  We investigate the interaction of femtosecond laser pulses with spins
including relativistic corrections. The time-ordered magneto-optical signals
corresponding to a pump-probe configuration are calculated in the case of one
electron submitted to a magnetic field and evolving in eight levels of the fine
structure of a hydrogen-like atom. Our simulations explain the origin of the
coherent magneto-optical response and ultrafast spin dynamics in ferromagnets
excited by intense laser pulses as recently reported in Ni and CoPt3
ferromagnetic thin films. Our detailed analysis allows identifying the
respective roles of the coherent spin-photon interaction and spin dynamics
unraveling recent controversies about the laser induced ultrafast magnetization
dynamics.
