Single-valley quantum Hall ferromagnet in a dilute Mg _xZn _1-xO/ZnO strongly correlated two-dimensional electron system

We investigate the spin susceptibility (g ^*m ^*) of dilute two-dimensional (2D) electrons confined at the Mg _xZn _1-xO/ZnO heterointerface. Magnetotransport measurements show a four-fold enhancement of g ^*m ^*, dominated by the increase in the Landé g-factor. The g-factor enhancement leads to a ferromagnetic instability of the electron gas as evidenced by sharp resistance spikes. At high magnetic field, the large g ^*m ^* leads to full spin polarization, where we found sudden increase in resistance around the filling factors of half-integer, accompanied by complete disappearance of fractional quantum Hall (QH) states. Along with its large effective mass and the high electron mobility, our result indicates that the ZnO 2D system is ideal for investigating the effect of electron correlations in the QH regime.