Large anomalous Hall effect in topological insulators with a proximitized ferromagnetic insulator

Ferromagnetism in topological insulators could lead to exotic quantum phenomena such as quantum anomalous Hall effect. A predominant requirement for such phenomena is the formation of an exchange gap at the Dirac surface states, being realized not only by magnetic doping but also by proximitizing ferromagnetic insulators. Here, we report on the effective gap formation driven by proximity coupling via detection of large anomalous Hall conductivity in heterostructures consisting of ferromagnetic insulator Cr2Ge2Te6 and topological insulator (Bi,Sb)2Te3. While no discernible magnetization in the (Bi,Sb)2Te3 layer is probed by spin-polarized neutron reflectometry, the emergence of large anomalous Hall effect implies that the wavefunction of the Dirac surface state gains an exchange field by penetrating into the Cr2Ge2Te6 by approximately 2-3 nm, finally activating a large Berry curvature. The presently verified mechanism of strong ferromagnetic proximity effect paves a way to enrich the proximity coupling phenomena in versatile topological quantum materials.