  Femtosecond magneto-optical pump-probe measurements of ultrafast
demagnetization show an intriguing difference in the first 100 fs of the
magneto-optical Kerr response depending on whether the polarization of the pump
and probe beams are in parallel or perpendicular configuration [Bigot et al.,
Nature Phys. 5, 515 (2009)]. Starting from a most general relativistic
Hamiltonian we focus on the ultra-relativistic light-spin interaction and show
that this coupling term leads to different light-induced opto-magnetic fields
when pump and probe polarization are parallel and perpendicular to each other,
providing thus an explanation for the measurements. We also analyze other
pump-probe configurations where the pump laser is circularly polarized and the
employed probe contains only linearly polarized light and show that similar
opto-magnetic effects can be anticipated.
