Single-LayerMagnet Phase in Intrinsic Magnetic TopologicalInsulators, [MnTe][Bi₂Te₃]_<i>n</i>, Far beyond the Thermodynamic Limit

The intrinsic magnetic topological insulator (IMTI) family [MnTe]­[Bi₂Te₃]_<i>n</i> has demonstrated magneto-topological properties dependent on <i>n</i>, making it a promising platform for advanced electronics and spintronics. However, due to technical barriers in sample synthesis, their properties in the large <i>n</i> limit remain unknown. To overcome this, we utilized the atomic layer-by-layer molecular beam epitaxy (ALL-MBE) technique and achieved IMTIs with <i>n</i> as large as 15, far beyond that previously reported in bulk crystals or thin films. Then, we discover that the “single-layer magnet (SLM)” phase, primarily determined by intralayer ferromagnetic coupling, emerges for <i>n</i> > ∼4 and remains little affected up to <i>n</i> = 15. Nonetheless, still, nonzero, interlayer ferromagnetic coupling is necessary to stabilize the SLM phase, suggesting that the SLM phase eventually disappears in the <i>n</i> → ∞ limit. This study uncovers the secrets of IMTIs beyond the thermodynamic limit and opens a door to diverse magneto-topological applications.