Dirac-fermion-mediated ferromagnetism in a topological insulator

Topological insulators are a newly discovered class of materials in which helical conducting modes exist on the surface of a bulk insulator(1-6). Recently, theoretical works have shown that breaking gauge symmetry(7) or time-reversal symmetry(8) in these materials produces exotic states that, if realized, represent substantial steps towards realizing new magnetoelectric effects(9,10) and tools useful for quantum computing(11). Here we demonstrate the latter symmetry breaking in the form of ferromagnetism arising from the interaction between magnetic impurities and the Dirac fermions(12,13). Using devices based on cleaved single crystals of Mn-doped Bi2Te3-ySey, the application of both solid-dielectric and ionic-liquid gating allows us to measure the transport response of the surface states within the bulk bandgap in the presence of magnetic ions. By tracking the anomalous Hall effect we find that the surface modes support robust ferromagnetism as well as magnetoconductance that is consistent with enhanced one-dimensional edge-state transport on the magnetic domain wall. Observation of this evidence for quantum transport phenomena demonstrates the accessibility of these exotics states in devices and may serve to focus the wide range of proposed methods for experimentally realizing the quantum anomalous Hall effect(8,10) and states required for quantum computing(14,15).