  Topological insulators are a class of solids in which the nontrivial inverted
bulk band structure gives rise to metallic surface states that are robust
against impurity scattering. In three-dimensional (3D) topological insulators,
however, the surface Dirac fermions intermix with the conducting bulk, thereby
complicating access to the low energy (Dirac point) charge transport or
magnetic response. Here we use differential magnetometry to probe spin rotation
in the 3D topological material family (Bi$_2$Se$_3$, Bi$_2$Te$_3$, and
Sb$_2$Te$_3$). We report a paramagnetic singularity in the magnetic
susceptibility at low magnetic fields which persists up to room temperature,
and which we demonstrate to arise from the surfaces of the samples. The
singularity is universal to the entire family, largely independent of the bulk
carrier density, and consistent with the existence of electronic states near
the spin-degenerate Dirac point of the 2D helical metal. The exceptional
thermal stability of the signal points to an intrinsic surface cooling process,
likely of thermoelectric origin, and establishes a sustainable platform for the
singular field-tunable Dirac spin response.
