  The quantum anomalous Hall effect has been theoretically predicted and
experimentally verified in magnetic topological insulators. In addition, the
surface states of these materials exhibit a hedgehog-like "spin" texture in
momentum space. Here, we apply the previously formulated low-energy model for
Bi$_2$Se$_3$, a parent compound for magnetic topological insulators, to a slab
geometry in which an exchange field acts only within one of the surface layers.
In this sample set up, the hedgehog transforms into a skyrmion texture beyond a
critical exchange field. This critical field marks a transition between two
topologically distinct phases. The topological phase transition takes place
without energy gap closing at the Fermi level and leaves the transverse Hall
conductance unchanged and quantized to $e^2/2h$. The momentum-space skyrmion
texture persists in a finite field range. It may find its realization in hybrid
heterostructures with an interface between a three-dimensional topological
insulator and a ferromagnetic insulator.
