Magnetization and ESR studies on Cu4(OH)6FCl: An antiferromagnet with a kagome lattice

We have synthesized a spin-1/2 antiferromagnet, $\mathrm{C}{\mathrm{u}}_{4}{(\mathrm{OH})}_{6}\mathrm{FCl}$, containing kagome lattices. The magnetic susceptibility gives a Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}=\ensuremath{-}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, nearest-neighbor exchange $J=\ensuremath{-}166\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and two magnetic anomalies at ${T}_{1}=15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{2}=4.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with weak ferromagnetism. However, the specific heat curve does not exhibit \ensuremath{\lambda}-like peaks expected for long-range magnetic order. The high-field ESR data are strongly correlated with the magnetism. There are a gapless mode ${\ensuremath{\omega}}_{1}$ observed at all temperatures and two gapped modes, ${\ensuremath{\omega}}_{2}$ and ${\ensuremath{\omega}}_{3}$, corresponding to different temperature regions: $T<{T}_{1}\phantom{\rule{0.28em}{0ex}}(=15\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and $T<{T}_{2}\phantom{\rule{0.28em}{0ex}}(=4.8\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, respectively. The possible origins of the gapped modes are discussed.