  NiO is a good candidate for ultrafast magnetic switching because of its large
spin density, antiferromagnetic order, and clearly separated intragap states.
In order to detect and monitor the switching dynamics, we develop a systematic
approach to study optical second harmonic generation (SHG) in NiO, both at the
(001) surface and in the bulk. In our calculations NiO is modeled as a doubly
embedded cluster. All intragap \emph{d}-states of the bulk and the (001)
surface are obtained with highly-correlational quantum chemistry and propagated
in time under the influence of a static magnetic field and a laser pulse. We
find that demagnetization and switching can be best achieved in a subpicosecond
regime with linearly rather than circularly polarized light. We also show the
importance of including an external magnetic field in order to distinguish
spin-up and spin-down states and the necessity of including magnetic-dipole
transitions in order to realize the $\Lambda$-process in the centrosymmetric
bulk. Having already shown the effects of phonons in the SHG for the bulk NiO
within the frozen-phonon approximation, and following the same trail of
thoughts, we discuss the role of phonons in a fully quantized picture as a
symmetry-lowering mechanism in the switching scenario and investigate the
electronic and lattice temperature effects.
