Local probe investigation of the spin dynamics in the kagome and inter-layers of orthorhombic barlowite Cu${}_4$(OD)${}_6$FBr: ${}^{79}$Br and ${}^{63}$Cu NQR study

We report ${}^{79}$Br and ${}^{63}$Cu nuclear quadrupole resonance (NQR) in the paramagnetic state above $T_{\text{N}}=15$ K of the antiferromagnetic orthorhombic phase of barlowite Cu${}_4$(OD)${}_6$FBr consisting of a layered kagome structure. The divergent behavior of the longitudinal ${}^{79}(1\mathrm{/}{T}_1)$ and transverse ${}^{79}(1\mathrm{/}{T}_2)$ relaxation rates observed at ${}^{79}$Br sites evidences that critical slowing down of Cu spin fluctuations sets in below $\sim 20$ K. This means that one or more Cu sites, most likely at the interlayer Cu(3,4,5) sites between the kagome planes, undergo the antiferromagnetic phase transition in a fairly conventional way. On the other hand, the ${}^{63}$Cu NQR signal intensity is gradually wiped out below $\sim 30$ K, pointing toward gradual spin freezing of the kagome layers instead. These contrasting findings suggest significant roles played by magnetic frustration effects within the kagome layers.