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By conducting helicity-dependent ultrafast magnetization dynamics in a CoTb ferrimagnetic alloy, we are able to quantitatively determine the magnetic circular dichroism (MCD) and resolve its role in the helicity-dependent all-optical switching (AOS). Unequivocal interpretation of the sign of the dichroism is provided by performing AOS and femtosecond laser-induced domain wall motion experiments. We demonstrate that AOS occurs when the magnetization is initially in the most absorbent state according to the light helicity. Moreover, we evidence that the MCD creates a thermal gradient that drives a domain wall towards hotter regions. Our experimental results are in agreement with the purely thermal models of AOS. The ultrafast optical control of the magnetic order rapidly emerged as a promising approach in ultrafast magnetism. Yet, a decade after the discovery of complete and deterministic magnetization switching induced by femtosecond circularly polarized laser pulses [1], many fundamental questions remained unanswered. In particular , the issue of the relative contribution of pure thermal or non-thermal effects induced by the laser pulses is crucial for the understanding of the magnetization reversal mechanism. Indeed, the interaction between light and matter may involve transfer of energy which results in an ultrafast heating of the electronic system [2-5]. Additionally , for circular polarization, transfer of light angular momentum and the emergence of an effective magnetic field via the inverse Faraday effect (IFE) may occur [6-9]. The ability to reverse the magnetization with ultra-short polarized laser pulses, a phenomenon known as all-optical switching (AOS), was evidenced in a broad variety of ferri-and ferro-magnetic materials [1, 10, 11]. In GdFeCo alloys, AOS can be single-pulse and helicity-independent [3, 4, 12]. On the other hand, in ferrimag-netic CoTb and ferromagnetic thin films such as Co/Pt multiplayers, AOS is found to be helicity-dependent [10, 11, 13, 14]. The latter was also demonstrated to be a multishot and cumulative process [15-17] which involves domain nucleation and helicity-dependent domain wall propagation [18]. Several theoretical models have been developed for multi-pulse all-optical helicity-dependent switching (AO-HDS) without reaching a consensus about the origin of the driving force [16, 19-22]. To explain the role of the photon helicity, the IFE was considered among non-thermal optomagnetic effects. Following the laser-induced thermal demagnetization, the effective field generated by circularly-polarized laser pulses would switch the magnetization according to the light handedness [9, 19, 20]. This magnetization reversal mechanism is mostly non-thermal. Yet, a precise characterization of the amplitude and lifetime of this optomagnetic field remains elusive in absorbing metallic media [9, 19]. In contrast, it has been argued that AO-HDS could originate from a purely heating mechanism [16, 21, 22]. In this case, the asymmetry would arise from the difference in light absorption between left-and right-circular polarization due to the magnetic circular dichroism (MCD). The multishot helicity-dependence of AOS in ferromagnets was theoretically reproduced with a purely thermal mechanism based on the MCD [16, 21]. In this process, the laser initially heats the system close to the Curie temperature. As a result of the MCD, some magnetic domains would be hotter and will experience stochastic switching, while domains of opposite magneti-zation direction would remain cooler and stable [16, 21]. Moreover, it was suggested that the temperature gradient arising from the MCD across a domain wall could explain the helicity-dependent domain wall motion reported in Co/Pt multilayers [18]. In all the thermal models of multishot AO-HDS [16, 21, 22], it was assumed that the final state is the least absorbent. This means that switching occurs if the mag-netization is initially in the higher absorption direction according to the light helicity. Hence, it comes that the sign of the MCD governs the helicity-dependence of AOS and the direction of the temperature gradient across a domain wall. Nevertheless, no experimental proof has been provided so far to corroborate the thermal models of AOS. Therefore, it appears legitimate to experimentally investigate the helicity dependence of light absorption in the magnetic thin films that exhibit multishot AO-HDS. A first attempt to reveal the MCD effects in (Co/Pt) multilayers upon laser pulse irradiation lacked of a direct comparison between the absorption states and the effect of the light helicity on the magnetization direction [23]. Besides, it is necessary to explore the effect of MCD on a domain wall as it was proved that domain wall motion plays an important role in the AOS mechanism [18]. In this letter, we aim to experimentally resolve the relevance of a purely thermal mechanism based on the

Resolving the role of magnetic circular dichroism in multishot helicity-dependent all-optical switching