Collinear magnetic structure and multiferroicity in the polar magnet Co2Mo3 O8

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice (Fe,Mn)2Mo3O8 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 TN=39K 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.