  Spintronics had a widespread impact over the past decades due to transferring
information by spin rather than electric currents. Its further development
requires miniaturization and reduction of characteristic timescales of spin
dynamics combining the sub-nanometer spatial and femtosecond temporal ranges.
These demands shift the focus of interest towards the fundamental open question
of the interaction of femtosecond spin current (SC) pulses with a ferromagnet
(FM). The spatio-temporal properties of the impulsive spin transfer torque
exerted by ultrashort SC pulses on the FM open the time domain for probing
non-uniform magnetization dynamics. Here we employ laser-generated ultrashort
SC pulses for driving ultrafast spin dynamics in FM and analyzing its transient
local source. Transverse spins injected into FM excite inhomogeneous
high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of
the FM magnetization is confined to 2 nm.
