  The presence of an exchange field in topological insulators reveals novel
spin related phenomena derived from the combination of topology and magnetism.
In the present work we show the controlled occurrence of either metallic or
gapped topological Dirac states at the interface between ultrathin Cr films and
the Bi$_2$Se$_3$ surface. The opening and closing of the gap at the Dirac point
is caused by the spin reorientation transitions arising in the Cr films. We
find that atom thin layers of Cr adhered to Bi$_2$Se$_3$ surfaces present a
magnetic ground state with ferromagnetic planes coupled antiferromagnetically.
As the thickness of the Cr film increases stepwise from one to three atomic
layers, the direction of the magnetization changes twice from out-of-plane to
in-plane and to out-of-plane again. The out of plane magnetization drives the
gap opening and the topological surface states acquire a circular meron spin
structure. Therefore, the Cr spin reorientation leads to the metal/insulator
transition in the Bi$_2$Se$_3$ surface and to the correlated modification of
the surface state spin texture. Consequently, the thickness of the Cr film
provides an effective and controllable mechanism to modify the metallic or
gapped nature, as well as the spin texture of the topological Dirac states.
