Spectroscopies of classical and quantum Heisenberg kagome antiferromagnets: from classical nematic spin liquid in Li9Fe3(P2O7)3(PO4)2 to quantum magnetism in the anisotropic Y-kapellasite.

The kagome lattice occupies a central place in the field of magnetic frustration. Very early on, pioneering theoretical studies on the kagome Heisenberg antiferromagnetic (KHAF) model highlighted its specificity even at a classical level. Since then, it has become a prime candidate for stabilizing a quantum spin liquid in real materials. In this presentation, I will focus on two kagome antiferromagnets recently investigated in our group.



In the first part, I will discuss the magnetic properties of a classical S = 5/2 Heisenberg kagome antiferromagnet, the layered monodiphosphate Li9Fe3(P2O7)3(PO4)2, using magnetization measurements and 31P nuclear magnetic resonance. Thanks to the moderate exchange interaction (J ~1 K) between spins, we could experimentally access the 1/3rd magnetization plateau stabilized by thermal fluctuations. A moderate NMR line broadening reveals the development of anisotropic short-range correlations, concomitantly with a gapless spin-lattice relaxation time T1 ~ k_BT/ħS, which points to the presence of a semiclassical nematic spin-liquid state predicted for the KHAF model.



In the second part, I will introduce our recent results on the quantum anisotropic kagome Heisenberg antiferromagnet Y-kapellasite Y3Cu9(OH)19Cl8. There, three different nearest neighbor interactions yield a rich phase diagram, which features a large spin liquid phase . besides two long range ordered ones. Noticeably the large difference in the Y and Cu radii prevents inter-site mixing and the anisotropic kagome planes are free from magnetic defects. I present a detailed investigation of large, phase pure, single crystals of this compound by neutron scattering, and local μSR and NMR techniques. Our study of single crystals gives evidence for a bulk magnetic transition at 2.1 K. Our analysis locate Y-kapellasite closer to the phase boundary to the spin-liquid phase than expected from ab initio calculations, where enhanced quantum fluctuations drastically reduced the ordered moment of the Cu²⁺.