfor overcoming the material constraints of high-anisotropy and high-packingdensity media, though much about the dynamics of the switching process remainsunexplored. We use ultrafast small-angle x-ray scattering at an x-rayfree-electron laser to probe the magnetic switching dynamics of FePtnanoparticles embedded in a carbon matrix following excitation by an opticalfemtosecond laser pulse. We observe that the combination of laser excitationand applied static magnetic field, one order of magnitude smaller than thecoercive field, can overcome the magnetic anisotropy barrier between "up" and"down" magnetization, enabling magnetization switching. This magnetic switchingis found to be inhomogeneous throughout the material, with some individual FePtnanoparticles neither switching nor demagnetizing. The origin of this behavioris identified as the near-field modification of the incident laser radiationaround FePt nanoparticles. The fraction of not-switching nanoparticles isinfluenced by the heat flow between FePt and a heat-sink layer.
展开▼
