Unconventional Spin–Orbit Torques by 2D Multilayered MXenes for Future Nonvolatile Magnetic Memories

MXenes have attracted attention in recent years owing to their 2D layered structures with various functionalities. To open a new application field for MXenes in the realm of electronic devices, such as ultrahigh-integrated magnetic memory, a spin-orbit torque (SOT) bilayer structure with MXene of Cr₂N is developed: substrate//Cr₂N/[Co/Pt]₃/MgO using the magnetron sputtering technique. Field-free current-induced magnetization switching in the bilayer structure is demonstrated, 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. 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>m</mml:mi> <mml:mi>Cr</mml:mi> <mml:mrow><mml:mi>UC</mml:mi> <mml:mo>.</mml:mo></mml:mrow> </mml:msubsup> <mml:annotation>$m_{{\mathrm{Cr}}}^{{\mathrm{UC}}.}$</mml:annotation></mml:semantics> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>m</mml:mi> <mml:mi>Cr</mml:mi> <mml:mrow><mml:mi>UC</mml:mi> <mml:mo>.</mml:mo></mml:mrow> </mml:msubsup> <mml:annotation>$m_{{\mathrm{Cr}}}^{{\mathrm{UC}}.}$</mml:annotation></mml:semantics> </mml:math> 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.