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 of ferromagnets is so far ultraslow. Here, we demonstrate a new mechanism to all-optically switch ferromagnetic spins on picosecond time scales by using a pair of one femtosecond (fs) linearly polarized (π) and one properly-delayed picosecond (ps) circularly polarized (σ) laser pulse for a Pt/Co/Pt multilayer. Our experiments show that the helicity dependent absorption of the ps-delayed ps σ pulse can deterministically steer the spin relaxation after destroying the ferromagnetic order by the fs π 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 π pulse preferentially switch via magnetic circular dichroism, 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 ultra-high-density recording is opened up.