Partially disordered state with short-range spin correlation in $S=5/2$ classical triangular antiferromagnet ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$

A triangular-lattice antiferromagnet of ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$ was synthesized under high pressure. Its magnetism was studied in terms of electrical resistivity, magnetic susceptibility, heat capacity, powder neutron scattering, and Mössbauer spectroscopy. The magnetic state of ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$ changes successively through a second-order phase transition at $T_{\mathrm{p}}=36$ K and a crossover at $T_{\mathrm{c}}=20$ K. A partially disordered (PD) state appears below $T_{\mathrm{p}}$, in which $\approx \mathrm{2/3}$ spins are ordered and the remaining $\approx \mathrm{1/3}$ spins fluctuate, which state persists at least down to 5 K. The spin correlation length starts to grow at $T_{\mathrm{p}}$; however, it remains very short ($\approx 27$ Å) below $T_{\mathrm{c}}$. This exotic magnetism is concerned with strong frustration in the classic antiferromagnetic triangular lattice.

Figure 1

Electrical resistivity (a), magnetic susceptibility (b), and heat capacity (c) of ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$. In panel (a), the 3PD magnetic structure and the average crystal structure consists of alternating stacks of a triangular lattice of edge-shared ${\mathrm{FeO}}_6$ octahedra and a double silver layer is illustrated. In panel (b), the red line indicates a linear fit to the inverse susceptibility. In panel (c), the solid curve shows the background heat capacity $C_{\mathrm{b}}$. The inset of panel (c) is a $C$/$T$ vs $T^2$ plot.

Figure 2

(a) Temperature dependence of magnetic susceptibility below 80 K, measured at $H$ = 1, 5 T in ZFC and FC conditions. (b) Magnetic heat capacity ($C_{\mathrm{m}}$/$T$ vs $T$ plot) at $H$ = 0, 7 T. The right axis indicates the estimated magnetic entropy (see text).

Figure 3

(a) Nuclear (N) and magnetic Bragg reflections with indices of $(1/3,1/3,L)$ for neutron diffraction measured at various temperatures. The profiles are shifted at $+15$-count intervals. The black lines show the calculated diffraction pattern assuming the 3PD structure with $S\parallel c$. The inset shows the temperature dependence of the spin correlation length $\xi$. (b) Color contour map of the inelastic neutron scattering intensity $S\left(\right|Q|;E)$ for ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$ powder sample measured with incident energy $E_i$ = 15 meV at $T$ = 5 K. Background signal measured using an empty cell was subtracted. The calculated excitation spectra of 3PD state with spin anisotropy $S\parallel c$ [(c), (d)] and ${120}^{\circ }$ structure with $S\parallel ab$ plane (e). $J_1$, $J_2$, $J_3$, $J_c$, and $D_c$ in the Hamiltonian (see text) are 5.1, 0.51, 2.6, 0, and $-1.2$ K (model I) for panel (c), 4.9, 0.49, 2.5, 0.8, and $-1.2$ K (model II) for panel (d), and 32.5, 0, 0, 0, and 5.9 K (model III) for panel (e), respectively. (f) $Q$ dependences of the inelastic scattering intensity. Filled circles are intensities observed at 5 K integrated in the range of 3.5 $\le E\le$ 6.0 meV. Solid, broken, and dotted lines represent $Q$ dependences of the calculated intensity integrated in the range of 3.5 $\le E\le$ 6.0 meV for panels (c) and (d) and 1.5 $\le E\le$ 5.0 meV for panel (e), respectively.

Figure 4

Mössbauer spectra of ${\mathrm{Ag}}_2{\mathrm{FeO}}_2$. The circles are our experimental data. The black line is the result of the fitting. The right panel is a magnified plot. The asterisks indicate the impurity (${\mathrm{Fe}}_2{\mathrm{O}}_3$) signal. The spectra at 30 and 25 K are divided into two components: C-1 (blue line) and C-2 (red line).