Unusual Magnetic State with Dual Magnetic Excitations in the Single Crystal of <i>S</i> = 1/2 Kagome Lattice Antiferromagnet CaCu₃(OH)₆Cl₂ ∙ 0.6H₂O

Abstract

We have succeeded in preparing single crystals of CaCu3(OH)6Cl2 ∙ 0.6H2O, a candidate for the S = 1/2 Kagome lattice antiferromagnet. Magnetic properties of the compound are dominated by the nearest neighbor antiferromagnetic interaction J1, and the next nearest neighbor ferromagnetic J2 and an antiferromagnetic Jd across a hexagon, which is different from related compounds Kapellasite and Haydeeite with ferromagnetic J1. Magnetic susceptibility exhibits a sudden increase below 13 K and a cusp anomaly at T* = 7.2 K in the ab-plane, whereas only a moderate enhancement is observed below T* along the c-axis. A tiny peak detected in heat capacity at T* indicates the occurrence of a magnetic phase transition. The low temperature magnetic heat capacity was reproduced by assuming a two-dimensional spin-wave component and a temperature-linear term. The spin-wave contribution suggests a magnon excitation in a short-range ordered region, whereas the relatively large T-linear term 5.9 mJ/(Cu-mol·K2) at H = 0 T of this insulating compound suggests the existence of an unusual quasi-particle excitation below T*. They apparently reveal the unconventionality of the ground state of this S = 1/2 Kagome lattice antiferromagnet.