  The Dzyaloshinskii-Moriya interaction (DMI) in magnetic systems stabilizes
spin textures with preferred chirality, applicable to next-generation memory
and computing architectures. In perpendicularly magnetized
heavy-metal/ferromagnet films, the interfacial DMI originating from structural
inversion asymmetry and strong spin-orbit coupling favors chiral N\'eel-type
domain walls (DWs) whose energetics and mobility remain at issue. Here, we
characterize a new effect in which domains expand unidirectionally in response
to a combination of out-of-plane and in-plane magnetic fields, with the growth
direction controlled by the in-plane field strength. These growth
directionalities and symmetries with applied fields cannot be understood from
static treatments alone. We theoretically demonstrate that perpendicular field
torques stabilize steady-state magnetization profiles highly asymmetric in
elastic energy, resulting in a dynamic symmetry breaking consistent with the
experimental findings. This phenomenon sheds light on the mechanisms governing
the dynamics of N\'eel-type DWs and expands the utility of field-driven DW
motion to probe and control chiral DWs.
