  We report on an absolute measurement of the electronic spin polarization of
the $\nu=1$ integer quantum Hall state. The spin polarization is extracted in
the vicinity of $\nu=1$ (including at exactly $\nu=1$) via resistive NMR
experiments performed at different magnetic fields (electron densities), and
Zeeman energy configurations. At the lowest magnetic fields, the polarization
is found to be complete in a narrow region around $\nu=1$. Increasing the
magnetic field (electron density) induces a significant depolarization of the
system, which we attribute to a transition between the quantum Hall ferromagnet
and the Skyrmion glass phase theoretically expected as the ratio between
Coulomb interactions and disorder is increased. These observations account for
the fragility of the polarization previously observed in high mobility 2D
electron gas, and experimentally demonstrate the existence of an optimal amount
of disorder to stabilize the ferromagnetic state.
