Dual nature of electron spin resonance in YbCo$_2$Zn$_{20}$ intermetallic compound

In single crystals of YbCo$_2$Zn$_{20}$ intermetallic compound, two coexisting types of electron spin resonance signals related to the localized magnetic moments of cobalt and to itinerant electrons have been observed in the $4.2$–$300$ K temperature range. It is shown that the relative contribution of itinerant electrons to the total magnetization does not exceed $9\%$. We argue that the electron dynamics in YbCo$_2$Zn$_{20}$ and YbCuAl heavy fermion systems is determined by the effects produced by the magnetic subsystem of the localized $3d$-electrons. We also discuss general aspects of the electron spin resonance spectroscopy in underdoped ytterbium-based intermetallics and the spectral manifestations of the interplay between the efficiency of the hybridization of $f$-electrons with the electrons filling outer atomic shells, crystal field effects, and the effects related to the proximity to the quantum critical point.