  In this work we analyze the instability conditions for spin-density-waves
(SDW) formation on the surface of strong topological insulators. We find that
for a certain range of Fermi-energies and strength of interactions the SDW
state is favored compared to the unmagnetized and the uniform-magnetization
states. We also find that the SDW are of spiral nature and for a certain range
of parameters a Skyrmion-lattice may form on the surface. We show that this
phase may have a non trivial Chern-number even in the absence of an external
magnetic field.
