Unconventional magnetic transitions in the mineral clinoatacamiteCu2Cl(OH)3

Unconventional magnetic properties are found for a quantum spin system of mineral clinoatacamite, ${\mathrm{Cu}}_2\mathrm{Cl}{\left(\mathrm{O}\mathrm{H}\right)}_3$. Clinoatacamite is found to undergo an antiferromagnetic transition at $18.1\mathrm{K}$ with a small entropy fall $\left(0.05R\ln 2\right)$. At temperatures below $6.4\mathrm{K}$, it transits into a disordered spin-glass-like state but with large specific heat anomalies. The entire entropy fall for the spin system is about $0.31R\ln 2$, which is smaller than those for dipolar spin ice $\left(0.67R\ln 2\right)$ and for ice $I_h$ $\left(0.71R\ln 2\right)$. Geometric frustration for the quantum ${\mathrm{Cu}}^{2+}$ spins is suggested based on its crystal structure, magnetization, and specific heat studies.