  Optical absorption measurements are used to probe the spin polarization in
the integer and fractional quantum Hall effect regimes. The system is fully
spin polarized only at filling factor $\nu=1$ and at very low
temperatures($\sim40$ mK). A small change in filling factor
($\delta\nu\approx\pm0.01$) leads to a significant depolarization. This
suggests that the itinerant quantum Hall ferromagnet at $\nu=1$ is surprisingly
fragile against increasing temperature, or against small changes in filling
factor.
