  In this work we study the spin accumulation due to an in-plane electric field
in an asymmetric topological insulator (TI) thin film system with an out of
plane magnetic field and an in-plane magnetization. A TI thin film differs from
the more typically studied thick TI system in that the former has both a top
and a bottom surface where the states localized at both surfaces can couple to
each other due to the finite thickness. In typical spin torque experiments on
TI thin film systems, the top and bottom surfaces of the film are asymmetric as
the former is in contact with a ferromagnetic layer while the latter is
adjacent to a non magnetic substrate. This may lead to differing (i) potentials
and (ii) magnetization strengths experienced by the top and bottom surface
states. We show, via Kubo formula calculations, that each of these two effects
can lead to in-plane spin accumulation perpendicular to the magnetization
direction which are otherwise absent in a top-bottom symmetric TI thin film
system. This spin accumulation results from the breaking of the antisymmetry of
the spin accumulation around the zero magnetic field equal energy contours.
