Magnetic Phase Diagram of Cu_4-xZn_x(OH)_6FBr Studied by Neutron-Diffraction and \muSR Techniques

We systematically investigate the magnetic properties of Cu_4-xZn_x(OH)_6FBr using the neutron diffraction and muon spin rotation and relaxation (μSR) techniques. Neutron-diffraction measurements suggest that the long-range magnetic order and the orthorhombic nuclear structure in the x = 0 sample can persist up to x = 0.23 and 0.43, respectively. The temperature dependence of the zero-field μSR spectra provides two characteristic temperatures, T_A_0 and T_\lambda, which are associated with the initial drop close to zero time and the long-time exponential decay of the muon relaxation, respectively. Comparison between T_A_0 and T_\rm M from previously reported magnetic-susceptibility measurements suggest that the former comes from the short-range interlayer-spin clusters that persist up to x = 0.82. On the other hand, the doping level where T_\lambda becomes zero is about 0.66, which is much higher than threshold of the long-range order, i.e., \sim0.4. Our results suggest that the change in the nuclear structure may alter the spin dynamics of the kagome layers and a gapped quantum-spin-liquid state may exist above x = 0.66 with the perfect kagome planes.