Magnetism and field-induced effects in the𝑆=52honeycomb lattice antiferromagnetFeP3⁢SiO11

Quantum magnets based on honeycomb lattices with a low coordination number offer a viable ground to realize exotic emergent quantum excitations and phenomena arising from the interplay between competing magnetic interactions, spin correlations, and spatial anisotropy. However, unlike their low-spin analogs, high-spin honeycomb lattice antiferromagnets have remained comparatively less explored in the context of capturing the classical limits of quantum phenomena. Herein, we report the crystal structure, magnetic susceptibility, specific heat, and electron spin resonance (ESR) measurements, complemented by ab initio density functional theory (DFT) calculations, on polycrystalline samples of FeP3⁢SiO11 in which the Fe3+ ions decorate a nearly perfect 𝑆=52 honeycomb lattice without any site disorder between constituent atoms. Above 150 K, an antiferromagnetic Weiss temperature 𝜃CW=−12K is observed, consistent with DFT calculations, which suggests the presence of strong intraplanar nearest-neighbor and weaker interplanar further neighbor exchange interactions. An anomaly at 𝑇𝑁=3.5K in specific heat and magnetic susceptibility reveals the presence of a long-range ordered ground state in zero field. Above 𝑇𝑁, ESR evidences short-range spin correlations and unsaturated magnetic entropy, while below 𝑇𝑁 unconventional excitations are seen via power-law specific heat. A spin-flop transition is observed at an applied field of 𝜇0⁢𝐻𝑐⁢1=0.2 T. At higher applied fields, 𝑇𝑁 is gradually suppressed down to zero at 𝜇0⁢𝐻c⁢2=5.6T. Above 𝜇0⁢𝐻c⁢2, a broad maximum in specific heat due to gapped magnon excitations indicates the emergence of an interesting nearly polarized state dressed by a disordered state in this honeycomb lattice antiferromagnet.