Collinear antiferromagnetic order in spin-$\frac52$ triangle lattice antiferromagnet Na$_3$Fe(PO$_4$)$_2$

Abstract: We set forth the structural and magnetic properties of the frustrated spin-$5/2$ triangle lattice antiferromagnet Na$3$Fe(PO$_4$)$_2$ 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 $J/k{\rm B} \simeq 1.8$ 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 $T_{\rm N} \simeq 10.4$ K. Neutron diffraction experiments elucidate a collinear antiferromagnetic ordering below $T_{\rm N}$ with the propagation vector $k = (1,0,0)$. An intermediate value of frustration ratio ($f \simeq 3.6$) reflects moderate frustration in the compound which is corroborated by a reduced ordered magnetic moment of $\sim 1.52$ $\mu_{\rm B}$ at 1.6 K, compared to its classical value ($5 \mu_{\rm B}$). Magnetic isotherms exhibit a change of slope envisaging a field induced spin-flop transition at $H_{\rm SF}\simeq3.2$ 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.