Field-induced magnetic phases in the Kitaev candidate Na$_3$Co$_2$SbO$_6$

TL;DR

The paper investigates field-induced magnetic phases in Na$_3$Co$_2$SbO$_6$, a honeycomb cobaltate Kitaev candidate, revealing an AFM ground state with a $j_{eff} = 1/2$ state and strong in-plane versus out-of-plane anisotropy. They map a rich anisotropic phase diagram via temperature- and field-dependent magnetization, specific heat, and magnetocaloric effect measurements on high-quality single crystals along three crystallographic directions. Classical Monte Carlo simulations of an extended Kitaev-Heisenberg model including ring exchange reproduce field-induced phases such as double-$q$, 1/3-AFM, zigzag, and vortex, and predict canting for certain field directions. The combined experimental and theoretical results provide a microscopic understanding of the field-induced orders and motivate neutron scattering and inelastic neutron scattering experiments to validate spin textures and determine the appropriate effective spin model.

Abstract

We report a rich anisotropic magnetic phase diagram of Na$_3$Co$_2$SbO$_6$, a previously proposed cobaltate Kitaev candidate, based on field- and temperature-dependent magnetization, specific heat, and magnetocaloric effect studies. At low temperatures, our experiments uncover a low-lying $j_{\textrm{eff}} = \frac{1}{2}$ state with an antiferromagnetic ground state and pronounced in-plane versus out-of-plane anisotropy. The experimentally identified magnetic phases are theoretically characterized through classical Monte Carlo simulations within an extended Kitaev-Heisenberg model with additional ring exchange interactions. The resulting phase diagram reveals a variety of exotic field-induced magnetic phases, including double-$\textbf{q}$, $\frac{1}{3}$-AFM, zigzag, and vortex phases.

Figures (4)

• Figure 1: (a) Room-temperature crystal structure of Na$_3$Co$_2$SbO$_6$. (b) Magnetic susceptibility $\chi$ as a function of temperature $T$ at a field of $\mu_0 H = 0.1\,\mathrm{T}$ along the crystallographic $\mathbf{a}$, $\mathbf{b}$, and $\mathbf{c^*} \perp \mathbf{a}, \mathbf{b}$ directions. The shaded areas highlight the bifurcation between field-cooled (upper curve) and zero-field-cooled (lower curve) magnetic susceptibility. (c) Isothermal magnetization $M$ as a function of field $H$ at $T = 1.8\,\mathrm{K}$ for $\mathbf H \parallel \mathbf {a}$ and $\mathbf H \parallel \mathbf {b}$ up to 2 T, and at $T = 2\,\mathrm{K}$ for $\mathbf H \parallel \mathbf {c^*}$ up to 14 T. Asterisks indicate the field-induced transitions (field-sweep up). (d) Magnetic entropy $S_\text{mag}$ as a function of temperature $T$ at zero field. The inset displays the zero-field heat capacity $C_p/T$.
• Figure 2: (a) Magnetic susceptibility $\partial M / \partial (\mu_0 H)$ for various fixed temperatures as a function of field $\mathbf H \parallel \mathbf a$ (field-sweep up) from SQUID magnetometry up to 2.5 T. Asterisks indicate the field-induced transitions. (b) Same as (a), but for $\mathbf H \parallel \mathbf b$ up to 1.5 T. (c) Same as (a), but for $\mathbf H \parallel \mathbf c^*$ from vibrating sample magnetometry in a PPMS up to 14 T.
• Figure 3: (a) Sample temperature $T_\text{S}$ (black) and its derivative $\partial T_\text{S}/\partial (\mu_0 H)$ (red) as a function of field $\mathbf H \parallel \mathbf a$ from low-temperature MCE measurements in pulsed fields under quasi-adiabatic conditions. Arrows indicate the field-up and field-down sweeps, while asterisks mark the field-induced transitions. (b) Same as (a), but for $\mathbf H \parallel \mathbf b$.
• Figure 4: Top: Na$_3$Co$_2$SbO$_6$ experimental magnetic phase diagram for (a) $\mathbf H \parallel \mathbf{a}$, (b) $\mathbf H \parallel \mathbf{b}$, and (c) $\mathbf H \parallel \mathbf{c^*}$. Note that the phase boundaries correspond to the critical fields observed for field-up sweeps. The field-induced phases predicted by theory are marked with an asterisk, while those without an asterisk are based on previous neutron diffraction experiments li2022giant. Insets illustrate the spin-excitation gap in the field-polarized (FP) state. Bottom: Ground-state phase diagram of extended Kitaev-Heisenberg model for (d) $\mathbf H \parallel \mathbf {a}$, (e) $\mathbf H \parallel \mathbf {b}$, and (f) $\mathbf H \parallel \mathbf {c^*}$. Insets indicate spin directions projected onto the $ab$ plane in the different phases.