  The surface states of topological insulators (TI) are protected by time
reversal symmetry and they display intrinsic spin helicity where the momentum
of the charge carriers decides their spin states. As a consequence, a current
injected through the surface states becomes spin polarized and this transport
spin-polarization leads to a proportionate suppression of Andreev reflection in
superconductor/TI junctions. Here we show that upon doping Bi2Se3 with Mn, the
transport spin-polarization is seen to be monotonically suppressed. The parent
compound Bi2Se3 is found to exhibit a transport spin-polarization of about 63%
whereas crystals with 10% Mn doping show transport spin-polarization of about
48%. This suppression is accompanied by an increasing ferromagnetic order of
the crystals with Mn doping. Scanning tunneling spectroscopy shows that the
topological protection of the surface states reduce due to Mn doping. The net
measured transport spin-polarization is due to a competition of this effect
with the increased magnetization on Mn doping. The present results provide
important insights for the choice of magnetic topological insulators for
spintronic applications.
