Ground state properties of a spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH${}_4$Fe(PO${}_3$F)${}_2$

Abstract

Structural and magnetic properties of a two-dimensional spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH${}_4$Fe(PO${}_3$F)${}_2$ are explored via x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and ${}^{31}$P nuclear magnetic resonance experiments on a polycrystalline sample. The compound portrays distorted triangular units of the Fe${}^{3+}$ ions with anisotropic bond lengths. The magnetic susceptibility shows a broad maxima around $T_{\chi }^{\max }\simeq 12$ K, mimicking the short-range antiferromagnetic order of a low-dimensional spin system. The magnetic susceptibility and NMR shift could be modeled assuming the spin-$5/2$ isotropic triangular lattice model and the average value of the exchange coupling is estimated to be $J/k_{\mathrm{B}}\simeq 1.7$ K. This value of the exchange coupling is reproduced well from the saturation field of the pulse field data. It shows the onset of a magnetic ordering at $T_{\mathrm{N}}\simeq 5.7$ K, setting the frustration ratio of $f=\frac{|{\theta }_{\mathrm{CW}}|}{T_{\mathrm{N}}}\simeq 5.7$. Such a value of $f$ reflects moderate magnetic frustration in the compound. The d$M$/d$H$ vs $H$ plots of the low temperature magnetic isotherms exhibit a sharp peak at $H_{\mathrm{SF}}\simeq 1.45$ T, suggesting a field-induced spin-flop transition and magnetic anisotropy. The rectangular shape of the ${}^{31}$P NMR spectra below $T_{\mathrm{N}}$ unfolds that the ordering is commensurate antiferromagnet type. Three distinct phase regimes are clearly discerned in the $H-T$ phase diagram, redolent of a frustrated magnet with in-plane (XY-type) anisotropy.

13 pages, 12 figures