Luca Maranzana, Naoto Nagaosa, Sergey Artyukhin
In spiral multiferroics, the magnetic order is tied to electric polarization, offering a promising route to novel memory devices where an electric field—not a magnetic one—switches magnetic bits. However, such a switching process, in which magnetic domain walls are moved electrically, remains poorly understood. Here we find the domain walls to contain arrays of meron strings, featuring zero- and one-dimensional topological defects that profoundly influence switching dynamics. Minimum energy walls host merons with alternating skyrmion charges and move as relativistic massive particles, with velocity limited by the magnon speed. At the ordering transition, walls with non-alternating meron charges appear, giving rise to defect lines within the domain wall. They result in a peculiar nonlocal dynamics where all the spins in the system rotate, and the wall mobility is suppressed. Meron strings also possess zero-dimensional hedgehog defects—analogous to Bloch points—that pin the wall to the lattice, increasing the coercive electric field. The results offer an interpretation of recent experiments and position spiral multiferroics as a new playground for electric manipulation of spins. In spiral multiferroics, electric polarization linked to magnetic order presents opportunities for electric-field-driven magnetic memory devices, yet the mechanism of electrically moving magnetic domain walls is not well understood. Here, the authors reveal that domain walls host meron strings with topological defects which significantly influence the switching dynamics, offering insights into experimental observations and enhancing spin manipulation potential.