Topological insulators are a state of matter where the bulk has an insulating band gap like an ordinary insulator, while on the surface there are conducting gapless surface states. Theory predicts that these surface states are protected against backscattering by time-reversal-symmetry (TRS). When TRS is violated, the surface states will open a gap, turning the surface states into massive Dirac fermions. 

One way to break TRS is to add magnetic impurities to the material. In this talk I'll present experimental evidence for TRS breaking in Fe doped Bi₂Se₃, which is a strong 3D topological insulator. We study the band structure and the nature of magnetic order in this system using Angle Resolved Photo-emission Spectroscopy (ARPES) and X-ray Magnetic Circular Dichroism (XMCD).