  The high coercive field ($\mathcal{E}_c$) of hafnia-based ferroelectrics
presents a major obstacle to their applications. The ferroelectric switching
mechanisms in hafnia that dictate $\mathcal{E}_c$, especially those related to
nucleation-and-growth at the domain wall (DW), have remained elusive. Through
deep-learning-assisted multiscale simulations, we determine the
finite-temperature thermodynamics and switching mechanisms for diverse types of
180$^\circ$ DWs, revealing a complex, stress-sensitive mobility landscape. The
propagation velocities for mobile DW types under various thermal conditions can
be characterized with a single creep equation, featuring a creep exponent of 2.
This unconventional critical exponent results from the nucleation of a
half-unit-cell-thin, elliptically-shaped critical nucleus. Our multiscale
approach not only reproduces the experimental thickness ($d$) scaling,
$\mathcal{E}_c\propto d^{-\frac{2}{3}}$, but also predicts that $\mathcal{E}_c$
of HfO$_2$ can be engineered to $\approx$0.1 MV/cm, even lower than perovskite
ferroelectrics. The theoretical lower bound of $\mathcal{E}_c$ afforded by
ferroelectric hafnia offers opportunities to realize power-efficient,
high-fidelity ferroelectric nanoelectronics.
