Unconventional spin freezing in the highly two-dimensional spin-1/2 kagome antiferromagnet Cd2Cu3(OH)6(SO4)24H2O: Evidence of partial order and coexisting spin singlet state on a distorted kagome lattice

S = 1/2 quantum-spin systems of the kagome lattice have received intense recent attention because they are expected to be promising candidates for novel quantum states such as spin liquid, spin nematic, and other unknown exotic states. Here we report an unusual transition in a highly two-dimensional and slightly distorted S = 1/2 quantum kagome system Cd2Cu3(OH)(6)(SO4)(24)H2O, which is a purified compound of the recently identified mineral edwardsite, Cd1.89Zn0.11Cu3(OH)(6)(SO4)(24)H2O. The S = 1/2 spins of Cu2+ ions form a highly two-dimensional kagome lattice with an extraordinarily large interlayer spacing >10 angstrom, thus providing a unique kagome lattice with minimum interlayer perturbation. The magnetization shows a Curie-Weiss temperature of 40.5 K and moment freezing below around T-F = 5 K and a magnetic transition feature near 3 K. The muon spin relaxation shows static magnetism below 5 K and saturation below 3 K. However, distinct features have been observed distinguishing it from a spin glass or a conventional order: High-field magnetization until 50 T showed a saturated value of only 1/3 of the full expected moment. Analysis of specific heat suggests that only 1/3 of the spins are related to the magnetic order; on the other hand, the order is destroyed in external field and 1/3 of the spins respond to the field with a Schottky-like behavior while the other 2/3 of the spins remain inactive. These experimental results suggest an unconventional coexistence of the partial order and most probably spin singlets.