High-magnetic-field X-ray absorption and magnetic circular dichroism spectroscopy in the mixed-valent compound YbAgCu₄

The X-ray absorption and magnetic circular dichroism (XMCD) spectra at the L₃ edge of Yb have been studied experimentally as well as theoretically in the mixed-valent heavy fermion compound YbAgCu₄ at high magnetic fields and low temperatures. It has been found that the absorption spectra depend on the temperature as well as the magnetic field, suggesting a significant change in valence with the temperature and magnetic field. The magnetic field-temperature (H-T) phase boundary determined by the valence state is in very good agreement with that determined by the magnetization. It is clearly found that the metamagnetism of this compound is due to the field-induced valence transition. A distinct positive peak of the XMCD spectra appears in the vicinity of the white line of the absorption due to the Yb ³⁺ state, while no feature is observed in the XMCD spectra corresponding to the Yb²⁺ state. On the other hand, a small negative XMCD peak was observed at a lower energy and the magnetic field dependence of the negative XMCD intensity was found to be qualitatively different from that of the main XMCD signal near the white line. By theoretical calculation, the quadrupole transition is found to explain the energy position and its peculiar magnetic field dependence.