Magnetic resonance as a local probe for kagomé magnetism in Barlowite Cu₄(OH)₆FBr

Temperature- and field-dependent ¹H-, ¹⁹F-, and ^79,81Br-NMR measurements together with zero - field ^79,81Br-NQR measurements on polycrystalline samples of barlowite, Cu₄(OH)₆FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the ^79,81Br-NMR spin-lattice relaxation rates 1/T₁ indicate a phase transition at T_N [Formula: see text] 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below T_N. Moreover, 1/T₁T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of v_Q ≃ 28.5 MHz and 24.7 MHz for ⁷⁹Br- and ⁸¹Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above T_N. This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.