Nonlinear 2D Spin Susceptibility in a Finite Magnetic Field:   Spin-Polarization Dependence
  By theoretically calculating the interacting spin susceptibility of a two-dimensional electron system in the presence of finite spin polarization, we show that the extensively employed technique of measuring the 2D spin susceptibility by linear extrapolation to a zero field from the finite-field experimental data is theoretically unjustified due to the strong nonlinear magnetic field dependence of the interacting susceptibility. Our work compellingly establishes that much of the prevailing interpretation of the 2D susceptibility measurements is incorrect, and, in general, the 2D interacting susceptibility cannot be extracted from the critical magnetic field for full spin polarization, as is routinely done experimentally. 
Ying Zhang, S. Das Sarma