Critical Behavior of the Pauli Spin Susceptibility in A Strongly Correlated 2d Electron Liquid

We have performed measurements of the thermodynamic magnetization and density of states in a low-disordered, strongly correlated 2D electron system in silicon. We have found that the spin susceptibility of band electrons (Pauli spin susceptibility) grows by almost an order of magnitude as the electron density ($n_s$) is reduced, behaving critically near $n_s=n_\chi\approx8\times10^{10}$~cm$^{-2}$. This provides thermodynamic evidence for the existence of a phase transition. The density $n_\chi$ is coincident within the experimental uncertainty with the critical density $n_c$ for the metal-insulator transition in clean samples. The nature of the low-density phase ($n_s