  Based on the chiral kinetic equations of motion, spin polarizations of
various quarks, due to the magnetic field induced by spectator protons as well
as the quark-antiquark vector interaction, are studied within a partonic
transport approach. Although the magnetic field in QGP enhances the splitting
of the spin polarizations of partons compared to the results under the magnetic
field in vacuum, the spin polarizations of $s$ and $\bar s$ quarks are also
sensitive to the quark-antiquark vector interaction, challenging that the
different $\Lambda$ and $\bar \Lambda$ spin polarization is a good measure of
the magnetic field in relativistic heavy-ion collisions. It is also found that
there is no way to obtain the large splitting of the spin polarization between
$\Lambda$ and $\bar \Lambda$ at $\sqrt{s_{NN}}=7.7$ GeV with partonic dynamics.
