  Inspired by a recent pressure experiment in fcc Ni, we propose a simple
method to use pressure to investigate the laser-induced femtosecond magnetism.
Since the pressure effect on the electronic and magnetic properties can be well
controlled experimentally, this leaves little room for ambiguity when compared
with theory. Here we report our theoretical pressure results in fcc Ni:
Pressure first suppresses the spin moment reduction, and then completely
diminishes it; further increase in pressure to 40 GPa induces a
demagnetization-to-magnetization transition. To reveal its microscopic origin,
we slide through the L-U line in the Brillouin zone and find two essential
transitions are responsible for this change, where the pressure lowers two
valence bands, resulting in an off-resonant excitation and thus a smaller spin
moment reduction. In the spin-richest L-W-W' plane, two spin contours are
formed; as pressure increases, the contour size retrieves and its intensity is
reduced to zero eventually, fully consistent with the spin-dipole factor
prediction. These striking features are detectable in time- and spin-resolved
photoemission experiments.
