We systematically investigate the magnetic properties of the triangular-lattice quantum magnet ${\mathrm{K}}_2\mathrm{CsYb}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$. The results reveal no evidence of long-range magnetic order down to 2 K. The Curie-Weiss temperatures obtained from low-temperature fits of magnetic susceptibility, ${\theta }_{\mathrm{CW},ab}=-0.014\mathrm{K}$ and ${\theta }_{\mathrm{CW},c}=-0.557\mathrm{K}$, indicate the presence of weak anisotropic antiferromagnetic exchange interactions. The crystal-field analysis yields an energy gap of 30.19 meV between the ground state and the first excited Kramers doublet, confirming that the low-temperature physics is well described by the spin-orbit entangled $J_{\text{eff}}=1/2$ state, which is also supported by the first-principles calculations. Furthermore, the frequency-field ($v\text{−}H$) relationship in the electron-spin-resonance spectra exhibits a distinct zero-field intercept, significantly deviating from the noninteracting paramagnetic behavior. This deviation, arising from the internal field caused by spin correlations at low temperatures, allows us to extract the anisotropic exchange interaction: $J_{\pm }/k_{\mathrm{B}}=-0.007\mathrm{K}$ and $J_{zz}/k_{\mathrm{B}}=-0.025\mathrm{K}$.

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• 2-dimensional systems
• Antiferromagnets
• Rare-earth magnetic materials
• Crystal growth
• Crystal-field theory
• Electron paramagnetic resonance
• Electron spin resonance
• First-principles calculations
• Magnetization measurements
• X-ray diffraction