Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}}_{\text{eff}}=\frac{1}{2}$ moments in the triangular-lattice magnet ${\mathrm{Na}}_2\mathrm{Ba}\mathrm{Co}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the ${\tilde{j}}_{\text{eff}}=\frac{1}{2}$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as an additional frustrating agent, altogether putting forward ${\mathrm{Na}}_2\mathrm{Ba}\mathrm{Co}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ as a promising Kitaev spin-liquid material.

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• Magnetic anisotropy
• Magnetic interactions
• Magnetic order
• Spin liquid
• Cobaltates
• Ab initio calculations
• Density functional theory
• Electron spin resonance
• Magnetization measurements