Unconventional Spin-orbit Torques by Two-dimensional Multilayered MXenes for Future Nonvolatile Magnetic Memories
MXenes have attracted considerable attention in recent years owing to their two-dimensional (2D) layered structures with various functionalities similar to those of graphene and transition metal dichalcogenides. To open a new application field for MXenes in the realm of electronic devices, such as ultrahigh-integrated magnetic memory, we have developed a spin-orbit torque (SOT) bilayer structure comprising bare MXene of Cr2N: substrate//Cr2N/[Co/Pt]3/MgO using the magnetron sputtering technique. We demonstrated field-free current-induced magnetization switching (CIMS) in the bilayer structure, regardless of the charge current directions with respect to the mirror symmetry lines of Cr2N crystal. This is a specific characteristic for the 2D MXene-based SOT-devices, originating from an unconventional out-of-plane SOT. As the SOT efficiency increases with increasing the Cr2N thickness, the first-principles calculations predict an intrinsic orbital-Hall conductivity with the dominant out-of-plane component, comparing to the spin-Hall conductivity in the Cr2N. X-ray magnetic circular dichroism reveals the out-of-plane uncompensated magnetic moment of Cr in the Cr2N layer at the interface, induced by contact with the Co in the [Co/Pt]3 ferromagnetic layer. Therefore, the intrinsic bulk orbital Hall effect in MXene and the interfacial contribution such as spin-filtering-like effect owing to uncompensated magnetic moment of Cr are considered as possible major mechanisms for the unconventional out-of-plane SOT in the device, rather than a crystal symmetry and/or an interlayer exchange coupling.