  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.
