  We show that the magnetic susceptibility of a dilute ensemble of magnetic
impurities in a conductor with a relativistic electronic spectrum is
non-analytic in the inverse tempertature at $1/T\to 0$. We derive a general
theory of this effect and construct the high-temperature expansion for the
disorder averaged susceptibility to any order, convergent at all tempertaures
down to a possible ordering transition. When applied to Ising impurities on a
surface of a topological insulator, the proposed general theory agrees with
Monte Carlo simulations, and it allows us to find the critical temperature of
the ferromagnetic phase transition.
