  We study the transport properties of the Dirac fermions with Fermi velocity
$v_F$ on the surface of a topological insulator across a ferromagnetic strip
providing an exchange field ${\mathcal J}$ over a region of width $d$. We show
that the conductance of such a junction changes from oscillatory to a
monotonically decreasing function of $d$ beyond a critical ${\mathcal J}$. This
leads to the possible realization of a magnetic switch using these junctions.
We also study the conductance of these Dirac fermions across a potential
barrier of width $d$ and potential $V_0$ in the presence of such a
ferromagnetic strip and show that beyond a critical ${\mathcal J}$, the
criteria of conductance maxima changes from $\chi= e V_0 d/\hbar v_F = n \pi$
to $\chi= (n+1/2)\pi$ for integer $n$. We point out that these novel phenomena
have no analogs in graphene and suggest experiments which can probe them.
