  In a GaAs/AlGaAs two-dimensional electron system with two occupied subbands,
the experimentally determined phase diagram in the density-magnetic field plane
exhibits rich topological features. Ring-like structures are observed at even
integer filling factors, in the phase diagram. Even with the identical
quantized Hall resistance values as those given rise by the ordinary integer
quantum Hall effect due to the Landau level quantization; the activation
energies of these states within the rings are much smaller. These ring
structures cannot account for by the simple single particle picture. We argue
that ferromagnetic quantum Hall states, due to the interaction of two energy
levels with opposite spin and different subband indices, are responsible for
these unusual structures.
