Coexistence of anomalous spin dynamics and weak magnetic order in the chiral trillium lattice K2FeSn(PO4)3

Trillium lattices, in which magnetic ions form a three-dimensional chiral network of corner-sharing equilateral triangular motifs, offer a prominent platform to explore exotic quantum states. In this work, we report ground-state properties of the $S=5/2$ trillium lattice compound K₂FeSn(PO₄)₃ through thermodynamic, electron spin resonance (ESR), and muon spin relaxation (μSR) experiments. Thermodynamic and ESR measurements reveal the two-step evolution of magnetic correlations across T*=110.16em0exK, which results from an interplay between dominant antiferromagnetic Heisenberg interactions and subleading interactions. Below T*, dc and ac magnetic susceptibilities indicate weak magnetic ordering at TN≈20.16em0exK under low fields, which is suppressed for μ₀H≥2 T, consistent with a power-law dependence of magnetic specific heat at low temperatures. μSR experiments confirm the dominance of persistent spin dynamics and the absence of conventional spin freezing, supporting the subtle nature of weak magnetic ordering coexisting with strong spin fluctuations. These findings underscore the potential for realizing a classical spin-liquid ground state with exotic excitations in high-spin trillium lattice systems.