  We employ an atomic spin model and present a systematic investigation from a
single spin to a large system of over a million spins. To have an efficient
spin switching, the electron initial momentum direction must closely follow the
spin's orientation, so the orbital angular momentum is transverse to the spin
and consequently the spin-orbit torque lies in the same direction as the spin.
The module of the spin-orbit torque is $\lambda |{\bf S}||{\bf r}||{\bf P}|
\sqrt{\cos^2\alpha+\cos^2\beta-2\cos\alpha \cos\beta \cos\gamma} $, where
$\alpha(\beta)$ is the angle between spin {\bf S} and position {\bf r}(momentum
{ \bf P}) and $\gamma$ is the angle between {\bf r} and {\bf P}. These findings
are manifested in a much larger system. The spin response depends on underlying
spin structures. A linearly polarized laser pulse creates a dip in a uniform
inplane-magnetized thin film, but has little effects on \neel and Bloch walls.
Both right- and left- circularly polarized light ($\sigma^+$ and $\sigma^-$)
have stronger but different effects in both uniform spin domains and Neel
walls. While $\sigma^+$ light creates a basin of spins pointing down,
$\sigma^-$ light creates a mound of spins pointing up. In the vicinity of the
structure spins are reversed, similar to the experimental observation.
$\sigma^+$ light has a dramatic effect, disrupting spins in Bloch walls. By
contrast, $\sigma^-$ light has a small effect on Bloch walls because $\sigma^-$
only switches down spins up and once the spins already point up, there is no
major effect.
