Frustration driven magnetic correlations in the spin-$5/2$ triangular lattice antiferromagnet RbFe(HPO$_{3}$)$_{2}$

Abstract

A detailed study of the structural and magnetic properties of a spin-$5/2$ triangular lattice antiferromagnet RbFe(HPO$_{3}$)$_{2}$ is presented using x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance (NMR) experiments on a polycrystalline sample. The crystal structure features an equilateral triangular lattice of Fe$^{3+}$ ions. The thermodynamic measurements reveal the onset of a magnetic long-range order at $T_{\rm N1} \simeq 7.8$ K in zero-field, followed by another low temperature field induced ordering at $T_{\rm N2}$ in higher fields. The transition at $T_{\rm N1}$ is further confirmed from the NMR spin lattice relaxation measurements. The value of the frustration ratio ($f \simeq 7$) implies moderate spin frustration in the compound. The $^{31}$P NMR spectra exhibit two distinct spectral lines corresponding to two inequivalent phosphorus sites (P1 and P2), consistent with the crystal structure. The P1 site is strongly coupled with an isotropic hyperfine coupling of $A_{\rm hf}^{\rm iso} = 0.55(2)$ T/$μ_{\rm B}$ while the P2 site is weakly coupled with $A_{\rm hf}^{\rm iso} = 0.25(3)$ T/$μ_{\rm B}$ with the Fe$^{3+}$ ions. The magnetic susceptibility and NMR shift data are described well assuming a spin-$5/2$ isotropic triangular lattice antiferromagnetic model with an average exchange coupling of $J/k_{\rm B} = 2.8(2)$ K. Below $T_{\rm N1}$, the spectra evolve into a nearly rectangular powder pattern, indicating a commensurate antiferromagnetic type order. The $^{31}$P spin-lattice relaxation rate well below $T_{\rm N1}$ follows a $T^3$ temperature dependence, implying a two-magnon Raman scattering mechanism in the ordered state. Three well-defined phase regimes are clearly ascertained in the $H-T$ phase diagram, reflecting a weak magnetic anisotropy in the compound.

Figures (13)

• Figure 1: (a) Crystal structure of RbFe(HPO$_3$)$_2$ showing triangular layers of Fe atoms formed by the corner-shared FeO$_6$ octahedra and HPO$_3$ pseudo-tetrahedra. Rb$^+$ ions separate the two adjacent triangular layers. (b) A section of the layer showing Fe$^{3+}$ ions connected through HPO$_3$ pseudo-tetrahedra in a triangular lattice. (c) Schematic picture showing different environments for P(1) and P(2) sites with respect to Fe atoms.
• Figure 2: Powder XRD data collected at room temperature. The red open circles are the experimental data, the black solid line is the Rietveld fit to the data, the vertical bars are the Bragg-peak positions, and the blue line at the bottom is the difference between the experimentally observed and calculated intensities.
• Figure 3: (a) The dc magnetic susceptibility $\chi$ and its inverse ($1/\chi$) as a function of temperature for $\mu_0 H = 0.5$ T in the left and right axes, respectively. The solid line represents the fit using the isotropic triangular lattice model to $\chi(T)$. The dash-dotted line is the simulation of $\chi(T)$ of a $S=5/2$ isotropic TLAF using FD method. The dashed line represents the CW fit to $1/\chi(T)$. (b) $\chi$ vs $T$ in various applied magnetic fields for $\mu_0 H \leq 2$ T. The arrows indicate the transitions at $T_{\rm N1}$ and $T^*$. Top inset: FC and ZFC $\chi(T)$ data measured at $\mu_0H=0.02$ T. Bottom inset: $M$ vs $H$ measured at $T=2$ K.
• Figure 4: Low temperature $\chi(T)$ in various applied fields 3 T $\leq \mu_0H \leq 9$ T. The vertical arrows indicate the two transitions, $T_{\rm N1}$ and $T_{\rm N2}$, respectively.
• Figure 5: (a) Temperature dependence of $\chi'$ measured in different frequencies. The transitions at $T_{\rm N1}$ and $T^*$ are highlighted. (b) $\chi"$ vs $T$ measured in different frequencies.