Magnetic phase diagram and spin Hamiltonian of antiferromagnet Cs$_2$CoI$_4$

Abstract

We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. The resulting symmetry reduction strongly affects both the magnetic exchange interactions and single-ion anisotropy. Despite the large parameter space, we propose a minimal magnetic Hamiltonian that reasonably captures the observed excitation spectrum, analyzed using extended SU(4) linear spin-wave theory.

Figures (17)

• Figure 1: Schematic overview of the (a) Room temperature unit cell and (b) low temperature unit cell. I$^{-}$ belonging to tetrahedras outside the shown unit cell have been omitted for visibility. (c) shows a zoom-in on the CoI$_4$ tetrahedras at room temperature (top) and two pairs of inequivalent ones below $T_s$ (Co A left and Co B right).
• Figure 2: Neutron powder diffraction pattern at (a) $T$ = 70 K and (c) $T$ = 40 K. Red circles show the measured data, black lines indicate the fits from Rietveld refinements, and blue lines indicate the difference between the measured data and fits. (b) and (d) show a restricted 2$\theta$ from (a) and (c), respectively. The vertical green lines represent the peak positions indicated by the accompanying ($h$$k$$l$) index.
• Figure 3: Magnetic susceptibility of Cs$_2$CoI$_4$ single crystal in applied fields of $\mu_0H$ = 0.1 T along the three crystallographic directions (symbols). The response along a and c is almost indistinguishable at high temperatures. The solid lines indicate fits to the data as described in the text. Dashed lines (from left to right) show positions of anomalies related to the Néel temperature $T_N$ and structural transition temperature $T_s$. Right inset: the same curves zoomed in between 50 and 58 K from 0.035 to 0.05 emu/mol$_\mathrm{Co^{2+}}$.
• Figure 4: Magnetization as function of applied magnetic field along the (a) a direction and (b) b direction. Circles are data points measured with the Faraday balance (FB) at $T$ = 200 mK, and lines are pulsed field magnetization measurements at $T$ = 500 mK. Dashed black lines indicate the saturation magnetization values. The black triangles indicate anomalies in the magnetization curve.
• Figure 5: Magnetization as function of temperature at several applied magnetic fields, from 1 to 14 T in steps of 1 T, along the (a) a direction and (c) b direction. Note that the y-axes do not start at zero, for visual purposes. Sub-plots (b) and (d) correspond to the derivative of the magnetization along the a and b directions, respectively, with respect to temperature. In (b) and (d) the curves at each consecutive field are vertically offset by 0.02 $\mu_\mathrm{B}$ K$^{-1}$T$^{-1}$ per Co$^{2+}$ ion. The black and gray lines in (b) and (d) correspond to a phenomenological fit to extract the peak positions of the anomalies.