Optical absorption to probe the quantum Hall ferromagnet at filling factor $\nu =1$

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($\sim 40$ mK). A small change in filling factor ($\delta \nu \approx \pm 0.01$) 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.