Antiferromagnetic ordering and glassy nature in NASICON type NaFe_2PO_4(SO_4)_2

We investigate crystal structure and magnetic properties including spin relaxation and magnetocaloric effect in NASICON type NaFe_2PO_4(SO_4)_2 sample. The Rietveld refinement of x-ray and neutron diffraction patterns show a rhombohedral crystal structure with the R3̅c space group. The core-level spectra confirm the desired oxidation state of constituent elements. The dc–magnetic susceptibility (χ) behavior in zero field-cooled (ZFC) and field-cooled (FC) modes show the ordering temperature ≈50 K. Interestingly, the analysis of temperature dependent neutron diffraction patterns reveal an A-type antiferromagnetic (AFM) structure with the ordered moment of 3.8 μ_B/Fe^3+ at 5 K, and a magnetostriction below T_ N= 50 K. Further, the peak position in the ac–χ is found to be invariant with the excitation frequency supporting the notion of dominating AFM transition. Also, the unsaturated isothermal magnetization curve supports the AFM ordering of the moments; however, the observed coercivity suggests the presence of weak ferromagnetic (FM) correlations at 5 K. On the other hand, a clear bifurcation between ZFC and FC curves of dc–χ and the observed decrease in peak height of ac–χ with frequency suggest for the complex magnetic interactions. The spin relaxation behavior in thermo-remanent magnetization and aging measurements indicate the glassy states at 5 K. Moreover, the Arrott plots and magnetocaloric analysis reveal the AFM–FM interactions in the sample at lower temperatures.