Low temperature magnetization of the $S=\frac{1}{2}$ kagome antiferromagnet $\mathrm{Zn}{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_6{\mathrm{Cl}}_2$

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