Local probe investigation of the spin dynamics in the kagome and interlayers of orthorhombic barlowite Cu₄(OD)₆FBr: ⁷⁹Br and ⁶³Cu NQR study

We report Br-79 and Cu-63 nuclear quadrupole resonance (NQR) in the paramagnetic state above T-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 Br-79 sites evidences that critical slowing down of Cu spin fluctuations sets in below similar to 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 Cu-63 NQR signal intensity is gradually wiped out below similar to 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.