Competing antiferromagnetic-ferromagnetic states in a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>d</mml:mi><mml:mn>7</mml:mn></mml:msup></mml:math> Kitaev honeycomb magnet

Abstract

The Kitaev model is a rare example of an analytically solvable and physically instantiable Hamiltonian yielding a topological quantum spin liquid ground state. Here we report signatures of Kitaev spin liquid physics in the honeycomb magnet ${\mathrm{Li}}_{3}{\mathrm{Co}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, built of high-spin ${d}^{7}$ (${\mathrm{Co}}^{2+}$) ions, in contrast to the more typical low-spin ${d}^{5}$ electron configurations in the presence of large spin-orbit coupling. Neutron powder diffraction measurements, heat capacity, and magnetization studies support the development of a long-range antiferromagnetic order space group of ${C}_{C}2/m$, below ${T}_{N}=11\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ${\ensuremath{\mu}}_{0}H=0\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The magnetic entropy recovered between $T=2$ and 50 K is estimated to be $0.6R\mathrm{ln}2$, in good agreement with the value expected for systems close to a Kitaev quantum spin liquid state. The temperature-dependent magnetic order parameter demonstrates a \ensuremath{\beta} value of 0.19(3), consistent with XY anisotropy and in-plane ordering, with Ising-like interactions between layers. Further, we observe a spin-flop-driven crossover to ferromagnetic order with space group of $C2/m$ under an applied magnetic field of ${\ensuremath{\mu}}_{0}H\ensuremath{\approx}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ at $T=2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the ${d}^{7}$ compass model, a quantitative comparison of the specific heat of ${\mathrm{Li}}_{3}{\mathrm{Co}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, and related honeycomb cobaltates to the anisotropic Kitaev model further support proximity to a Kitaev spin liquid state. This material demonstrates the rich playground of high-spin ${d}^{7}$ systems for spin liquid candidates and complements known ${d}^{5}$ Ir- and Ru-based materials.

Topics & Concepts