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

Abstract

We report a comprehensive muon spectroscopy study of the Zn-barlowite series of $$S=\frac{1}{2}$$ S = 1 2 kagomé antiferromagnets, ZnxCu4−x(OH)6FBr, for x = 0.00 to 0.99(1). By combining muon spin relaxation and rotation measurements with state-of-the-art density-functional theory muon-site calculations, we observe the formation of both μ–F and μ–OH complexes in Zn-barlowite. From these stopping sites, implanted muon spins reveal the suppression of long-range magnetic order into a possible quantum spin liquid state upon the increasing concentration of Zn-substitution. In the parent compound (x = 0), static long-range magnetic order below TN = 15 K manifests itself in the form of spontaneous oscillations in the time-dependent muon asymmetry signal consistent with the dipolar fields expected from the calculated muon stopping sites and the previously determined magnetic structure of barlowite. Meanwhile, in the x = 1.0 end-member of the series—in which antiferromagnetic kagomé layers of Cu2+$$S=\frac{1}{2}$$ S = 1 2 moments are decoupled by diamagnetic Zn2+ ions—we observe that dynamic magnetic moment fluctuations persist down to at least 50 mK, indicative of a quantum disordered ground state. We demonstrate that this crossover from a static to dynamic magnetic ground state occurs for compositions of Zn-barlowite with x > 0.5, which bears resemblance to the dynamical behaviour of the widely studied Zn-paratacamite series that contains the quantum spin liquid candidate herbertsmithite.

Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-barlowite ZnCu$_3$(OD)$_6$FBr

“…dom around 75 K in 19 F NMR [15] and µSR experiments [19]. Therefore, the observed NQR anomalies must be attributed to the EFG, and we conclude that the structural environments at 79 Br sites become somewhat different and more disordered below ∼ 75 K. We note that the spatially averaged crystal structure observed by diffraction techniques maintains the perfect kagome symmetry down to 3 K by neutron powder diffraction and 13 K by synchrotron x-ray diffraction [17].…”

“…Likewise, on the experimental side, each spin liquid candidate material has its own complications, too, often arising from structural disorders. For example, the non-magnetic interlayer Zn 2+ sites of the kagome lattice Heisenberg antiferromagnet (KLHA) herbertsmithite ZnCu 3 (OH) 6 Cl 2 [3][4][5][6][7][8][9][10][11][12][13][14][15] and Zn-barlowite ZnCu 3 (OH) 6 FBr [15][16][17][18][19] are occupied by Cu 2+ defect spins with ∼ 15% [5] and ∼ 5% [18] probability, respectively. These defect spins have been generally believed to account for the enhanced magnetic response observed at low temperatures, and mask the intrinsic behavior of the kagome planes.…”

“…In order to understand the mechanism behind these NQR anomalies across ∼ 75 K, we measured 79 1/T 1 at the main peak with high precision between 60 K and estimated from the results in (a). Also plotted using the right axis is 19 1/T1 measured at 19 F sites [15]. 280 K. We also measured 81 1/T 1 at 81 ν Q 24.1 MHz for the 81 Br sites.…”

“…For comparison, we also present 79 1/T spin 1 measured at the 19 F sites [15]. 19 F has nuclear spin 1/2 and lacks nuclear quadrupole moment, and probes only spin fluctuations with no influence of the EFG. The similarity in the observed temperature dependence between 79 1/T spin 1 and 19 1/T 1 assures us that our procedures to separate 79 1/T 1 into 79 1/T spin 1 and 79 1/T lattice 1 are working well.…”

Dynamic fingerprint of fractionalized excitations in single-crystalline Cu3Zn(OH)6FBr

“…Considering the fact that a lattice distortion has recently been confirmed in Herbersmithite, [24,25] which stimulates investigations on the subtle magneto-elastic effect in the kagome materials, [26,27] an alternative realization of the QSL compound with the ideal kagome lattice is still in urgent need. Zn-Barlowite [Cu 3 Zn(OH) 6 FBr] is another candidate for a kagome QSL ground state [28][29][30][31][32][33][34][35][36][37][38] with no lattice distortion being reported yet. Measurements on the powder samples didn't detect the long-range magnetic order down to temperatures of 0.02 K, four orders of magnitude lower than the Curie-Weiss temperature.…”