  The spin polarization (P) of high-density InSb two-dimensional electron
systems (2DESs) has been measured using both parallel and tilted magnetic
fields. P is found to exhibit a superlinear increase with the total field B.
This P-B nonlinearity results in a difference in spin susceptibility between
its real value Xs and Xgm ~ m*g*(m* and g* are the effective mass and g factor,
respectively) as routinely used in experiments. We demonstrate that such a P-B
nonlinearity originates from the linearly P-dependent g* due to the exchange
coupling of electrons rather than from the electron correlation as predicted
for the low-density 2DES.
