  This year the discovery of femtosecond demagnetization by laser pulses is 20
years old. For the first time this milestone work by Bigot and coworkers gave
insight in a very direct way into the time scales of microscopic interactions
that connect the spin and electron system. While intense discussions in the
field were fueled by the complexity of the processes in the past, it now became
evident that it is a puzzle of many different parts. Rather than giving an
overview that has been presented in previous reviews on ultrafast processes in
ferromagnets, this perspective will show that with our current depth of
knowledge the first real applications are on their way: THz spintronics and
all-optical spin manipulation are becoming more and more feasible. The aim of
this perspective is to point out where we can connect the different puzzle
pieces of understanding gathered over 20 years to develop novel applications.
based on many observations in a large number of experiments. Differences in the
theoretical models arise from the localized and delocalized nature of
ferromagnetism. Transport effects are intrinsically non-local in spintronic
devices and at interfaces. We review the need for multiscale modeling to
address processes starting from electronic excitation of the spin system on the
picometer length scale and sub-femtosecond time scale, to spin wave generation,
and towards the modeling of ultrafast phase transitions that altogether
determine the response time of the ferromagnetic system. Today, our current
understanding gives rise to the first real applications of ultrafast spin
physics for ultrafast magnetism control: THz spintronic devices. This makes the
field of ultrafast spin-dynamics an emerging topic open for many researchers
right now.
