We investigate the low temperature magnetic properties of a $S=\frac{ 5}{ 2}$ Heisenberg kagom\'e antiferromagnet, the layered monodiphosphate Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and $^{ 31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at $T_{ \rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below $T^* \sim 5$ K. A moderate $^{ 31}$P NMR line broadening reveals the development of anisotropic short-range correlations within the plateau phase concomitantly with a gapless spin-lattice relaxation time $T_1 \sim k_B T / \hbar S$, which both point to the presence of a semiclassical nematic spin liquid state predicted for the Heisenberg kagom\'e antiferromagnetic model or to the persistence of the zero-energy modes of the kagome lattice under large magnetic fields.
Categories: cond-mat.str-el cond-mat.mtrl-sci