Local probe investigation of the spin dynamics in the kagome and inter-layers of orthorhombic barlowite Cu_4(OD)_6FBr: ^79Br and ^63Cu NQR study

We report ^79Br and ^63Cu nuclear quadrupole resonance (NQR) in the paramagnetic state above T_N = 15 K of the antiferromagnetic orthorhombic phase of barlowite Cu_4(OD)_6FBr 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 ^79Br 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 ^63Cu 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.