Magnetic resonance as a local probe for kagomé magnetism in Barlowite Cu${}_4$(OH)${}_6$FBr

Abstract

Temperature- and field- dependent ${}^1$H-, ${}^{19}$F-, and ${}^{79,81}$Br- NMR measurements together with zero - field ${}^{79,81}$Br-NQR measurements on polycrystalline samples of barlowite, Cu${}_4$(OH)${}_6$FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the ${}^{79,81}$Br- NMR spin-lattice relaxation rates 1/$T_1$ indicate a phase transition at $T_{\mathrm{N}}\simeq$15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below $T_{\mathrm{N}}$. Moreover, 1/$T_{1}T$ 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 ${\nu }_Q\simeq$ 28.5~MHz and 24.7~MHz for ${}^{79}$Br- and ${}^{81}$Br- nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to $\eta \simeq 0.2$. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above $T_{\mathrm{N}}$. This broadening and the relative large $\eta$ 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 $\eta$. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.