  Laser-induced femtosecond magnetism or femtomagnetism simultaneously relies
on two distinctive contributions: (a) the optical dipole interaction (ODI)
between a laser field and a magnetic system and (b) the spin expectation value
change (SEC) between two transition states. Surprisingly, up to now, no study
has taken both contributions into account simultaneously. Here we do so by
introducing a new concept of the optical spin generator, a product of SEC and
ODI between transition states. In ferromagnetic nickel, our first-principles
calculation demonstrates that the larger the value of optical spin generator
is, the larger the dynamic spin moment change is. This simple generator
directly links the time-dependent spin moment change {\Delta}Mk z (t) at every
crystal- momentum k point to its intrinsic electronic structure and magnetic
properties. Those hot spin spots are a direct manifestation of the optical spin
generator, and should be the focus of future research.
