Exchange splitting of surface and bulk electronic states in excited magnetic states of Gd: First-principles study

Abstract

Recent pump-probe experiments provide time evolution of the bulk and surface electronic states in Gd excited by the laser pulse. These experiments are in close connection with earlier spectroscopic experiments probing temperature dependence of the exchange splitting of the electronic states. We report first-principles study of the electronic states in excited Gd modeled by the noncollinearity of the $4f$ spin moments. In agreement with experiments we obtain a strong difference in the properties of the bulk and surface electronic states. To reveal the origin of this behavior we apply the concept of spin mixing to characterize the electronic states of the excited system. The surface states remain weakly spin mixed with respect to the local atomic spin axis of the surface layer that explains the persistence of the exchange splitting in highly excited Gd. On the other hand, a smaller part of the surface state localized in the second layer becomes strongly spin mixed leading to decreased value of the exchange splitting. In contrast to the surface states the bulk states are strongly spin mixed and average the influence of the atomic spin-up and spin-down potentials. This leads to the properties of the bulk states that are usually associated with the Stoner model. The good agreement between calculational results and the results of the pump-probe experiment support the assumption of ultrafast disordering of the $4f$ moments after laser irradiation.