Spin ordering: two different scenarios for the single and double layer structures in the fractional and integer quantum Hall effect regimes

We investigate the ground state competition at the transition from the spin unpolarized to spin ordered phase at filling factor $\nu=2/3$ in single layer heterostructure and at $\nu=2$ in double layer quantum well. To trace the quantum Hall phase we use the minimum in the dissipative conductivity $\sigma_{xx}$. We observe two different transition scenarios in two investigated situations. For one of them we propose a qualitative explanation, based on the domain structure evolution in the vicinity of the transition point. The origin for the second scenario, corresponding to the experimental situation at $\nu=2$ in double layer 2DES, still remains unclear.