High-Magnetic-Field X-ray Absorption Spectroscopy of Field-Induced Valence Transition in EuNi2(Si1-xGex)2

The magnetic-field-induced valence transition in EuNi 2 (Si 1- x Ge x ) 2 ( x =0.82, 0.85) has been studied by X-ray absorption spectroscopy at low temperatures. The field dependence of the Eu valence is directly observed for the first time. We find a significant decrease in the valences at high magnetic fields above 40 T. The valence of Eu changes from v * =2.75 at a zero field to v * =2.33 at 40 T for x =0.82. The results are analyzed using a theoretical model. The hybridization parameter V between the Eu 2+ and Eu 3+ states and the energy separation Δ E are determined, which are crucial for quantum mechanical mixing.