Control of speed and efficiency of ultrafast demagnetization by direct transfer of spin angular momentum

Since the discovery in 1996 that the magnetization of a nickel thin film is reduced within a few picoseconds after femtosecond-laser excitation(1), ultrafast demagnetization has attracted a thriving interest. This attraction is driven by the twofold challenge of understanding magnetization dynamics in a strongly out-of-equilibrium regime and controlling the magnetic properties of materials on the subpicosecond timescale, with potential applications in spintronics. In the past decade significant progress has been made in understanding the microscopic processes that govern ultrafast demagnetization(2-7). The discussion has been particularly focused on the role of angular-momentum conservation during the demagnetization process(8-10). Here, using the time-resolved magneto-optical Kerr effect, we demonstrate that interlayer transfer of spin angular momentum in specially engineered Co/Pt multilayers speeds up the demagnetization process, bypassing the mechanism responsible for the conservation of total angular momentum taking place in a single ferromagnetic layer. This new channel for spin-angular-momentum dissipation leads to a reduction of the demagnetization time of up to 25%, accompanied by an increase of the total demagnetization by almost the same amount.