Exotic magnetism and persistent short-range spin correlations in a frustrated honeycomb lattice antiferromagnet

Abstract: Two-dimensional high-spin bipartite honeycomb networks, where anisotropy, competing exchange interactions, and spin fluctuations interplay, provide an alternative platform to test theoretical models that distinguish between classical and quantum magnetism in the context of emergent many-body phenomena and exotic excitations. Here, we report the crystal structure, magnetization, specific heat, and inelastic neutron scattering measurements of the $S = 5/2$ distorted honeycomb magnet $\mathrm{CaZn_2Fe(PO_4)_3}$. Magnetization measurements reveal dominant antiferromagnetic interactions between the $\mathrm{Fe<sup>{3+}}$ ($S = 5/2$) moments. The development and field evolution of a dip in the magnetic susceptibility under an external magnetic field indicate an unconventional field-induced transition, further supported by anomalies observed in magnetization isotherms. Zero-field specific heat measurements show an antiferromagnetic transition at $T_N \approx 1.67 \mathrm{K}$, which evolves under applied magnetic field, suggesting stabilization of a field-induced spin-canted state. Thermodynamic measurements reveal short-range spin correlations above the transition temperature. Inelastic neutron scattering results further corroborate antiferromagnetic ordering, consistent with specific heat data. Spin-wave calculations indicate competing exchange interactions that introduce magnetic frustration, along with weak Ising-like anisotropy. The interplay of competing interactions and anisotropy gives rise to exotic field-induced behavior and places the system in close proximity to a mean-field tricritical point in the $J_2/J_1$--$J_3/J_1$ phase diagram, opening a route to unconventional states in high-spin frustrated honeycomb magnets.