Coexistence of Long-Range Order and Spin Fluctuation in Geometrically Frustrated ClinoatacamiteCu2Cl(OH)3

Muon spin rotation experiments are carried out on clinoatacamite, ${\mathrm{Cu}}_2\mathrm{Cl}(\mathrm{OH}{)}_3$, which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral $3d$ ${\mathrm{Cu}}^{2+}$ spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about $0.05R\ln 2/\mathrm{Cu}$. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.