Néel-type antiferromagnetic order and magnetic field–temperature phase diagram in the spin-$\frac{1}{2}$ rare-earth honeycomb compound $\mathrm{YbCl}{}_3$

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled $J=1/2$ local moments such as iridates and $\alpha \text{−}{\mathrm{RuCl}}_3$. Here we propose the monoclinic ${\mathrm{YbCl}}_3$ with a ${\mathrm{Yb}}^{3+}$ honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on ${\mathrm{YbCl}}_3$ single crystal. We find that the ${\mathrm{Yb}}^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin $J_{\text{eff}}=1/2$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range Néel-type antiferromagnetic order at 0.60 K, below which the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)${}^{\circ }$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the Néel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) ${\mu }_B$ at 0.25 K, all indicate that ${\mathrm{YbCl}}_3$ could be a two-dimensional spin system to proximate the Kitaev physics.

Figure 1

(a) The crystal structure of the $ab$ plane of ${\mathrm{YbCl}}_3$. (b) The temperature dependent specific heat $C$ for ${\mathrm{YbCl}}_3$ and ${\mathrm{LuCl}}_3$. The 3D Debye model fitting is shown in blue. Inset: $C/T$ vs $T^2$ for ${\mathrm{LuCl}}_3$. (c) $C_M/T$ vs $T$ for ${\mathrm{YbCl}}_3$. (d) The temperature dependent magnetic entropy $S_M$ with error bars.

Figure 2

(a) The temperature dependent magnetic susceptibility of ${\mathrm{YbCl}}_3$ at $H=1\mathrm{T}$ with $H\parallel ab$ and $H\perp ab$. Inset: the inverse magnetic susceptibility 1/$\chi$ from 3 K to 15 K for $H\parallel ab$ and $H\perp ab$. (b) The isothermal magnetization data taken at 1.8 K with $H\parallel ab$ and $H\perp ab$. (c),(d) The field dependent $ac$ susceptibility ${\chi }_{ac}^{\prime }$ with $H\parallel ab$ (c) and $H\perp ab$ (d) at various temperatures. We used a frequency of 577 Hz and current of 0.5 mA.

Figure 3

(a) $C/T$ vs $T$ at various magnetic fields. Each data set was offset by 1.5 J/mol $\mathrm{K}{}^2$. The dashed line is a guide to the eyes to track the evolution of the SRO features shown in $C_p$ under field. (b) The $H\text{−}T$ phase diagram of ${\mathrm{YbCl}}_3$. (c) The polar plot of angle-dependent magnetic torque at $T=2.1$ K and $H=5\mathrm{T}$ when the field rotates in the $ab$ plane. The zero-degree-crystal axis $l$ was arbitrarily chosen.

Figure 4

(a) $d$ scan of the peak (0 2 0) in the monoclinic setting at 0.25 K and 10 K. The inset shows the difference between 10 K and 0.25 K. (b) Temperature dependence of the integrated intensity of the peak (0 2 0). (c),(d) Néel-type antiferromagnetic structure of ${\mathrm{Yb}}^{3+}$ at 0.25 K, the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with the angle between the moment and the $a$ axis as 16(11)${}^{\circ }$, nearly parallel to the honeycomb $ab$ plane.

Figure 5

(a)–(e) The $d$ scan of the peak (0 0 1), (0 $-2$ 0), (1 $-1$ 0), (0 2 1), and (1 $-1$ $-1$) at 0.25 K and 10 K. Most of the error bars are smaller than the size of spheres. The solid line represents the fitting of the peaks by a bi-Gaussian peak function. (f) Comparison of the squared magnetic structure factors between observations and calculations. The magnetic intensities have been normalized to their nuclear intensities.