  Electron Spin Resonance (ESR) measurements of the ferromagnetic Kondo lattice
system CeRuPO show a well defined ESR signal which is related to the magnetic
properties of the Ce3+ moment. In contrast, no ESR signal could be observed in
the antiferromagnetic homologue CeOsPO. Additionally, we detect an ESR signal
in a further ferromagnetic Yb compound, YbRh, while it was absent in a number
of Ce or Yb intermetallic compounds with dominant antiferromagnetic exchange,
independently of the presence of a strong Kondo interaction or the proximity to
a (quantum) critical point. Thus, the observation of an ESR signal in a Kondo
lattice is neither specific to Yb nor to the proximity of a quantum critical
point, but seems to be connected to the presence of ferromagnetic fluctuations.
These conclusions not only provide a basic concept to understand the ESR in
Kondo lattice systems even well below the Kondo temperature as observed in the
heavy fermion metal YbRh2Si2 but point out ESR as a prime method to investigate
directly the spin dynamics of the Kondo ion.
