Timescales and contribution of heating and helicity effect in helicity-dependent all-optical switching

The manipulation of the magnetic direction by using the ultrafast laser pulse is attractive for its great advantages in terms of speed and energy efficiency for information storage applications. However, the heating and helicity effects induced by circularly polarized laser excitation are entangled in the helicity-dependent all-optical switching (HD-AOS), which hinders the understanding of magnetization dynamics involved. Here, by applying a dual-pump laser excitation, first with a linearly polarized (LP) laser pulse followed by a circularly polarized (CP) laser pulse, we identify the timescales and contribution from heating and helicity effects in HD-AOS with a Pt/Co/Pt triple layer. When the sample is preheated by the LP laser pulses to a nearly fully demagnetized state, CP laser pulses with a much-reduced power switches the sample's magnetization. By varying the time delay between the two pump pulses, we show that the helicity effect, which gives rise to the deterministic helicity induced switching, onsets instantly upon laser excitation, and only exists for less than 0.2 ps close to the laser pulse duration of 0.15 ps. The results reveal that that the transient magnetization state upon which CP laser pulses impinge is the key factor for achieving HD-AOS, and importantly, the tunability between heating and helicity effects with the unique dual-pump laser excitation approach will enable HD-AOS in a wide range of magnetic material systems for the potential ultrafast spintronics applications.