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.