Local probe investigation of the spin dynamics in the kagome and interlayers of orthorhombic barlowite Cu4(OD)6FBr: Br79 and Cu63 NQR study

We report Br79 and Cu63 nuclear quadrupole resonance (NQR) in the paramagnetic state above TN=15 K of the antiferromagnetic orthorhombic phase of barlowite Cu4(OD)6FBr consisting of a layered kagome structure. The divergent behavior of the longitudinal (1/T1)79 and transverse (1/T2)79 relaxation rates observed at Br79 sites evidences that critical slowing down of Cu spin fluctuations sets in below ∼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 Cu63 NQR signal intensity is gradually wiped out below ∼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.