Collinear magnetic structure and multiferroicity in the polar magnet Co2Mo3O8

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice (Fe_{,Mn ) 2}Mo₃O₈ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound Co₂Mo₃O₈ , a structural analogue of Fe₂Mo₃O₈ , 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 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 Fe₂Mo₃O₈ with ferrimagnetic transition or Mn₂Mo₃O₈ 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 Co₂Mo₃O₈ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.