Fractional Quantum Hall Effect and Wigner Crystal of Interacting Composite Fermions

In two-dimensional electron systems confined to GaAs quantum wells, as a function of either tilting the sample in a magnetic field or increasing density, we observe multiple spin-polarization transitions of the fractional quantum Hall states at filling factors $\ensuremath{\nu}=4/5$ and $5/7$. The number of observed transitions provides evidence that these are fractional quantum Hall states of interacting two-flux composite fermions. Moreover, the fact that the reentrant integer quantum Hall effect near $\ensuremath{\nu}=4/5$ always develops following the transition to full spin polarization of the $\ensuremath{\nu}=4/5$ fractional quantum Hall state links the reentrant phase to a pinned ferromagnetic Wigner crystal of composite fermions.