Magnetization and ESR studies on Cu-4(OH)(6)FCl: An antiferromagnet with a kagome lattice

We have synthesized a spin-1/2 antiferromagnet, Cu-4(OH)(6)FCl, containing kagome lattices. The magnetic susceptibility gives a Curie-Weiss temperature theta(CW) = -135 K, nearest-neighbor exchange J = -166 K, and two magnetic anomalies at T-1 = 15 K and T-2 = 4.8 K with weak ferromagnetism. However, the specific heat curve does not exhibit.-like peaks expected for long-range magnetic order. The high-field ESR data are strongly correlated with the magnetism. There are a gapless mode omega(1) observed at all temperatures and two gapped modes, omega(2) and omega(3), corresponding to different temperature regions: T < T-1 (=15 K) andT < T-2 (=4.8 K), respectively. The possible origins of the gapped modes are discussed.