  We investigate charge transport in two-dimensional ferromagnet/feromagnet
junction on a topological insulator. The conductance across the interface
depends sensitively on the directions of the magnetizations of the two
ferromagnets, showing anomalous behaviors compared with the conventional
spin-valve. It is found that the conductance depends strongly on the in-plane
direction of the magnetization. Moreover, in sharp contrast to the conventional
magnetoresistance effect, in the p-n junction, the conductance at the parallel
configuration is much smaller than that at the antiparallel configuration. This
stems from the way how the wavefunctions connect between both sides.
