Spontaneous Topological Locking and Symmetry Restoration of Meron Lattices in Synthetic Antiferromagnets

Abstract

Synthetic antiferromagnets offer a robust platform for stabilizing fractional topological textures, effectively circumventing the limitations of ferromagnetic systems. In this study, we utilize large-scale Monte Carlo simulations to investigate the spontaneous topological locking and structural symmetry restoration of meron-antimeron crystals within SAF bilayers subjected to easy-plane magnetic anisotropy. In the uncoupled monolayer limit, increasing anisotropy induces an extreme core-shrinking effect that physically expands the inter-core distance and triggers a $C_4 \rightarrow C_2$ symmetry breaking. However, the introduction of an ultra-weak interlayer antiferromagnetic exchange acts as an active structural scaffold. For rigid crystals, this coupling strictly enforces vertical synchronization, forming robust antiferromagnetic bimeron dipoles and fully restoring the macroscopic $C_4$ rotational symmetry. Furthermore, in highly expanded, pre-collapse crystals, we observe an anomalous interlayer-induced lattice compression that actively maximizes the exchange energy. At extreme anisotropy limits where macroscopic crystalline order irrecoverably collapses, the bilayer coupling continues to enforce a strict local topological locking of surviving isolated defects. These findings reveal a fundamental decoupling between local vertical synchronization and global structural order, providing a comprehensive theoretical roadmap for stabilizing and manipulating fractional topological textures in beyond-skyrmion spintronic architectures.