Emergent domain topology in the multiferroic hexagonal manganites

Abstract: Emergent topological phenomena in multiferroic materials arise from the intricate coupling between structural, electric, and magnetic order parameters. Hexagonal manganites provide a paradigmatic platform for such studies. These compounds exhibit a strongly coupled distortive-improper ferroelectric order, arising from trimerizing lattice distortions, and a 120{\deg} noncollinear antiferromagnetic spin structure. While their two-dimensional domain topology has been extensively studied, the full three-dimensional multiferroic domain architecture has remained largely unexplored, mainly due to the experimental challenges of probing bulk structures beyond surfaces. Here, we employ a Landau free-energy framework combined with large-scale phase-field simulations to reveal the intricate three-dimensional multiferroic domain network of hexagonal manganites. We demonstrate that the coupling between the structural and antiferromagnetic order parameters gives rise to a rich variety of three-dimensional topological features. In particular, these features give rise to an attraction between different types of domain walls. Moreover, we identify bifurcations of vortex-like lines at domain-wall intersections, a phenomenon that can exist only in three dimensions and fundamentally alters the topology of the domain network. Our results provide a comprehensive theoretical basis for understanding three-dimensional domain interactions in multiferroics and highlight the essential role of dimensionality in coupling improper ferroelectricity, magnetism, and topology in hexagonal manganites.