  Experimental and theoretical studies of the coherent spin dynamics of
two-dimensional GaAs/AlGaAs electron gas were performed. The system in the
quantum Hall ferromagnet state exhibits a spin relaxation mechanism that is
determined by many-particle Coulomb interactions. In addition to the spin
exciton with changes in the spin quantum numbers of $\delta S\!=\!\delta S_z
\!=\!-1$, the quantum Hall ferromagnet supports a Goldstone spin exciton that
changes the spin quantum numbers to $\delta S\!=\!0$ and $\delta S_z\!=\!-1$,
which corresponds to a coherent spin rotation of the entire electron system to
a certain angle. The Goldstone spin exciton decays through a specific
relaxation mechanism that is unlike any other collective spin state.
