  Electrons' multiple quantum degrees of freedom can lead to rich physics,
including a competition between various exotic ground states, as well as novel
applications such as spintronics and valleytronics. Here we report
magneto-transport experiments demonstrating how the valley degree of freedom
impacts the fractional quantum states (FQHSs), and the related
magnetic-flux-electron composite fermions (CFs), at very high magnetic fields
in the extreme quantum limit when only the lowest Landau level is occupied.
Unlike in other multivalley two-dimensional electron systems such as Si or
monolayer graphene and transition-metal dichalcogenides, in our AlAs sample we
can continuously tune the valley polarization via the application of in-situ
strain. We find that the FQHSs remain exceptionally strong even as they make
valley polarization transitions, revealing a surprisingly robust ferromagnetism
of the FQHSs and the underlying CFs. Our observation implies that the CFs are
strongly interacting in our system. We are also able to obtain a phase diagram
for the FQHS and CF valley polarization in the extreme quantum limit as we
monitor transitions of the FHQSs with different valley polarizations.
