Specific heat study of geometrically frustrated magnet clinoatacamite Cu₂Cl(OH)₃

We have performed the specific heat study in a new geometrically frustrated system, clinoatacamite Cu₂Cl(OH)₃ with the corner-sharing tetrahedron structure of the Cu²⁺ ions with S=1/2 Heisenberg spin. At H=0 T, two anomalies are observed at T₁=18.1 K and at T ₂=6.2 K. The specific heat decreases rapidly below T₂ and shows no anomaly down to T=150 mK despite the existence of the spin fluctuation shown in the μSR experiments. As the magnetic field is increased, the sharp peak at T₂ is broadened and shows a small reentrant behavior in the T - H phase diagram. On the other hand, the peak at T₁ shows no obvious change up to H=5 T. The entropy at T₁ is estimated as ∼0.35Rln2 at H=0 T. These features may be caused by the two dimensional nature of the kagome antiferromagnets which are weakly coupled via Cu ²⁺ ions at the triangular sites located in between the kagome layers.