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

TL;DR

This study uses $^{79}$Br and $^{63}$Cu NQR to investigate spin dynamics in orthorhombic barlowite, revealing critical slowing down of Cu spins and gradual spin freezing, highlighting magnetic frustration effects.

Contribution

It provides the first detailed NQR analysis of spin dynamics in orthorhombic barlowite, elucidating the distinct behaviors of interlayer and kagome layer spins.

Findings

Critical slowing down of Cu spin fluctuations below ~20 K.

Gradual spin freezing of kagome layers below ~30 K.

Interlayer Cu sites undergo conventional antiferromagnetic transition.

Abstract

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 $^{79}(1/T_{1})$ and transverse $^{79}(1/T_{2})$ relaxation rates observed at $^{79}$Br sites evidences that critical slowing down of Cu spin fluctuations sets in below $\sim20$ 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 $\sim30$ 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.