Spin order in the classical spin kagome antiferromagnet M g x M n 4 − x ( OH ) 6 C l 2

Geometrically frustrated kagome-lattice antiferromagnets have received much attention because of their exotic magnetic behaviors. Here we report the synthesis, structure, and magnetism of an $S=5/2$ quasiclassical new spin kagome compound. Polycrystalline compounds of $\\mathrm{M}{\\mathrm{g}}_{x}\\mathrm{M}{\\mathrm{n}}_{4\\ensuremath{-}x}{(\\mathrm{OH})}_{6}\\mathrm{C}{\\mathrm{l}}_{2}$, with a nominal ideal chemical formula of $\\mathrm{MgM}{\\mathrm{n}}_{3}{(\\mathrm{OH})}_{6}\\mathrm{C}{\\mathrm{l}}_{2}$, have been successfully synthesized by selectively substituting the triangular-lattice-plane $\\mathrm{M}{\\mathrm{n}}^{2+}$ with nonmagnetic $\\mathrm{M}{\\mathrm{g}}^{2+}$. The compounds studied crystallize in rhombohedral structure in space group $R\\text{\\ensuremath{-}}3m$, in a similar crystal structure to the much-researched quantum spin liquid candidates herbertsmithite $\\mathrm{ZnC}{\\mathrm{u}}_{3}{(\\mathrm{OH})}_{6}\\mathrm{C}{\\mathrm{l}}_{2}$ and tondiite $\\mathrm{MgC}{\\mathrm{u}}_{3}{(\\mathrm{OH})}_{6}\\mathrm{C}{\\mathrm{l}}_{2}$. Antiferromagnetic transition below ${T}_{\\mathrm{N}}\\ensuremath{\\sim}8\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}$ and a 120\\ifmmode^\\circ\\else\\textdegree\\fi{} nearest-neighbor spin ordering confined in the kagome plane with positive spin chirality have been revealed by magnetic susceptibility measurements and neutron diffraction experiment. The obtained value of critical exponent $\\ensuremath{\\beta}=0.35$ agrees with a three-dimensional Heisenberg spin model. The present work suggests the intrinsic nature of long-range order in classical Heisenberg kagome aniferromagnet and provides a classical reference system to quantum kagome antiferromagnets.