Spatiotemporal observation of surface plasmon polariton mediated ultrafast demagnetization

Demagnetization Dynamics in Magnetic Thin Films: An Ultrafast Journey

At the intersection of magnetism and optics, a fascinating dance unfolds: the ultrafast demagnetization of thin magnetic films, subject to a laser-induced transient optical grating (TMG). Bodying like a heartbeat, this demagnetization phenomenon exhibits complex spatiotemporal characteristics, governed by intricate interplay between light, magnetic fields, and electron dynamics.

The stage is set with a nickel-iron (Permalloy) film, teetering on a ‘-5 to 46.2 ps’ equilibrist. A laser-driven optical TG inadvertently sets the magnetization in motion, with the in-plane component parallel to the film’s edge. An initial static magnetic field, in excess of the material’s coercive field, ensures uniform alignment of magnetic moments before the laser’s time-zero interrogation.

As the laser strikes, dynamic evolution transpires. The LUEM (Lorentz Ultrafast Electron Microscopy) technique, akin to aleun slowed-down camera, captures the sequence: a magnetic grating contrast appears after time zero, peaks at 2.2 ps, and gradually fades, interspersed with partial recovery. Thisoprocess entail swift energy transfer from electron subsystem top spin system, followed bydissipative interactions with the lattice, in agreement with the three-temperature model for ultrafast demagnetization.

The TG’s interference pattern yields a sinusoidal periodic modulation of the Lorentz force experienced by relativistic electrons, translating to a striped TMG contrast post-timze. FFT (Fast Fourier Transform) analysis confirms the temporal scale constraining ultrafast demagnetization reflects the instrumental time resolution, not inherent limitations.

PINEM (Photo-Induced Near-Field Electron Microscopy), akin to a whispering sponge, detects evanescent electromagnetic fields induced by the TG. SPP (Surface Plasmon Polaritons), evanescent field propagators, are preferentially excited at the film’s edge, their intensity decaying exponentially as distance increases.

Meticulously mapping demagnetization dynamics, we divide LUEM images into slices parallel to the edge, analyzing TMG-induced demagnetization. Proximity to the edge amplifies demagnetization magnitude, an additional magnetic periodicity emerges, and time to reach maximum demagnetization increases with distance.

Controlling experiments, sans plasmonic hotspot, verify the TG’s governing role in demagnetization processes, whilst highlighting SPP’s unique influence: enhanced demagnetization near the edge, a magnetic beat signature (’17 μm’ periodicity), and spatial delay in reaching maximum demagnetization. Spatiotemporal distributions reflect not only TG-induced demagnetization dynamics but also SPP mediation, accentuating and modulating the demagnetization process.