Nonlinear Spin Polarization Dependence of Spin Susceptibility in Interacting Two-Dimensional Electron Systems

We measure the spin polarization (P) of two-dimensional electron gases confined to an InSb quantum well using parallel and tilted magnetic fields. The nonlinear field dependence of P is prominent, leading to a direct deduction of the spin susceptibility (chi(s)) over a wide range of P from 0.07 to 1. chi(s) is found to increase nonlinearly with P and exceed chi(gm) infinity m*g* (where m* and g* are the effective mass and g factor) as commonly used in experiments. We show that chi(s) and chi(gm) obey a square law chi(s)/chi(0) = (chi(gm)/chi(0))(2), where chi(0) is the paramagnetic spin susceptibility.