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

Abstract

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 TN 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below TN. 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 vQ ≃ 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 TN. 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.

From magnetic order to quantum disorder in the Zn-barlowite series of S = 1/2 kagomé antiferromagnets

“…Also of note is that we find no evidence in our μSR measurements for the coexistence of long-range magnetic order and dynamic magnetic moment fluctuations in barlowite, which has been previously observed by other groups via NMR measurements 53 and in the structurally related clinoatacamite via μSR measurements 33 . Indeed, the reduced ordered moments obtained from the magnetic structure refinement of barlowite also indicate the presence of persistent dynamics in its ordered state 23 .…”

Tuning of a Kagome Magnet: Insulating Ground State in Ga‐Substituted Cu4(OH)6Cl2

“…We assume that the increase of the line broadening when lowering temperature is of magnetic origin. If so the width of the line should follow the bulk susceptibility . The 71 Ga NMR line width (full width at half maximum, ) is plotted in Figure c as a function of temperature.…”

Field-induced instability of the quantum spin liquid ground state in the Jeff=12 triangular-lattice compound NaYbO2

“…The coefficient β from a stretched exponential fit function is constant above 2 K, indicating a homogeneous distribution of fluctuating moments [39]. Below 2 K and at 3.1 T, β changes and shows a dip at T N1 , which is typical for strongly frustrated low-dimensional spin systems [52][53][54]. In contrast, at 0.8 T there is only a smooth change in β, which might indicate the onset of correlations among the Yb 3+ ions.…”