  The valence state of Ce in a canonical heavy fermion compound CeRu2Si2 has
been investigated by synchrotron X-ray absorption spectroscopy at 1.8 K in high
magnetic fields of up to 40 T. The valence was slightly larger than for the
pure trivalent state (Ce3+: f1), as expected in heavy fermion compounds, and it
decreased toward the trivalent state as the magnetic field was increased. The
field-induced valence reduction indicates that the itinerant character of the
4f electrons in CeRu2Si2 was suppressed by a strong magnetic field. The
suppression was gradual and showed characteristic magnetic field dependence,
which reflects the metamagnetism around Hm \sim 8 T. The itinerant character
persisted, even at 40 T (\sim 5Hm), suggesting that the Kondo bound state is
continuously broken by magnetic fields and that it should completely collapse
at fields exceeding 200 T.
