  Laser-induced ultrafast demagnetization has puzzled researchers around the
world for over two decades. Intrinsic complexity in electronic, magnetic, and
phononic subsystems is difficult to understand microscopically. So far it is
not possible to explain demagnetization using a single mechanism, which
suggests a crucial piece of information still missing. In this paper, we return
to a fundamental aspect of physics: spin and its change within each band in the
entire Brillouin zone. We employ fcc Ni as an example and use an extremely
dense {\bf k} mesh to map out spin changes for every band close to the Fermi
level along all the high symmetry lines. To our surprise, spin angular momentum
at some special {\bf k} points abruptly changes from $\pm \hbar/2$ to $\mp
\hbar/2$ simply by moving from one crystal momentum point to the next. This
explains why intraband transitions, which the spin superdiffusion model is
based upon, can induce a sharp spin moment reduction, and why electric current
can change spin orientation in spintronics. These special {\bf k} points, which
are called spin Berry points, are not random and appear when several bands are
close to each other, so the Berry potential of spin majority states is
different from that of spin minority states. Although within a single band,
spin Berry points jump, when we group several neighboring bands together, they
form distinctive smooth spin Berry lines. It is the band structure that
disrupts those lines. Spin Berry points are crucial to laser-induced ultrafast
demagnetization and spintronics.
