Exchange-coupled interfaces offer a powerful route to stabilising and enhancing ferromagnetic properties in two-dimensional materials, such as transition metal chalcogenides. These materials exhibit strong correlations among charge, spin, orbital, and lattice degrees of freedom, making them an exciting area for emergent quantum phenomena.

Cr₂Te₃’s crystal structure naturally forms layers that behave like two-dimensional sheets of magnetic material. Each layer has magnetic ordering (ferromagnetism), but the layers are not tightly bonded in the third dimension and are considered “quasi-2D.” These layers are useful for interface engineering. Using a vacuum-based technique for atomically precise thin-film growth, known as molecular beam epitaxy, the researchers demonstrate wafer-scale synthesis of Cr₂Te₃ down to monolayer thickness on insulating substrates. Remarkably, robust ferromagnetism persists even at the monolayer limit, a critical milestone for 2D magnetism.

When Cr₂Te₃ is proximitized (an effect that occurs when one material is placed in close physical contact with another so that its properties are influenced by the neighbouring material) to a topological insulator, specifically (Bi,Sb)₂Te₃, the Curie temperature, the threshold between ferromagnetic and paramagnetic phases, increases from ~100 K to ~120 K. This enhancement is experimentally confirmed via polarized neutron reflectometry, which reveals a substantial boost in magnetization at the interface.

Theoretical modelling attributes this magnetic enhancement to the Bloembergen–Rowland interaction which is a long-range exchange mechanism mediated by virtual intraband transitions. Crucially, this interaction is facilitated by the topological insulator’s topologically protected surface states, which are spin-polarized and robust against disorder. These states enable long-distance magnetic coupling across the interface, suggesting a universal mechanism for Curie temperature enhancement in topological insulator-coupled magnetic heterostructures.

This work not only demonstrates a method for stabilizing 2D ferromagnetism but also opens the door to topological electronics, where magnetism and topology are co-engineered at the interface. Such systems could enable novel quantum hybrid devices, including spintronic components, topological transistors, and platforms for realizing exotic quasiparticles like Majorana fermions.

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Interacting topological insulators: a review by Stephan Rachel (2018)