Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$

Very recently, a $3d$ based honeycomb cobaltate ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ has garnered tremendous attention due to the proposed proximity to the Kitaev spin-liquid state as its $4d/5d$ counterparts. Here, we use Zn to substitute Co in a broad range and perform systematic studies on ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$ by structural, magnetic, and thermodynamic measurements, and track the doping evolution of its magnetic ground states. Due to the extremely close radii of ${\mathrm{Zn}}^{2+}$ and high-spin ${\mathrm{Co}}^{2+}$ ions, the substitution can be easily achieved. X-ray diffractions reveal no structural transition but only minor changes on the lattice parameter $c$ over a wide substitution range $0\le x\le 1.5$. Magnetic susceptibility and specific heat measurements both suggest an antiferromagnetic ground state which is gradually suppressed with doping. It can survive with $x$ up to $\sim 1.0$. Then it evolves into a spin-glass phase with short-range order that is rapidly supplanted by a magnetically disordered state when $x\ge 1.3$. By summarizing all these data, we construct a magnetic phase diagram of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$. Our results demonstrate that the Zn doping can effectively suppress the magnetic order and induce a possible quantum paramagnetic state. These may serve as a platform to investigate the Kitaev physics in this system.

Figure 1

(a) Schematic crystal structure of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$. (b) Top view of the honeycomb layer of ${\mathrm{CoO}}_6$ octahedra. (c) XRD patterns of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$ with $0\le x\le 1.5$. Green ticks denote Bragg peak positions of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ with space group $P{6}_{3}22$ (No. 182). Asterisks mark the impurity peak of NaOH. (d) Evolution of the lattice parameters $a$ and $c$ with varying zinc concentration $x$. Solid lines are guides to the eye. (e) Zoom-in view of the most intense diffraction peak (002) for different $x$. The dashed line indicates a minor shift to the low-angle direction with increasing $x$. Errors represent one standard deviation throughout the paper.

Figure 2

(a) Temperature dependence of the magnetic susceptibility ($\chi$) of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$ with $0\le x\le 1.3$. The inset shows the inverse susceptibility data of two representative zinc concentrations, which are $x=0$ and 1.3. Dashed lines are the fits with the Curie-Weiss law. (b) Extracted effective moment ${\mu }_{\mathrm{eff}}$ and Curie-Weiss temperature ${\mathrm{\Theta }}_{\mathrm{CW}}$ as a function of $x$. Solid lines are guides to the eye. (c) Derivative of the magnetic susceptibility versus temperature for different $x$. Arrows mark the positions of $dM$/$dT=0$, from which $T_{\mathrm{N}}$ is extracted.

Figure 3

(a) Temperature dependence of the specific heat ($C_{\mathrm{p}}$) of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$ within a temperature range of 2–30 K. The dashed line with an arrow denotes an evolution tendency of the AFM phase transition with increasing $x$. (b) Temperature dependence of $C_{\mathrm{p}}/T$ of ${\mathrm{Na}}_2{\mathrm{Co}}_{0.7}{\mathrm{Zn}}_{1.3}{\mathrm{TeO}}_6$ in several different magnetic fields.

Figure 4

Temperature dependence of the magnetic susceptibility of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$ when $1.0\le x\le 1.4$. The data are collected under zero-field-cooling (ZFC; open symbols) and field-cooling (FC; filled symbols) conditions with a small magnetic field of 0.1 T.

Figure 5

Magnetic phase diagram of ${\mathrm{Na}}_2{\mathrm{Co}}_{2-x}{\mathrm{Zn}}_x{\mathrm{TeO}}_6$. The high white zone represents the paramagnetic (PM) state. The bottom-left blue zone and bottom-right yellow zone denote AFM and quantum paramagnetic (QPM) states, respectively, where a spin-glass (SG) phase is located between them. The dashed line is a guide to the eye.