Ab initio investigation of light-induced relativistic spin-flip effects in magneto-optics

TL;DR

This study uses ab initio methods to investigate whether relativistic light-induced spin-flip effects significantly contribute to ultrafast demagnetization in nickel, finding their impact to be minimal.

Contribution

The paper develops a response theory for ultra-relativistic terms in the Dirac Hamiltonian and quantifies their effect on magneto-optical responses in Ni.

Findings

Relativistic spin-flip effects contribute less than 0.1% to magnetization change.

Ab initio calculations show minimal impact of relativistic effects on ultrafast demagnetization.

The developed theory enables detailed analysis of relativistic contributions in magneto-optics.

Abstract

Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, we develop, first, a response theory formulation of the additional appearing ultra-relativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and, second, compute the influence of relativistic light-induced spin-flip transitions on the magneto-optics. Our ab initio calculations of relativistic spin-flip optical excitations predict that these can give only a very small contribution ($\leq 0.1$%) to the laser-induced magnetization change in Ni.