  We study the magneto-transport properties on the disordered surface of a
topological insulator attached with a ferromagnet/ferromagnet junction. Since,
in the surface Dirac Hamiltonian, out-of-plane magnetization induces a mass
gap, while in-plane magnetization has a role of the effective vector potential,
the mechanism of magneto-transport is different between these two cases. The
former is similar to the conventional one in ferromagnetic metals, while the
latter is due to the shift of Fermi circles in momentum space. Our numerical
calculations show that the magnetoconductance in in-plane configuration is
robust against disorder compared to that in out-of-plane configuration.
