  A chiral compound FeSi is a nonmagnetic narrow-gap insulator, exhibiting
peculiar charge and spin dynamics beyond a simple band-structure picture. Those
unusual features have been attracting renewed attention from topological
aspects. Although a signature of surface conduction was indicated according to
size-dependent resistivity in bulk crystals, its existence and topological
properties remain elusive. Here we demonstrate an inherent surface
ferromagnetic-metal state of FeSi thin films and its strong spin-orbit-coupling
(SOC) properties through multiple characterizations of the two-dimensional (2D)
conductance, magnetization and spintronic functionality. Terminated
covalent-bonding orbitals constitute the polar surface state with
momentum-dependent spin textures due to Rashba-type spin splitting, as
corroborated by unidirectional magnetoresistance measurements and
first-principles calculations. As a consequence of the spin-momentum locking,
non-equilibrium spin accumulation causes magnetization switching. These surface
properties are closely related to the Zak phase of the bulk band topology. Our
findings propose another route to explore noble-metal-free materials for
SOC-based spin manipulation.
