  We report experimental studies on enhancing the magnetoelectric (ME) coupling
of Co$_4$Nb$_2$O$_9$ by substitution of non-magnetic metal Mg for Co. A series
of single crystal Co$_{4-x}$Mg$_x$Nb$_2$O$_9$ (x=0, 1, 2, 3) with a
single-phase corundum-type structure are synthesized by the optical floating
zone method, and the good quality and crystallographic orientations of the
synthesized samples are confirmed by Laue spots and sharp XRD peaks. Although
the Neel temperature ($T_N$) of the Mg substituted crystals are slightly
decreased from 27 K for pure Co$_4$Nb$_2$O$_9$ to 19 K and 11 K for
Co$_3$MgNb$_2$O$_9$ and Co$_2$Mg$_2$Nb$_2$O$_9$, respectively, the ME coupling
is double enhanced by Mg substitution when x=1. For the magnetic field
(electric field) control of electric polarization (magnetization), the ME
coefficient $\alpha_{ME}$ of Co$_3$MgNb$_2$O$_9$ is measured to be 12.8 ps/m
(13.7 ps/m). These results indicate that the Mg substituted
Co$_{4-x}$Mg$_x$Nb$_2$O$_9$ (x=1) could serve as a potential material candidate
for applications in future logic spintronics and logic devices.
