Nonanalytic Magnetic Response of Fermi- and non-Fermi Liquids

We study the nonanalytic behavior of the static spin susceptibility of 2D fermions as a function of temperature and magnetic field. For a generic Fermi liquid, \chi_s (T, H)= const+c_1 max (T,\mu_B|H|), where c_1 is shown to be expressed via complicated combinations of the Landau parameters, rather than via the backscattering amplitude, contrary to the case of the specific heat. Near a ferromagnetic quantum critical point, the field dependence acquires a universal form \chi^{-1}_s(H)= const- c_2|H|^{3/2}$, with $c_2>0$. This behavior implies a first-order transition into a ferromagnetic state. We establish a criterion for such a transition to win over the transition into an incommensurate phase.