Emergence of Frustrated Short-Range Order above Long-Range Order in the $S=1/2$ Kagome Antiferromagnet CaCu$_3$(OD)$_6$Cl$_2\cdot0.6$D$_2$O

TL;DR

This study investigates the low-energy spin dynamics in the kagome antiferromagnet CaCu$_3$(OD)$_6$Cl$_2ullet0.6$D$_2$O using $^2$D-NMR, revealing a frustration-induced short-range order above the long-range magnetic order.

Contribution

It provides new insights into the nature of short-range order and its relation to long-range magnetic order in a kagome antiferromagnet through detailed NMR measurements.

Findings

No peak at $T_s$ in $1/T_1$ for D sites, indicating no molecular dynamics origin.

Evidence of frustration-induced short-range order below $T_s$.

Confirmation of a negative-chirality $q=0$ magnetic structure below $T^{ ext{*}}$.

Abstract

We report on the low-energy dynamics in the kagome antiferromagnet CaCu$_3$(OD)$_6$Cl$_2\cdot0.6$D$_2$O (Ca-kapellasite) as studied by use of $^2$D-NMR measurements. Previous $^{35}$Cl-NMR measurements revealed that the nuclear spin-lattice relaxation rate ($1/T_1$) shows two peaks at temperatures, $T^{\ast} = 7.2$ K and $T_s \simeq 25$ K. While the low-temperature peak at $T^{\ast}$ 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 $1/T_1$ measurements on the D sites at the OD groups (D$_{\rm OD}$), 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 D$_{\rm OD}$ site in the long-range ordered state below $T^{\ast}$, and confirm the previously proposed negative-chirality $q=0$ magnetic structure.