Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca₃CoNb₂O₉

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca₃CoNb₂O₉, in which the effective spin of Co²⁺ is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θ_CW～-55 K and the low Neel temperature T_N～ 1.45 K give a frustration factor f=|θ_CW/T_N|≈ 38, suggesting that Ca₃CoNb₂O₉ resides in strong frustration regime. Slightly below T_N, deviation between the susceptibility data under zero-field cooling (ZFC) and field cooling (FC) is observed. A new magnetic state with 1/3 of the saturate magnetization M_s is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca₃CoNb₂O₉ is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.