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

Abstract

The dynamics of ultrafast demagnetisation in 3$d$ magnets is complicated by the presence of both spin $\check{S}$ and orbital $\check{L}$ 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 $\check{L}$ due to laser excitation, followed post pulse by (ii) an increase of $\check{L}$ as $\check{S}$ transfers to $\check{L}$ during subsequent ($>15$~fs) SO induced spin-flip demagnetisation. We also show that to see an unambiguous transfer of $\check{S}$ to $\check{L}$ a short pulse is required.

5 pages 4 figures