  We report a study of enhancing the magnetic ordering in a model magnetically
doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a
high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source
of exchange interaction yet without removing the nontrivial surface state. By
performing the elemental specific X-ray magnetic circular dichroism (XMCD)
measurements, we have unequivocally observed an enhanced TC of 50 K in this
magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm
at 30 K) but faster decreasing (by 80% from 30 K to 50 K) penetration depth
compared to that of diluted ferromagnetic semiconductors (DMSs), which could
indicate a novel mechanism for the interaction between FMIs and the nontrivial
TIs surface.
