Unconventional magnetic transitions in the mineral clinoatacamite ${\mathrm{Cu}}_2\mathrm{Cl}{\left(\mathrm{O}\mathrm{H}\right)}_3$

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.

Figure 1

Crystal structure of clinoatacamite showing the 3D network of corner-sharing tetrahedrons of ${\mathrm{Cu}}^{2+}$ ions (black balls).

Figure 2

(A) Temperature dependence of the dc susceptibility. The arrow helps to show a magnetic transition at $18.1\mathrm{K}$. (B) temperature dependence of reciprocal dc susceptibility. The dashed line follows a Curie-Weiss law of $\chi \sim A∕(T+190\mathrm{K})$.

Figure 3

(A) Temperature dependence of magnetization under zero-field cooling and field cooling conditions; (B) field dependence of magnetization.

Figure 4

(A) Real and imaginary parts of ac magnetic susceptibilities ${\chi }^{\prime }$ and ${\chi }^{″}$ measured at varied frequencies (10, 100, and $1000\mathrm{Hz}$, respectively); (B) ac susceptibilities (plotted on a logarithm scale) in dc fields at $1\mathrm{Hz}$. Dashed lines denote freezing temperatures.

Figure 5

(A) Temperature dependence of the specific heat (per mol of the molecule) at zero field and $5\mathrm{T}$, respectively, and the magnetic entropy; the dashed line represents the lattice contribution according to Debye’s law. (B) An enlarged view of the zero-field specific heat in the low temperature range showing consistency to the anomalies in the ac susceptibility.