  Disentangling orbital (OAM) and spin (SAM) angular momenta in the ultrafast
spin dynamics of two-dimensional (2D) ferromagnets on subfemtoseconds is a
challenge in the field of ultrafast magnetism. Herein, we employed
non-collinear spin version of real-time time-dependent density functional
theory to investigate the orbital and spin dynamics of 2D ferromagnets Fe3GeTe2
(FGT) induced by circularly polarized light. Our results show the
demagnetization of Fe sublattice in FGT is accompanied by helicity-dependent
precession of OAM and SAM excited by circularly polarized lasers. We further
identify that precession of OAM and SAM in FGT is faster than the
demagnetization within a few femtoseconds. Remarkably, circularly polarized
lasers can significantly induce a periodically transverse response of OAM and
SAM on very ultrafast timescales of ~250 attoseconds. Our finding suggests a
powerful new route for attosecond regimes of the angular momentum manipulation
to coherently control helicity-dependent orbital and spin dynamics in 2D
limits.
