On/Off Ultra‐Short Spin Current for Single Pulse Magnetization Reversal in a Magnetic Memory Using VO 2 Phase Transition

International audienceBeaurepaire and Bigot when they discovered the ultrafast demagnetization in Ni thin film by femtosecond laser pulse. [1] A decade later, the full reversal of a ferrimagnet, GdFeCo, by polarized laser pulses was first experimentally realized namely All Optical Switching (AOS), [2] which was later observed on a large variety of magnetic materials. [3] It has also been demonstrated that All Optical Switching (AOS) can be dependent on the light helicity, which is called all-optical helicitydependent switching (AO-HDS) but it can also be independent on the light helicity and it is then called all-optical helicityindependent switching (AO-HIS). [4-11] All-optical helicity-dependent switching (AO-HDS) is reported for many different types of material, especially in ferromagnetic multilayers, such as Co/Pt as well as granular recording media, but it requires multiple laser pulses to switch the magnetization which limits its operating speed and increases energy consumption for application. [12,13] AO-HIS, on the other hand, can be completed with only one pulse. So far, AO-HIS has been reported in some specific materials, including Gd-based ferrimagnetic alloys or multilayers, Tb/Co multilayers and half-metallic compensated ferrimagnetic Heusler alloy Mn 2 Ru x Ga. [14-19] Until now, the physical mechanism of AO-HIS has been discussed widely. For GdFeCo as well as some other Gd-based alloys, it has been explained by different demagnetization time and the antiferromagnetic exchange interaction between rare-earth (RE) and transition metal (TM) sublattices which may result in a transient ferromagnetic like state. [14,20-25] It has also been demonstrated that Gd-based sample could be switched using one femtosecond electron pulse proving that it is a thermal effect. [26,27] However, restricted by the practical application problem of Gd-based materials, such as low tunneling magnetoresistance ratio (TMR) in GdFeCo/MgO-based magnetic tunnel junction (MTJ), more materials for AO-HIS, especially the ferromagnetic layer, have attracted intensive interests. Up to now no direct single laser pulse switching has been reported for a ferromagnetic layer. Nevertheless, in 2018, Satoshi et al. unveiled a method to switch a single ferromagnetic layer, Co/Pt, with only one light pulse inducing the GdFeCo demagnetization and consequently generating an ultra-short spin current pulse from GdFeCo in a GdFeCo/Cu/[Co/Pt] spin valve structure. [28] They show that the final state of Co/Pt is determined by the polarize