Beyond Superexchange: Emergent Unconventional Ferromagnetism in Thin-Film Sandwich Structures of Intrinsic Magnetic Topological Insulators
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An intrinsic magnetic topological insulator Mn(Bi 1 − x Sb x ) 2 Te 4 features a highly ordered layered structure with a ferromagnetic Mn monolayer embedded in a septuple-layer unit, offering a promising platform for exotic quantum phenomena such as the quantum anomalous Hall effect at elevated temperatures. We explore its magnetic interactions via two structures: (i) Mn(Bi 1 − x Sb x ) 2 Te 4 /(Bi 1 − x Sb x ) 2 Te 3 heterostructures and (ii) Mn(Bi 1 − x Sb x ) 2 Te 4 /(Bi 1 − x Sb x ) 2 Te 3 /Mn(Bi 1 − x Sb x ) 2 Te 4 sandwich structures. The sandwiches exhibit significantly enhanced magnetization, indicating that a ferromagnetic interlayer interaction contributes beyond the previously reported antiferromagnetic behavior. Furthermore, reducing the in-plane Mn–Mn distance via Sb doping enhances Curie temperature T C , revealing intralayer ferromagnetic enhancement inconsistent with conventional direct/superexchange competition models. In sandwiches with x = 0, T C increases with the Bi 2 Te 3 spacer thickness (2–9 Quintuple layers: QLs), suggesting transition to a topologically nontrivial state, consistent with a van Vleck–type mechanism. The anomalous Hall effect shows negligible dependence on gate-induced carrier modulation, reinforcing the carrier-independent origin of the magnetism. These findings demonstrate topology-enhanced interlayer coupling and lattice-driven intralayer exchange, offering a new microscopic picture of magnetism in Mn(Bi 1 − x Sb x ) 2 Te 4 systems. Our study shows new principles for tailoring magnetic order in topological materials and opens pathways toward robust and tunable platforms for next-generation quantum and spintronic devices.
