Picosecond localization dynamics following ultrafast nanoscale magnetic switching

TL;DR

This study reveals that ultrafast magnetic switching in a periodically patterned Co/Pt multilayer nucleates via high-temperature fluctuations and subsequently localizes into a nanometer-scale skyrmion lattice within less than 1 ns. By combining shot-resolved pump–probe resonant SAXS at an XFEL with atomistic spin dynamics and micromagnetic modeling, the authors show that nucleation and localization are distinct, sequential processes governed by different energy scales: exchange- and chirality-dominated fluctuations drive nucleation near Tc, while micromagnetic energy balance and local anisotropy variations govern post-nucleation localization at room temperature. A quasi-particle, lifetime-gradient framework explains how strong lateral variations in spin-texture lifetimes across sub-100 nm regions steer deterministic nanometer-scale localization, enabling precise control over ultrafast nanoscale order through energy-landscape engineering. These findings establish a multiscale approach to nonequilibrium phase transitions, offering insights and tools for ultrafast control in nanoscale magnetic devices.

Abstract

Ultrashort laser pulses provide the fastest known way to switch magnetic order. Such excitation commonly creates nanometer-scale domains, even after homogeneous illumination when the position of nucleated domains is not externally defined. However, the physics of domain localization during such ultrafast phase transitions remains unresolved. Here, we use shot-resolved pump-probe resonant x-ray scattering together with a material featuring a periodically modulated magnetic anisotropy landscape to track, in real time, the laser-driven nucleation and localization of nanometer-scale spin textures. We find that nucleation and localization are two distinct processes. Nucleation occurs homogeneously via fluctuations at early times, whereas spatially periodic structures emerge only later and, under suitable conditions, localize in less than one nanosecond. Real-space simulations show that this localization is governed by strong lateral variations in spin-texture lifetimes. Our results demonstrate that ultrafast phase-transition dynamics fundamentally differ from conventional transitions, yet still can be controlled through moderate nanometer-scale tailoring of the energy landscape.

Figures (14)

• Figure 0: Characterization of the irradiated sample.a Out-of-plane hysteresis loops recorded with Kerr-microscopy within $15\times\qty{15}{\micro\meter\squared}$-large regions irradiated at the specified Ga-irradiation dose. The values correspond to the pristine film (red), the dose used in the time-resolved experiment (2.3ions, blue) and a significantly higher dose (13ions, green). b Domain state at 0mT applied field for each dose. The XMCD contrast images, showing the out-of-plane magnetization, were recorded with STXM. Scalebar 1.
• Figure 0: Schematic of the time resolved SAXS experiment.a Conceptual sketch of the experimental setup. b Control sequence to record time-resolved data. After resetting the magnetic state at $B_{\text{reset}}$ and consequently reducing the applied field to $B_{\text{meas}}$, the optical pump and x-ray probe pulse hit the sample with the specified time delay $\tau$. c Example of a raw final-state camera frame, showing magnetic scattering and the artifacts that need to be removed in post-processing.
• Figure 0: Resonant magnetic scattering patterns for all recorded fields and delays. For displaying purposes, a Gaussian filter ($\sigma=\qty{2}{px}$) was applied. Gray panels indicate that no data was recorded at that combination of delay and field. Scalebar 0.05.
• Figure 0: Isolated isotropic magnetic scattering patterns for all recorded fields and delays. For displaying purposes, a Gaussian filter ($\sigma=\qty{2}{px}$) was applied. Gray panels indicate that no data was recorded at that combination of delay and field. Scalebar 0.05.
• Figure 0: Isolated Bragg scattering patterns for all recorded fields and delays. For displaying purposes, a Gaussian filter ($\sigma=\qty{2}{px}$) was applied. Gray panels indicate that no data was recorded at that combination of delay and field. Scalebar 0.05.