  An interface electron state at the junction between a three-dimensional
topological insulator (TI) film of Bi2Se3 and a ferrimagnetic insulator film of
Y3Fe5O12 (YIG) was investigated by measurements of angle-resolved photoelectron
spectroscopy and X-ray absorption magnetic circular dichroism (XMCD). The
surface state of the Bi2Se3 film was directly observed and localized 3d spin
states of the Fe3+ state in the YIG film were confirmed. The proximity effect
is likely described in terms of the exchange interaction between the localized
Fe 3d electrons in the YIG film and delocalized electrons of the surface and
bulk states in the Bi2Se3 film. The Curie temperature (TC) may be increased by
reducing the amount of the interface Fe2+ ions with opposite spin direction
observable as a pre-edge in the XMCD spectra.
