Crossover from Universal Depinning to Free Domain-Wall Dynamics in Ultrathin Iron Garnet Films

Abstract

Magnetic domain walls display universal, disorder-controlled elastic dynamics at low drive, and texture-governed free motion at high drive. Here, we establish the crossover mechanism between these two regimes. Using experiments in ultrathin epitaxial iron garnet films and Landau-Lifshitz-Gilbert simulations, including disorder, thermal, and internal texture effects, we uncover a disorder- and temperature-dependent precessional flow that bridges pinned and free dynamics. We further demonstrate that the exceptionally low pinning in garnets arises from the weak coupling between domain walls and disorder, together with a correlation length that exceeds the wall width.

Figures (2)

• Figure 1: Domain wall dynamics in garnet films. (a) DW velocity versus magnetic field ($\mu_0 H$) showing the flow regimes. The short-dot lines reflect the asymtotic precessional flow regime. (b) Zoom of the same curves highlighting the depinning regime and the crossover to the flow regime. (c) Same curves presented in log scale versus $(\mu_0 H)^{-1/4}$ revealing the creep regime. In (b) and (c), the solid and dash lines represent the predictions of Eqs. \ref{['eq: 1']} for the creep and depinning, respectively.
• Figure 2: Comparison between numerical simulations and experimental results for the TmIG film. (a) DW velocity versus $\mu_0 H$ in log-log scale, showing the full range of DW dynamics. Simulations were performed for $T=293~\mathrm{K}$ with (green triangles) and without disorder (half-filled down magenta diamonds) and for $T=0~\mathrm{K}$ without disorder (half filled up violet diamonds). The green solid and dash lines correspond to predictions of Eqs. \ref{['eq: 1']}, for the creep and depinning regime, respectively. Inset: Propagation front within the instability domain observed at $\mu_0H=40.0~\mathrm{mT}$ with closed areas located in front and behind. The image size is $L_x L_y = 5.12 \times 5.12 ~\mathrm{\mu m^2}$. (b) Close-up of the velocity curves highlighting the crossover between pinned and free DW dynamics. Inset: Same curve as presented in log scale versus $(\mu_0 H)^{-1/4}$ to emphasize the creep regime. (c) Simulated DW displacement $\Delta x(t)$ versus time for $T=0~\mathrm{K}$ and $T=293~\mathrm{K}$, and under different magnetic fields ($\mu_0H=0.395$, $0.500$, and $0.700~\mathrm{mT}$), corresponding to the creep, depinning, and flow regimes. Displacements are normalized to $\Delta x(t=10~\mathrm{\mu s}, \mu_0H)$, with the normalization values indicated at the bottom of the panel.