  Quantum magnets based on honeycomb lattices with 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
analogues, high-spin honeycomb lattice antiferromagnets have remained
comparatively less explored in the context of capturing the classical analogs
of quantum phenomena. Herein, we report the crystal structure, magnetic
susceptibility, specific heat, and electron spin resonance (ESR), complemented
by ab initio density functional theory (DFT) calculations on polycrystalline
samples of FeP3SiO11 wherein the Fe3+ ions decorate a nearly-perfect S = 5/2
honeycomb lattice without any site disorder among constituent atoms. Above 150
K, an antiferromagnetic Weiss temperature of - 12 K is observed consistent with
DFT calculations, which suggest the presence of strong intra-planar
nearest-neighbor and weaker inter-planar further nearest-neighbor exchange
interactions. An anomaly at TN = 3.5 K 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 in an
applied field of Hc1 = 0.2 T. At higher applied fields, TN is gradually
suppressed down to zero at Hc2 = 5.6 T with a 2D critical exponent 0.255. Above
Hc2, 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 the honeycomb lattice antiferromagnet FeP3SiO11.
