Laser-induced helicity and texture-dependent switching of nanoscale stochastic domains in a ferromagnetic film

Controlling magnetic textures at ever smaller length and time scales is of key fundamental and technological interest. Achieving nanoscale control often relies on finding an external stimulus that is able to act on that small length scales, which is highly challenging. A promising alternative is to achieve nanoscale control using the inhomogeneity of the magnetic texture itself. Using a multilayered ferromagnetic Pt/Co/Pt thin-film structure as a model system, we employ a magnetic force microscope to investigate the change in magnetic nanotextures induced by circularly polarized picosecond laser pulses. Starting from a saturated magnetic state, we find stochastic nucleation of complex nanotextured domain networks. In particular, the growth of these domains depends not only on their macroscopic magnetization but also on the complexity of the domain texture. This helicity and texture-dependent effect contrasts with the typical homogeneous growth of magnetic domains initiated by an effective magnetic field of a circularly polarized laser pulse. We corroborate our findings with a stochastic model for the nucleation of magnetic domains, in which the nucleation and annihilation probability not only depends on the helicity of light but also on the relative magnetization orientation of neighboring domains. Our results establish a new approach to investigate ultrafast nanoscale magnetism and photo-excitation across first-order phase transitions.

💡 Research Summary

The authors investigate how circularly polarized picosecond laser pulses can manipulate magnetic textures at the nanometer scale in a Pt/Co/Pt multilayer with perpendicular magnetic anisotropy. Using magnetic force microscopy (MFM) they directly image the evolution of magnetic domains after exposure to a controlled number of laser pulses (N) and fluence (F). Starting from a saturated state, the first few pulses (2–4) stochastically nucleate sub‑micrometer domains (≈1–2 µm). With additional pulses these domains coalesce into intricate stochastic domain networks (SDN). The total switched area (As) grows roughly linearly with N, but the fractal dimension (D) of the domain pattern—used as a measure of texture complexity—rises sharply up to N≈6 (D≈1.27) and then declines as the system approaches a deterministic switching (DS) state.

Conventional all‑optical switching models for Co/Pt films invoke the inverse Faraday effect or a temperature gradient across domain walls generated by magnetic circular dichroism (MCD). Such models predict uniform domain growth driven by an effective magnetic field. The authors test this by first creating isolated domains with a fluence above the demagnetization threshold (F_DM≈2.6 mJ cm⁻²) and then applying subsequent sub‑threshold pulses (F=2.4 mJ cm⁻²). Contrary to expectations, the pre‑existing domains shrink and eventually disappear, indicating that a simple MCD‑induced thermal gradient cannot dominate the nanoscale dynamics.

To explain the observations, the authors develop a phenomenological stochastic model adapted from granular magnetic media. The film is discretized into a 2‑D lattice; each cell carries magnetization M = ±1. The energy barrier for switching a cell depends linearly on the number n of neighboring cells with the same orientation (ΔE = E₀ − α n). Laser heating raises the electronic temperature, giving an Arrhenius switching probability P = Δt/τ, where τ⁻¹ ∝ exp(−ΔE/k_BT). Circular polarization introduces a 5 % higher temperature rise for M = +1 cells, embodying the MCD effect. Simulations with this model reproduce the experimental As(N) and D(N) trends, showing that cells with few aligned neighbors have low barriers and high nucleation probability, while cells surrounded by similarly oriented neighbors become more stable, leading to the observed texture‑dependent growth and eventual coalescence.

The work demonstrates two key insights: (1) helicity of ultrafast light provides a direct probabilistic bias for domain nucleation at the nanoscale, beyond the average effective magnetic field picture; (2) the pre‑existing magnetic texture itself modulates the energy landscape, creating a feedback loop where domain complexity influences further switching dynamics. These findings challenge the prevailing view that all‑optical switching in ferromagnetic multilayers is governed solely by inverse Faraday fields or thermal gradients.

Beyond fundamental interest, the demonstrated control of stochastic domain nucleation and texture‑dependent switching opens pathways for neuromorphic and probabilistic computing architectures that exploit intrinsic randomness, as well as for ultrafast magnetic memory concepts where sub‑micron bits can be written or erased with light of specific helicity. The methodology—combining picosecond laser excitation with high‑resolution MFM and a simple yet powerful stochastic model—provides a versatile platform to explore photo‑induced first‑order phase transitions in a wide range of magnetic and multiferroic materials.