Magnetic behavior near the boundary of 4f delocalization in ferromagnetic CeRu ₂Ge ₂ and paramagnetic CeRu ₂Si ₂ observed by Ce M _4,5 XAS and XMCD

X-ray absorption (XAS) and its magnetic circular dichroism (XMCD) were measured at the Ce M _4,5 absorption edges of ferromagnetic CeRu ₂Ge ₂ and paramagnetic CeRu ₂Si ₂: both compounds are considered to be located near the boundary of delocalization of Ce 4f electrons. While the XAS line shape varies clearly reflecting the variation in the 4f delocalization, the line-shape variation in XMCD is hardly discernible under various conditions of temperature and magnetic field. The XAS line-shape variation can be explained as effects of the variations in the 4f occupation number and in the ratio of J=7/2 states in the ground states, both of which are closely related to the 4f delocalization. The 4f delocalization also causes a decrease in the ratio of the orbital magnetic moment to the spin magnetic moment. The magnetic-field dependence of XAS suggests that the Ce 4f electrons retain a delocalized character across the metamagnetic transition in CeRu ₂Si ₂.