We numerically study the dynamics of extended domain walls in homogeneous ferromagnets driven by a uniform magnetic field at zero temperature. Using both micromagnetic Landau-Lifshitz-Gilbert simulations and a collective-coordinate description, we show that flat chiral domain walls become linearly unstable above the Walker breakdown field and below a higher threshold, provided their length exceeds a characteristic scale. This instability is captured by a quasiuniversal spectral stability diagram, parametrized solely by the Gilbert damping, which predicts the onset of deviations from rigid-wall behavior. Beyond the linear regime, large domain walls with bands of unstable modes develop spatiotemporal chaos, intricate Bloch-line dynamics, and deterministic roughening. At a critical field, the system undergoes a dynamical phase transition from a flat to a rough moving phase with universal features. Our results provide a framework for addressing domain-wall dynamics in the presence of thermal fluctuations and quenched disorder by disentangling their effects from intrinsic deterministic instabilities.

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• Domain wall motion
• Line defects
• Magnetization dynamics
• Micromagnetism
• Pattern formation
• Phase diagrams
• Roughness
• Spatiotemporal chaos
• Collective dynamics
• Ferromagnets
• Floquet systems
• Magnetic thin films
• Nonequilibrium systems
• Surface growth
• Chaos & nonlinear dynamics
• Landau-Lifschitz-Gilbert equation
• Micromagnetic modeling
• Multiscale modeling