Tuning the4fstate occupancy of Ce in highly correlatedCeSi∕Femultilayers: An x-ray absorption spectroscopy study

Abstract

Spectra of x-ray absorption were measured at the ${L}_{2,3}(2p)$ and ${M}_{4,5}(3d)$ edges of Ce in multilayers $[{\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}∕\mathrm{Fe}]xn$ ($x$ between 0.1 and 0.65), with single-phase amorphous ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ sublayers. The study uncovers the highly correlated nature of this layered system; an $\ensuremath{\alpha}$-phase-like electronic configuration of Ce is observed, which indicates considerable hybridization between the $4f$ and conduction-band states. This is at variance with single alloy films ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ which show a $\ensuremath{\gamma}$-phase-like Ce configuration already at $x=0.1$. X-ray magnetic circular dichroism measured at the ${L}_{2,3}$ edges of Ce in the multilayers reveals magnetic order on the $5d$ electrons, induced by the interaction with Fe at the interfaces. With increasing the Si content, the strength of the $4f$-conduction-band hybridization is reduced, which is reflected in a growing occupation of the $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ states. Variations of the line shape and intensity of the ${L}_{2,3}$-edge dichroism spectra are very complex. The spectra not only are related to the magnetic $5d$ polarization in the ground state but are largely controlled by the exchange interaction between the photoexcited $5d$ electron and the $4f$ electron, which generates a spin-dependent enhancement of the radial parts of the $2p$-to-$5d$ matrix element. The strength of the $4f\text{\ensuremath{-}}5d$ exchange interaction can be controlled by varying the composition of the ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ sublayers. At high Si concentration and low temperature, it induces a change in sign of the dichroic signal. We present a detailed discussion within a simple phenomenological model.