New S=1/2 Kagomé antiferromagnets A₂Cu₃SnF₁₂: A=Cs and Rb

We synthesized single crystals of the new hexagonal compounds A₂Cu₃SnF₁₂ with A=Cs and Rb, and investigated their magnetic properties. These compounds are composed of Kagomé layers of corner-sharing CuF₆-octahedra. Cs₂Cu₃SnF₁₂ has the proper Kagomé layer at room temperature, and undergoes structural phase transition at T_t ≊ 185 K. The temperature dependence of the magnetic susceptibility in Cs₂Cu₃SnF₁₂ agrees well with the result of the numerical calculation for S = 1/2 two-dimensional Heisenberg Kagomé antiferromagnet down to T_t with the nearest exchange interaction J/k_B ≊ 240 K. Although the magnetic susceptibility deviates from the calculated result below T < T_t, the rounded maxima were observed at approximately T ≊ (1/6)J/k_B as predicted by the theory. Cs₂Cu₃SnF₁₂ undergoes three-dimensional magnetic ordering at T_N = 20 K. Rb₂Cu₃SnF₁₂ has the Kagomé layer, whose unit cell is enlarged by 2a × 2a as compared with the proper Kagomé layer even at room temperature. From the viewpoint of crystal structure, the exchange interactions between nearest neighbor Cu²⁺-ions are classified into four kinds. From the magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted by recent theory. Exact diagonalization method with 12-site Kagomé cluster was performed to analyze the magnetic susceptibility. By comparing the calculated results with the experimental data, the individual exchange interactions were evaluated.