  The ultimate control of magnetic states of matter at femtosecond (or even
faster) timescales defines one of the most pursued paradigm shifts for future
information technology. In this context, ultrafast laser pulses developed into
extremely valuable stimuli for the all-optical magnetisation reversal in
ferrimagnetic and ferromagnetic alloys and multilayers, while this remains
elusive in elementary ferromagnets. Here we demonstrate that a single laser
pulse with sub-picosecond duration can lead to the reversal of the
magnetisation of bulk nickel, in tandem with the expected demagnetisation. As
revealed by realistic time-dependent electronic structure simulations, the
central mechanism is ultrafast light-induced torques acting on the
magnetisation, which are only effective if the laser pulse is circularly
polarised on a plane that contains the initial orientation of the
magnetisation. We map the laser pulse parameter space enabling the
magnetisation switching and unveil rich intra-atomic orbital-dependent
magnetisation dynamics featuring transient inter-orbital non-collinear states.
Our findings open further perspectives for the efficient implementation of
optically-based spintronic devices.
