Collinear magnetic structure and multiferroicity in the polar magnet Co2Mo3O8

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice ${(\mathrm{Fe},\phantom{\rule{0.28em}{0ex}}\mathrm{Mn})}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, a structural analogue of ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, and present a series of characterizations on its structural, magnetic, and electric properties. An antiferromagnetic transition takes place at the Neel temperature ${T}_{N}=39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with appearance of electric polarization and dielectric anomaly, which provides clear evidence of simultaneous magnetic and ferroelectric transitions. The neutron powder diffraction (NPD) and magnetic susceptibility data confirm the $c$-axis collinear antiferromagnetic orders and emergent ferroelectric polarization. In particular, such antiferromagnetic order is relatively robust against magnetic field up to 9 T, different from ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with ferrimagnetic transition or ${\mathrm{Mn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with spin flop in the low-field region. Our data on single crystals demonstrate the second-order magnetoelectric effect in terms of magnetic field dependence of ferroelectric polarization response, while no linear magnetoelectric response is allowed. It is suggested that ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.