  Monolayer transition metal dichalcogenides are recently emerged 2D electronic
systems with various novel properties, such as spin-valley locking, circular
dichroism, valley Hall effects, Ising superconductivity. The reduced
dimensionality and large effective masses further produce unconventional
many-body interaction effects. Although recent hole transport measurements in
WSe$_2$ indicate strong interactions in the valence bands, many-body
interaction effects, particularly in the conduction bands, remain elusive to
date. Here, for the first time, we perform transport measurements up to a
magnetic field of $29$T to study the massive Dirac electron Landau levels (LL)
in layer-polarized MoS$_2$ samples with mobilities of $22000$cm$^2$/(V$\cdot$s)
at $1.5$K and densities of $\sim10^{12}$cm$^{-2}$. With decreasing the density,
we observe LL crossing induced valley ferrimagnet-to-ferromagnet transitions,
as a result of the interaction enhancement of the g-factor from $5.64$ to
$21.82$. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL
anticrossings due to the formation of quantum Hall Ising ferromagnets, the
valley polarizations of which appear to be reversible by tuning the density or
an in-plane magnetic field. Our results provide compelling evidence for
many-body interaction effects in the conduction bands of monolayer MoS$_2$ and
establish a fertile ground for exploring strongly correlated phenomena of
massive Dirac electrons.
