Where does the spin angular momentum go in laser induced demagnetisation?

TL;DR

This paper uses advanced simulations to reveal a two-step process in laser-induced demagnetisation, showing how orbital and spin angular momenta evolve over ultrafast timescales in 3d magnets.

Contribution

It uncovers the microscopic mechanism of angular momentum transfer involving spin and orbital moments during ultrafast demagnetisation, highlighting the importance of pulse duration.

Findings

Initial orbital angular momentum loss due to laser excitation

Post-pulse increase in orbital angular momentum from spin transfer

Short pulses are essential to observe clear spin-to-orbital transfer

Abstract

The dynamics of ultrafast demagnetisation in 3$d$ magnets is complicated by the presence of both spin ${\v S}$ and orbital ${\v L}$ angular momentum, with the microscopic mechanism by which the magnetic moment is redistributed to the lattice, and at what time scales, yet to be resolved. Employing state-of-the-art time dependent density function theory we disentangle the dynamics of these two momenta. Utilising ultra short (5~fs) pulses that separate spin-orbit (SO) and direct optical excitation time scales, we demonstrate a two-step microscopic mechanism: (i) an initial loss of ${\v L}$ due to laser excitation, followed post pulse by (ii) an increase of ${\v L}$ as ${\v S}$ transfers to ${\v L}$ during subsequent ($> 15$~fs) SO induced spin-flip demagnetisation. We also show that to see an unambiguous transfer of ${\v S}$ to ${\v L}$ a short pulse is required.