<i>Ab initio</i>investigation of light-induced relativistic spin-flip effects in magneto-optics

TLDR

Ultrafast demagnetization of metallic ferromagnets such as Ni occurs within ~300 fs after excitation by an intense femtosecond laser pulse, and has been suggested to arise from relativistic light‑induced processes. This study conducts an ab initio investigation of how relativistic effects influence the magneto‑optical response of Ni. We formulate a response‑theory description of the ultrarelativistic terms that appear in the Foldy–Wouthuysen transformed Dirac Hamiltonian under an electromagnetic field and compute the impact of relativistic light‑induced spin‑flip transitions on the magneto‑optics. The calculations show that relativistic spin‑flip optical excitations contribute at most 0.1 % to the laser‑induced magnetization change in Ni.

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, first, we develop a response theory formulation of the additional appearing ultrarelativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and second, we 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 $(\ensuremath{\le}0.1%)$ to the laser-induced magnetization change in Ni.