Push a domain wall with a current, and it moves in the direction of the push. Increase the current, and it moves faster. This is the standard paradigm: the magnitude of the current controls speed, the direction controls direction. In most magnetic systems, inertia affects only transient dynamics — the wall wobbles briefly, then settles into steady motion determined by the applied drive.
This paper shows the paradigm breaks down in ferrimagnets near the angular momentum compensation point. A domain wall driven by a direct current can propagate steadily forward or backward, with the direction controlled solely by the current strength — not its sign. The same current, applied the same way, produces opposite motion depending on how hard you push.
The mechanism is inertial. An internal collective coordinate of the domain wall behaves as a massive particle in a current-dependent double-well potential. At one current strength, the particle relaxes into the left well (forward motion). At another, it relaxes into the right well (backward motion). The final state depends on which basin of attraction the system falls into, and the basins rearrange with current magnitude.
The result enables one-port devices — components where a single current terminal produces bidirectional control. You don't need to reverse the current to reverse the wall. You just need to change how much current you send. Direction becomes a function of amplitude, not sign.