Ferromagnetic Phase Transition in a Multivalley Electron System

The quantum phase transition has been discovered in a multivalley two-dimensional electron system confined in an AlAs quantum well at low temperatures and high magnetic fields. The transition has occurred with a successive increase in the tilt angle of the magnetic field and has been accompanied by the appearance of a sharp peak in the longitudinal resistance of the sample. The electron spin resonance technique has made it possible to identify the type of the observed phase transition, which is extremely difficult to determine by conventional transport approaches. At the same time, the component of the magnetic field in the plane of the quantum well is directed in such a way that the spin resonance of the electrons occupying each of the valleys can be measured independently. Analysis of the evolution of the relative amplitude of resonances has demonstrated that the observed phase transition is attributed to the spin type; i.e., it is accompanied by a macroscopic change in the spin polarization without changing the relative valley population.