Collinear antiferromagnetic order in spin-\frac52 triangle lattice antiferromagnet Na₃Fe(PO₄)₂

Abstract

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-dependant 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/kB 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 TN 10.4 K. Neutron diffraction experiments elucidate a collinear antiferromagnetic ordering below TN 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 ∼ 1.52 µB at 1.6 K, compared to its classical value (5 µB). Magnetic isotherms exhibit a change of slope envisaging a field induced spin-flop transition at HSF 3.2 T. The magnetic field vs temperature phase diagram clearly unfold O5 three distinct phase regimes, reminiscent of a frustrated magnet with in-plane (XY-type) anisotropy.