Magnetism study on a triangular lattice antiferromagnet Cu$_2$(OH)$_3$Br

Magnetism of Cu$_2$(OH)$_3$Br single crystals based on a triangular lattice is studied by means of magnetic susceptibility, pulsed-field magnetization, and specific heat measurements. There are two inequivalent Cu$^{2+}$ sites in an asymmetric unit. Both Cu$^{2+}$ sublattices undergo a long-range antiferromagnetic (AFM) order at $T\rm_N$ = 9.3 K. Upon cooling, an anisotropy crossover from Heisenberg to $XY$ behavior is observed below 7.5 K from the anisotropic magnetic susceptibility. The magnetic field applied within the $XY$ plane induces a spin-flop transition of Cu$^{2+}$ ions between 4.9 T and 5.3 T. With further increasing fields, the magnetic moment is gradually increased but is only about half of the saturation of a Cu$^{2+}$ ion even in 30 T. The individual reorientation of the inequivalent Cu$^{2+}$ spins under field is proposed to account for the magnetization behavior. The observed spin-flop transition is likely related to one Cu site, and the AFM coupling among the rest Cu spins is so strong that the 30-T field cannot overcome the anisotropy. The temperature dependence of the magnetic specific heat, which is well described by a sum of two gapped AFM contributions, is a further support for the proposed scenario.