  In this paper, we performed thermodynamic and electron spin resonance (ESR)
measurements to study low-energy magnetic excitations, which were significantly
affected by crystalline electric field (CEF) excitations due to relatively
small gaps between the CEF ground state and the excited states. Based on the
CEF and mean-field (MF) theories, we analyzed systematically and consistently
the ESR experiments and thermodynamic measurements including susceptibility,
magnetization, and heat capacity. The CEF parameters were successfully
extracted by fitting high-temperature (> 20 K) susceptibilities in the ab-plane
and along the c-axis, allowing to determine the Lande factors ($g_{ab,calc}$ =
5.98(7) and $g_{c,calc}$ = 2.73(3)). These values were consistent with the
values of Lande factors determined by ESR experiments ($g_{ab,exp}$ = 5.69 and
$g_{c,exp}$ = 2.75). By applying the CEF and MF theories to the susceptibility
and magnetization results, we estimated the anisotropic spin-exchange energies
and found that the CEF excitations in \ce{KErTe2} played a decisive role in the
magnetism above 3 K, while the low-temperature magnetism below 10 K was
gradually correlated with the anisotropic spin-exchange interactions. The CEF
excitations were demonstrated in the low-temperature heat capacity, where both
the positions of two broad peaks and their magnetic field dependence well
corroborated our calculations. The present study provides a basis to explore
the enriched magnetic and electronic properties of the QSL family.
