Collinear order in the spin-5/2 triangular-lattice antiferromagnet Na₃Fe(PO₄)₂

We set forth the structural and magnetic properties of the frustrated spin-5/2 triangle lattice antiferromagnet Na₃Fe(PO₄)₂ examined via x-ray diffraction, magnetization, heat capacity, and neutron diffraction measurements on the polycrystalline sample. No structural distortion was detected from the temperature-dependent x-ray diffraction down to 12.5 K, except a systematic lattice contraction. The magnetic susceptibility at high temperatures agrees well with the high-temperature series expansion for a spin-5/2 isotropic triangular lattice antiferromagnet with an average exchange coupling of J/k_B ≃ 1.8 K rather than a one-dimensional spin-5/2 chain model. This value of the exchange coupling is consistently reproduced by the saturation field of the pulse field magnetization data. It undergoes a magnetic long-range order at T_N ≃ 10.4 K. Neutron diffraction experiments elucidate a collinear antiferromagnetic ordering below T_N with the propagation vector k = (1, 0, 0). An intermediate value of frustration ratio ( f ≃ 3.6) reflects moderate frustration in the compound which is corroborated by a reduced ordered magnetic moment of ∼1.52μ_B at 1.6 K, compared to its classical value (5μ_B). Magnetic isotherms exhibit a change of slope envisaging a field induced spin-flop transition at H_SF ≃ 3.2 T. The magnetic field vs temperature phase diagram clearly unfold three distinct phase regimes, reminiscent of a frustrated magnet with in-plane (XY-type) anisotropy.