Ultrafast demagnetization dynamics in Ni: role of electron correlations

TL;DR

This paper demonstrates that including electron correlations and memory effects in advanced theoretical models explains the ultrafast (less than 50 fs) demagnetization in nickel, aligning with experimental results and highlighting the importance of spin-flip transitions.

Contribution

The study introduces a combined spin-flip time-dependent density-functional theory and dynamical mean-field theory approach to explain femtosecond demagnetization in Ni, emphasizing electron correlations and memory effects.

Findings

Ultrafast demagnetization occurs within 50 fs, matching experiments.

Electron correlations and memory effects are crucial for accurate modeling.

Spin-flip transitions from occupied to unoccupied orbitals drive demagnetization.

Abstract

Experimental observations of the ultrafast (less than 50 fs) demagnetization of Ni have so far defied theoretical explanations particularly since its spin-flipping time is much less than that resulting from spin-orbit and electron-lattice interactions. Through the application of an approach that benefits from spin-flip time-dependent density-functional theory and dynamical mean-field theory, we show that proper inclusion of electron correlations and memory (time-dependence of electron-electron interaction) effects leads to demagnetization at the femtosecond scale, in good agreement with experimental observations. Furthermore, our calculations reveal that this ultrafast demagnetization results mainly from spin-flip transitions from occupied to unoccupied orbitals implying a dynamical reduction of exchange splitting. These conclusions are found to be valid for a wide range of laser pulse amplitudes. They also pave the way for ab initio investigations of ultrafast charge and spin dynamics in a variety of quantum materials in which electron correlations may play a definitive role.