Ultrafast Ferromagnetic Spin Switching by a Single Pair of Optical Pulses

All-optical magnetic recording potentially realizes orders of magnitude faster and more energy-efficient writing of magnetic bits but all-optical switching (AOS) of ferromagnets is so far ultraslow. Here, we demonstrate a new mechanism to all-optically switch ferromagnetic spins on picosecond (ps) time scales by using a pair of one femtosecond (fs) linearly polarized $(\pi)$ and one properly delayed ps circularly polarized $(\sigma)$ laser pulse for a Pt/Co/Pt multilayer. Our experiments show that the helicity-dependent absorption of the ps-delayed ps $\sigma $ pulse can deterministically steer the spin relaxation after destroying the ferromagnetic order by the fs $\pi $ pulse. Atomistic spin-dynamics simulations show that the reversal occurs within approximately 3 ps through an unprecedented mechanism, where nanoscale spin textures created by the fs $\pi $ pulse preferentially switch via magnetic circular dichroism (MCD), producing a spin- and helicity-dependent temperature difference of only a few Kelvin. Thus, a new route to deterministic ultrafast switching of nanoscale ferromagnetic media for ultrahigh-density recording is opened up.