  Competition of crystal field splitting and Hund's rule coupling in magnetic
metal-insulator transitions of half-filled two-orbital Hubbard model is
investigated by multi-orbital slave-boson mean field theory. We show that with
the increase of Coulomb correlation, the system firstly transits from a
paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott
insulator, or a nonmagnetic orbital insulator, depending on the competition of
crystal field splitting and the Hund's rule coupling. The different AFM Mott
insulator, PM metal and orbital insulating phase are none, partially and fully
orbital polarized, respectively. For a small $J_{H}$ and a finite crystal
field, the orbital insulator is robust. Although the system is nonmagnetic, the
phase boundary of the orbital insulator transition obviously shifts to the
small $U$ regime after the magnetic correlations is taken into account. These
results demonstrate that large crystal field splitting favors the formation of
the orbital insulating phase, while large Hund's rule coupling tends to destroy
it, driving the low-spin to high-spin transition.
