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arXiv · 1804.00893

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

K. M. Ranjith·C. Klein·A. A. Tsirlin·H. Rosner·C. Krellner·M. Baenitz
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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_{\rm N}\simeq$15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below $T_{\rm N}$. Moreover, 1/$T_1T$ 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 $T_{\rm N}$. 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.

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K. M. Ranjith, C. Klein, A. A. Tsirlin, H. Rosner, C. Krellner, M. Baenitz. 2018-04-03. Magnetic resonance as a local probe for kagom\'{e} magnetism in Barlowite Cu$_4$(OH)$_6$FBr. https://doi.org/10.1038/s41598-018-29080-8

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