  The electronic correlation may have essential influence on electronic
structures in some materials with special structure and localized orbital
distribution. In this work, taking Janus monolayer FeClF as a concrete example,
the correlation effects on its electronic structures are investigated by using
generalized gradient approximation plus $U$ (GGA+$U$) approach. For
perpendicular magnetic anisotropy (PMA), the increasing electron correlation
effect can induce the ferrovalley (FV) to half-valley-metal (HVM) to quantum
anomalous Hall (QAH) to HVM to FV transitions. For QAH state, there are a unit
Chern number and a chiral edge state connecting the conduction and valence
bands. The HVM state is at the boundary of the QAH phase, whose carriers are
intrinsically 100\% valley polarized. With the in-plane magnetic anisotropy, no
special QAH states and prominent valley polarization are observed. However, for
both out-of-plane and in-plane magnetic anisotropy, sign-reversible Berry
curvature can be observed with increasing $U$. It is found that these
phenomenons are related with the change of $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$
orbital distributions and different magnetocrystalline directions. It is also
found that the magnetic anisotropy energy (MAE) and Curie temperature strongly
depend on the $U$. With PMA, taking typical $U=$2.5 eV, the electron valley
polarization can be observed with valley splitting of 109 meV, which can be
switched by reversing the magnetization direction. The analysis and results can
be readily extended to other nine members of monolayer FeXY (X/Y=F, Cl, Br and
I) due to sharing the same Fe-dominated low-energy states and electronic
correlations with FeClF monolayer.
