Spin dynamics and magnetic ordering in the quasi-one-dimensional S = 1 2 antiferromagnet Na 2 CuSO 4 Cl 2

The $S=\\frac{1}{2}$ quasi-one-dimensional antiferromagnet ${\\mathrm{K}}_{2}{\\mathrm{CuSO}}_{4}{X}_{2}\\phantom{\\rule{4pt}{0ex}}(X=\\mathrm{Cl}$, Br) exhibits peculiar Dzyaloshinskii-Moriya (DM) interactions that are uniform along its spin chains and antiparallel with respect to neighboring chains. This feature has received much attention recently because it leads to spin frustration, however, the spin dynamics around ${T}_{\\mathrm{N}}$ and magnetic structure have not been reported. Here we report magnetic behaviors of ${\\mathrm{Na}}_{2}{\\mathrm{CuSO}}_{4}{\\mathrm{Cl}}_{2}$. The orthorhombic crystal structure of ${\\mathrm{Na}}_{2}{\\mathrm{CuSO}}_{4}{\\mathrm{Cl}}_{2}$, which is identical to $\\mathrm{K}{\\phantom{\\rule{0.16em}{0ex}}}_{2}{\\mathrm{CuSO}}_{4}{X}_{2}$, was verified. The results of the thermodynamic measurements suggest that this compound has moderately strong intra- and interchain interaction for investigation of the spin state around ${T}_{\\mathrm{N}}$. The inelastic neutron scattering and muon spin relaxation and rotation measurements reveal the presence of a two-spinon continuum, and below ${T}_{\\mathrm{N}}$, the long-range order develops. However, the obtained critical exponent is $\\ensuremath{\\beta}=0.18$, which is not indicative of a three-dimensional magnetic system; instead, one-dimensional (1D) spin correlation likely affects the formation of magnetic ordering in ${\\mathrm{Na}}_{2}{\\mathrm{CuSO}}_{4}{\\mathrm{Cl}}_{2}$. There is a possibility that ${\\mathrm{Na}}_{2}{\\mathrm{CuSO}}_{4}{\\mathrm{Cl}}_{2}$ is a model compound for investigation of DM-induced frustration effects in a 1D quantum spin system.