  Gapless surface states that are protected by time reversal symmetry and
charge conservation are among the manifestations of 3D topological insulators.
In this work we study how electron-electron interaction may lead to spontaneous
breaking of time reversal symmetry on surfaces of such insulators. We find that
a critical interaction strength exists, above which the surface is unstable to
spontaneous formation of magnetization, and study the dependence of this
critical interaction strength on temperature and chemical potential.
