Proximity Magnetism in Mn(Bi,Sb)₂Te₄-(Bi,Sb)₂Te₃/MnTe Natural Heterostructures

Magnetic topological insulators and their heterostructures provide significant opportunities to couple band topology with a nontrivial spin configuration for enhanced spintronic device performance, as well as designing magnetoelectric systems and functionalities. We find that Mn interdiffusion from MnTe when interfaced with (Bi,Sb)₂Te₃ stabilizes as self-organized Mn(Bi,Sb)₂Te₄ septuple lamellae among alternating (Bi,Sb)₂Te₃ quintuple layers, as observed using scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. We further demonstrate a valuable combination of magnetic and topological orders in these naturally formed Mn(Bi,Sb)₂Te₄-(Bi,Sb)₂Te₃ heterostructures, which are exchange-coupled with MnTe. Magnetotransport experiments and quantum magnetism simulations reveal that, above its own Néel temperature T_N ∼ 20 K, Mn(Bi,Sb)₂Te₄ mediates the exchange field leading to an anomalous Hall effect at the (Bi,Sb)₂Te₃/MnTe interface, with an enhanced interfacial T_N exceeding 200 K, approaching that of the bulk MnTe. This magnetic interface, in turn, allows a robust and deterministic spin-orbit torque switching without an external magnetic field at a low critical current density of 3 × 10⁵ A cm^-2. The antiferromagnetically coupled architecture of Mn(Bi,Sb)₂Te₄-(Bi,Sb)₂Te₃/MnTe, featuring magnetic and topological proximity effects across a chalcogenide backbone, is rich in fundamental interface physics and holds the potential for practical applications in spintronics.