Thermodynamics of the Ising quantum hall ferromagnets at $\nu=2$

In the two-dimensional electron systems with strong coupling in MgZnO/ZnO heterostructures, the thermal behavior of Ising quantum Hall ferromagnets at the filling factor $\nu=2$ has been studied. The spin polarization of Hall ferromagnets has been detected by measuring the signal related to the inelastic light scattering by intrasubband spin excitons. A stepwise change in the spin polarization at the phase transition at the filling factors $\nu=2$, $3$, and $4$ in the heterostructures with different electron densities has been observed. The thermal stability of the Hall ferromagnetic phases at $\nu=2$ has been studied and the Curie temperature has been estimated. It has been shown that the Curie temperature is determined by the formation energy for domain walls in the Ising quantum Hall ferromagnets.