Emerging weak localization effects on a topological insulator–insulating ferromagnet (Bi<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>Se<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>3</mml:mn></mml:msub></mml:math>-EuS) interface

Thin films of topological insulator Bi${}_{2}$Se${}_{3}$ were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature ${T}_{C}$, resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above ${T}_{C}$. Such negative magnetoresistance was only observed for Bi${}_{2}$Se${}_{3}$ layers thinner than $t\ensuremath{\sim}4$ nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.