Direct Imaging of the Defect-Mediated Magnetic Phase Transition in a Topological Superlattice MnSb₆Te₁₀

Structural disorder, specifically cation site-mixing, fundamentally dictates the magnetic energy landscape of van der Waals topological insulators. In this work, we resolve the real-space dynamics of the temperature-driven ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in the superlattice MnSb₆Te₁₀. We demonstrate that MnSb antisite defects introduce competing interlayer exchange pathways, which trap the system in a metastable FM state at low temperatures. Using cryogenic magnetic force microscopy (MFM), we reveal that this state relaxes upon warming not through homogeneous melting, but via first-order spatial fragmentation into microscale magnetic islands, eventually yielding the intrinsic AFM ground state. Monte Carlo simulations based on a spatially inhomogeneous Heisenberg model reproduce these fragmentation dynamics, identifying defect density as a deterministic control knob for the magnetic phase competition. These results provide a comprehensive nanoscale picture of defect-modulated spin textures, establishing structural site-mixing as a viable strategy for manipulating magnetic symmetry and engineering non-trivial topological phases.