Ultrafast Laser-induced Spin Dynamics in advanced materials

Helicity-dependent all-optical induced magnetization switching has been achieved in Pt/Co/Pt samples with perpendicular magnetic anisotropy. Applying a dual-pump laser excitation, first with a linearly polarized (LP) laser pulse followed by a circularly polarized (CP) laser pulse, the timescales and contribution from heating and helicity effects in HD-AOS were identified with a Pt /Co /Pt triple-layer. When the LP laser pulses preheat the sample to a nearly fully demagnetized state, the CP laser pulses with a power reduced by 80% switch the sample’s magnetization. By varying the time delay between the two pump pulses, the results show that the helicity effect, which gives rise to the deterministic helicity-induced switching, arises almost instantly within 200fs close to the pulse width upon laser excitation.
All-optical-induced magnetization switching in Pt/Co/Pt triple layers has been further explored as a function of the thickness of the Co and Pt layers, respectively. The Ab initio calculation shows that the Pt atom significantly improves the spin-orbit coupling of Co’s 3d electrons. This enhanced spin-orbital coupling only occurs at the Pt/Co interface. Moreover, the ab initio calculation gives a surprising interlayer antiferromagnetic coupling in the Co/Pt multilayer system, which is due to the oscillation of the RKKY coupling.
The effect of ultrafast laser heating on the interlayer antiferromagnetic coupling of synthetic antiferromagnetic tri-layers has been investigated by time-resolved pump-probe measurements together with transient hysteresis loop measurements at selected time delays. Time-resolved Kerr hysteresis loop measurements have shown that the interlayer antiferromagnetic coupling can be transiently suppressed by the laser’s heating effect within the first picosecond after laser excitation. While the Kerr rotation time scans show, the antiferromagnetic coupling can be transferred into an apparent ferromagnetic coupling state and then recovered to the antiferromagnetic coupling state as the recovery of the magnetization of each Co layer.