Phase transition near the filling factor $\nu=3$

A phase transition accompanied by the appearance of a spike in the longitudinal resistance of a two-dimensional electron system has been studied using the electron spin resonance near the filling factor $\nu=3$ in the ZnO/MgZnO heterojunction. This transition occurs when the tilt angle $\theta$ of the magnetic field is increased to some critical value $\theta_c$. An analysis of the spin resonance amplitude has made it possible to demonstrate the spin nature of this phenomenon. For example, the ground state of the system on both sides of the transition has a nonzero spin polarization, which changes by several times when the phase of the system is changed. Strong spin resonance is observed both at $\theta<\theta_c$ and at $\theta>\theta_c$. Surprisingly, the spin resonance at the critical angle $\theta_c$ has been detected in only one phase, which lies in the region of magnetic fields below the critical field $B_c$ corresponding to the spike position in the longitudinal resistance. An increase in the magnetic field to this value leads to a decrease in the resonance amplitude and an increase in the resonance width. In the field region above $B_c$, the spin resonance disappears completely. Such behavior of the spin resonance is most likely due to the formation of domains with different spin polarizations in the electron system.