ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Below the Kondo temperature the heavy Fermion compound YbRh${}_2$Si${}_2$ shows a well defined Electron Spin Resonance (ESR) with local Yb${}^{3+}$ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb${}^{3+}$ in YPd${}_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic ${}^{174}$YbRh${}_2$Si${}_2$ we could exclude unresolved hyperfine contributions to the lineshape.