  The surface of topological insulators is proposed as a promising platform for
spintronics and quantum information applications. In particular, when time-
reversal symmetry is broken, topological surface states are expected to exhibit
a wide range of exotic spin phenomena for potential implementation in
electronics. Such devices need to be fabricated using nanoscale artificial thin
films. It is of critical importance to study the spin behavior of artificial
topological MBE thin films associated with magnetic dopants, and with regards
to quantum size effects related to surface-to-surface tunneling as well as
experimentally isolate time-reversal breaking from non-intrinsic surface
electronic gaps. Here we present observation of the first (and thorough) study
of magnetically induced spin reorientation phenomena on the surface of a
topological insulator. Our results reveal dramatic rearrangements of the spin
configuration upon magnetic doping contrasted with chemically similar
nonmagnetic doping as well as with quantum tunneling phenomena in ultra-thin
high quality MBE films. While we observe that the spin rearrangement induced by
quantum tunneling occurs in a time-reversal invariant fashion, we present
critical and systematic observation of an out-of-plane spin texture evolution
correlated with magnetic interactions, which breaks time-reversal symmetry,
demonstrating microscopic TRB at a Kramers' point on the surface.
