Fe2Co2Nb2O9: a magnetoelectric honeycomb antiferromagnet

Abstract

Fe₄Nb₂O₉ and Co₄Nb₂O₉ are antiferromagnetic compounds belonging to the M₄M′₂O₉ series (M = Mn, Fe, Co; M′ = Nb, Ta), whose structure (Pc1) derives from the Cr₂O₃ corundum one. By mixing cobalt and iron in a 1 : 1 ratio, an Fe₂Co₂Nb₂O₉ oxide has been synthesized. Its structural, magnetic and electric characterization studies confirm that its unit cell volume (V = 331.19(1) ų), Néel temperature (T_N = 58 K) and dielectric permittivity are intermediate between those of the Fe₄Nb₂O₉ and Co₄Nb₂O₉ end members. The neutron diffraction study evidences an Fe:Co random occupation of the (4d) Wyckoff sites. Its antiferromagnetic structure, refined in the C2/c′ magnetic space group, shows magnetic moments in the planes of the honeycomb (HC) layers of the M₄M′₂O₉ structure, and stacked along c, as for Fe₄Nb₂O₉ and Co₄Nb₂O₉. However, the magnetic field induced magnetization M(H) of Fe₂Co₂Nb₂O₉ below T_N differs strongly from both end members, exhibiting a sublinear dependence. A similar rounded shape of the electric polarization P(H) curve is evidenced, in marked contrast to the linear M(H) and P(H) behaviours observed below the spin-flop magnetic field for this class of linear magnetoelectric materials. As a result, Fe₂Co₂Nb₂O₉ exhibits larger P values and steeper P responses to H at low H than both Fe₄Nb₂O₉ and Co₄Nb₂O₉ end members. This result is explained by a microscopic model based on the Fe and Co mixed occupation. Substitutional disorder locally allows for stronger magnetoelectric couplings and its effect does not cancel on average. This chemical substitution effect opens a new route to enhance the magnetoelectric response of honeycomb antiferromagnets.