Electron spin resonance under conditions of a ferromagnetic phase transition

The spin resonance of two-dimensional conduction electrons in a ZnO/MgZnO heterojunction in tilted magnetic fields is studied near the filling factor $\nu = 2$. The analysis of the spin resonance intensity at various $\nu$ values indicates that a phase transition accompanied by drastic change in the spin polarization occurs in a two-dimensional electron system at a certain angle near $\nu = 2$. For $\nu$ values larger than a certain critical value $\nu_c$, an intense spin resonance is observed, clearly demonstrating that the system turns out to be in a spin-polarized state. For $\nu<\nu_c$, the resonance amplitude drops by more than an order of magnitude, and the spin polarization of the ground state decreases significantly. In the immediate vicinity of the transition, the spin resonance is broadened significantly and split into several independent peaks. Such behavior of the resonance can most likely be explained by the division of the system into domains with different spin polarizations.