the Dzyaloshinskii-Moriya interaction (DMI) [6][7][8] with synthetic antiferromagnets (SAFs), which resulted in reported DW velocities close to 750 m s -1 . [9] Despite the large improvement, the energy efficiency is still limited due to the weak strength of the antiferromagnetic (AF) coupling. Therefore, the materials platform of rare earth (RE)-transition metal (TM) compounds garnered considerable attention, [1] promising faster CIDWM due to the much stronger direct AF coupling than the indirect exchange coupling [10] utilized in SAFs. Furthermore, the SOTs used to drive the DW promise to be highly efficient in the RE-TM systems as a result of the long spin coherence length. [11] Cosequently, high velocity CIDWM has been reported in Co-Gdbased ferrimagnetic alloy systems [12,13] when the angular momentum in the magnetic material is compensated, being at least a factor of three faster than that of the previously reported SAFs.Besides the efficient CIDWM, single pulse all-optical switching (AOS) of the magnetization [14,15] in the RE-TM systems has obtained significant attention thanks to its subpicosecond [16] energy efficient [14,17,18] magnetization switching enabled by the ultrafast angular momentum transfer upon laser excitation. [19] This can be useful as a new generation of ultrafast magnetic memory, as well as a data buffer between electronics and integrated photonics. [14,20,21] Recently, a synthetic ferrimagnetic system based on a Pt/Co/Gd- [18,22] layered structure has shown high robustness [23] for such a hybrid integration. These kinds of synthetic ferrimagnets have some distinct advantages over RE-TM alloys. For instance, AOS is not limited by the exact composition. [24] They also withstand thermal annealing [25] and offer easier magnetic composition control at wafer scale than the alloy system, as well as better access to interface engineering. Therefore, it has been proposed that such a materials platform has high potential to realize a hybrid integration of DW memory in photonic platforms to further enhance their storage density. [20,26] So far, the CIDWM of Co/Gd bilayers [26,27] has been investigated. However, the highest reported velocity, achieved at cryogenic conditions, [27] was several times lower than that reported in alloys, [12] in part due to large net angular momentum, low compensation temperature as well as DW pinning effects. In this report, we therefore propose a materials platform based on the [Co/Gd] 2 synthetic ferrimagnet capable of accommodating both efficient CIDWM of over 2 km s -1 ) at room temperature
Flexibility for interface engineering and access to all-optical switching of the magnetization make synthetic ferrimagnets an interesting candidate for advanced optospintronic devices. Moreover, due to their layered structure and disordered interfaces, they also bear promise for the emerging field of graded magnetic materials. The fastest and most efficient spin–orbit torque driven manipulation of the magnetic order in this material system generally takes place at compensation. Here, we present a systematic experimental and modeling study of the conditions for magnetization compensation and perpendicular magnetic anisotropy in the synthetic ferrimagnetic Co/Gd/Co/Gd system. A model based on partial intermixing at the Co/Gd interfaces of this system has been developed which explains the experiments well and provides a tool to understand its magnetic characteristics. More specifically, this work provides further insight into the decay of the Co proximity-induced magnetization in the Gd, and the role the capping layer plays in the Gd magnetization.
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