From spin liquid to magnetic ordering in the anisotropic kagome Y-kapellasite Y3Cu9(OH)19Cl8: A single-crystal study

Y3Cu9(OH)19Cl8 realizes an original anisotropic kagome model hosting a rich magnetic phase diagram [Hering et al., npj Comput. Mater. 8, 10 (2022)]. We present an improved synthesis of large phase-pure single crystals via an external gradient method. These crystals were investigated in detail by susceptibility, specific heat, thermal expansion, neutron scattering, and local mu SR and NMR techniques. At variance with polycrystalline samples, the study of single crystals gives evidence for subtle structural instabilities at 33 and 13 K which preserve the global symmetry of the system and thus the magnetic model. At 2.1 K the compound shows a magnetic transition to a coplanar (1/3,1/3) long-range order as predicted theoretically. However, our analysis of the spin-wave excitations yields magnetic interactions which locate the compound closer to the phase boundary to a classical jammed spin-liquid phase. Enhanced quantum fluctuations at this boundary may be responsible for the strongly reduced ordered moment of the Cu2+.