  When a topological insulator (TI) is in contact with a ferromagnet, both time
reversal and inversion symmetries are broken at the interface. An energy gap is
formed at the TI surface, and its electrons gain a net magnetic moment through
short-range exchange interactions. Magnetic TIs can host various exotic quantum
phenomena, such as massive Dirac fermions, Majorana fermions, the quantum
anomalous Hall effect and chiral edge currents along the domain boundaries.
However, selective measurement of induced magnetism at the buried interface has
remained a challenge. Using magnetic second harmonic generation, we directly
probe both the in-plane and out-of-plane magnetizations induced at the
interface between the ferromagnetic insulator (FMI) EuS and the
three-dimensional TI Bi2Se3. Our findings not only allow characterizing
magnetism at the TI-FMI interface but also lay the groundwork for imaging
magnetic domains and domain boundaries at the magnetic TI surfaces.
