Classical Spin Liquid State in the S=52 Heisenberg Kagome Antiferromagnet Li9Fe3(P2O7)3(PO4)2

We investigate the low temperature magnetic properties of a $S=\frac{5}{2}$ Heisenberg kagome antiferromagnet, the layered monodiphosphate ${\mathrm{Li}}_{9}{\mathrm{Fe}}_{3}({\mathrm{P}}_{2}{\mathrm{O}}_{7}{)}_{3}({\mathrm{PO}}_{4}{)}_{2}$, using magnetization measurements and $^{31}\mathrm{P}$ nuclear magnetic resonance. An antiferromagnetic-type order sets in at ${T}_{N}=1.3\text{ }\text{ }\mathrm{K}$ and a characteristic magnetization plateau is observed at $1/3$ of the saturation magnetization below ${T}^{*}\ensuremath{\sim}5\text{ }\text{ }\mathrm{K}$. A moderate $^{31}\mathrm{P}$ NMR line broadening reveals the development of anisotropic short-range correlations concomitantly with a gapless spin-lattice relaxation time ${T}_{1}\ensuremath{\sim}{k}_{B}T/\ensuremath{\hbar}S$, which may point to the presence of a semiclassical nematic spin-liquid state predicted for the Heisenberg kagome antiferromagnetic model or to the persistence of the zero-energy modes of the kagome lattice under large magnetic fields.