Low temperature magnetization of the S=1/2 kagome antiferromagnet ZnCu3(OH)6Cl2

The dc magnetization of the unique S=1/2 kagome antiferromagnet Herbertsmithite has been measured down to 0.1 K. No sign of spin freezing is observed in agreement with former mu SR and ac-susceptibility results. The low temperature magnetic response is dominated by a defect contribution which exhibits an energy scale similar or equal to 1 K, likely reflecting the coupling of the defects. The defect component is saturated at low temperature by H greater than or similar to 8 T applied magnetic fields which enables us to estimate an upper bound for the nonsaturated intrinsic kagome susceptibility at T=1.7 K.