Emergence of Frustrated Short-Range Order above Long-Range Order in the S = 1/2 Kagome Antiferromagnet CaCu₃(OD)₆Cl₂·0.6D₂O

We report on the low-energy dynamics in the kagome antiferromagnet CaCu₃(OD)₆Cl₂·0.6D₂O (Ca-kapellasite) as studied by use of ²D-NMR measurements. Previous ³⁵Cl-NMR measurements revealed that the nuclear spin–lattice relaxation rate (1/T₁) shows two peaks at temperatures, T* = 7.2 K and T_s ≃ 25 K. While the low-temperature peak at T* is ascribed to the critical fluctuations near the long-range magnetic ordering, the origin of the high-temperature peak has not been fully understood. From the 1/T₁ measurements on the D sites at the OD groups (D_OD), we find no peak at T_s, evidencing that the high-temperature peak is not related to the molecular dynamics of the OD groups. We discuss the possibility of a frustration-induced short-range ordered state below T_s before the long-range order is stabilized by the Dzyaloshinskii–Moriya interaction. We also observed static internal fields at the DOD site in the long-range ordered state below T*, and confirm the previously proposed negative-chirality q = 0 magnetic structure.