  The magnetization and electric polarization of a polar antiferromagnet
Fe$_2$Mo$_3$O$_8$ are studied up to 66 T for spin-saturation magnetic fields
applied along the polar axis. The magnetization process at 1.4 K exhibited
multistep structures below the saturation field of 65 T. The electric
polarization along the polar axis exhibits a similar multistep behavior with a
total change of 1.2 $\rm{\mu}C/cm^{2}$. A combined triangular-lattice
antiferromagnetic model with strong Ising-type spin anisotropy reproduces this
multistep magnetoelectric (ME) effect. The exchange striction mechanism
explains the remarkable ME response in the two sub-lattice type-I multiferroic
materials. These results and interpretation demonstrate a method for realizing
multistage magnetoelectric effects in hybrid spin systems.
