Topological rescue and structural symmetry breaking of skyrmion lattices in synthetic antiferromagnetic bilayers.

Synthetic antiferromagnetic bilayers provide a highly promising platform for next-generation spintronics by eliminating stray fields and the skyrmion Hall effect. In this study, we investigate the field-driven phase evolution and thermodynamic stability of topological spin textures in synthetic antiferromagnetic bilayers using Monte Carlo simulations based on a minimal, purely isotropic two-dimensional classical Heisenberg model. By systematically varying the interlayer exchange coupling across different external magnetic field regimes, we uncover distinct topological phase transitions dictated by the energetic competition between Zeeman alignment, Dzyaloshinskii-Moriya interaction, and interlayer coupling. Crucially, despite the absence of any structural asymmetries or staggered interactions, we observe a striking spontaneous structural symmetry breaking at intermediate fields, where the skyrmion lattice distorts into a frustrated multi-$Q$ state with a unique 8-peak structure factor. This energetic struggle further culminates at extreme magnetic fields, where the synthetic antiferromagnetic bilayer demonstrates a profound ``Topological Rescue'' effect in a regime where standard monolayers are completely field-polarized and topologically dead. We show that the antiferromagnetic coupling acts as a thermodynamic shield, reviving a dense and stable skyrmion lattice from the saturated background. A comprehensive differential energy analysis proves that this rescue mechanism is governed by a strict energetic trade-off: the system accepts a penalty in Zeeman and intralayer exchange energies to unlock massive synergistic gains from the DMI and interlayer exchange. Our findings establish the exact thermodynamic boundaries of these transitions in a perfectly aligned lattice, highlighting the immense potential of tuning interlayer coupling for the design of robust, field-immune topological memory and logic switches.&#xD.