Antiferromagnetic ordering and glassy nature in the sodium superionic conductor NaFe2PO4(SO4)2

We investigate the crystal structure and magnetic properties, including spin relaxation and magnetocaloric effect, in a NASICON-type ${\mathrm{NaFe}}_{2}{\mathrm{PO}}_{4}{({\mathrm{SO}}_{4})}_{2}$ sample. The Rietveld refinement of x-ray and neutron diffraction patterns shows a rhombohedral crystal structure with the $R\overline{3}c$ space group. The core-level spectra confirm the desired oxidation state of constituent elements. The dc-magnetic susceptibility $(\ensuremath{\chi})$ behavior in zero field-cooled (ZFC) and field-cooled (FC) modes shows the ordering temperature $\ensuremath{\approx}50$ K. Interestingly, the analysis of temperature-dependent neutron diffraction patterns reveal an A-type antiferromagnetic (AFM) structure with the ordered moment of 3.8 ${\ensuremath{\mu}}_{B}/{\mathrm{Fe}}^{3+}$ at 5 K, and a magnetostriction below ${T}_{\mathrm{N}}=50$ K. Further, the peak position in the ac-$\ensuremath{\chi}$ is found to be invariant with the excitation frequency, supporting the notion of a 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-$\ensuremath{\chi}$ and the observed decrease in a peak height of ac-$\ensuremath{\chi}$ with frequency suggests complex magnetic interactions. The spin relaxation behavior in thermoremanent 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.