  The understanding of how spins move at pico- and femtosecond time scales is
the goal of much of modern research in condensed matter physics, with
implications for ultrafast and more energy-efficient data storage. However, the
limited comprehension of the physics behind this phenomenon has hampered the
possibility of realising a commercial technology based on it. Recently, it has
been suggested that inertial effects should be considered in the full
description of the spin dynamics at these ultrafast time scales, but a clear
observation of such effects in ferromagnets is still lacking. Here, we report
the first direct experimental evidence of inertial spin dynamics in
ferromagnetic thin films in the form of a nutation of the magnetisation at a
frequency of approximately 0.6 THz. This allows us to evince that the angular
momentum relaxation time in ferromagnets is on the order of 10 ps.
