Spectroscopy Studies of Temperature-Induced Valence Transition on EuNi ₂(Si_1-xGe_x)₂ around Eu 3d-4f, 4d-4f and Ni 2p-3d Excitation Regions

The resonant photoemission spectra and X-ray absorption spectra of temperature-induced valence transition material EuNi₂(Si _1-xGe_x)₂ around Eu 3d-4f, 4d-4f and Ni 2p-3d resonant excitation regions have been measured. From the comparison between the resonant photoemission spectra around the 3d-4f (bulk sensitive) and 4d-4f (surface sensitive) excitation regions, the Eu divalent component originating from the surface is separated from the bulk one. The divalent and the trivalent features of 4f electrons are obviously distinguished depending on the excitation conditions. It was confirmed that the intensity ratio of the 4f electronic structures between the Eu divalent and trivalent ions changes as a function of the temperature. The mean valence values estimated from the Eu M-edge X-ray absorption spectra and from the photoemission spectra were rather smaller than those obtained previously from the L-edge X-ray absorption spectra. The reason of the discrepancy is discussed. The spectral features and the resonant behaviors are well explained by a theoretical calculation based on the atomic model except for the strong contribution from the Ni 3d bands. In the resonant condition of the Eu divalent component, the spin flip satellite beside the 4f divalent states was observed. The photoemission spectra around the Ni 2p-3d excitation region show similar behavior to that of Ni pure metal.