The surface magnetic property plays a key role in determining magnetic related quantum phenomena of magnetic topological insulators. Using spin-polarized scanning tunneling microscopy, we investigate the surface magnetism and anisotropy of a Cr doped topological insulator: ${\mathrm{Cr}}_{0.05}{\mathrm{Sb}}_{1.95}{\mathrm{Te}}_3$. It is found that the topological surface state of ${\mathrm{Cr}}_{0.05}{\mathrm{Sb}}_{1.95}{\mathrm{Te}}_3$ is spin polarized in the surface plane while the bulk shows a ferromagnetism with an out-of-plane easy axis. The upper and lower branch of the helical Dirac cone harbors the opposite spin polarization and the polarization at the Dirac point is zero. Our results show the complexity of surface magnetism of magnetic doped topological insulators.

• Received 2 April 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.176802