Enhancement of Out‐of‐Plane Spin–Orbit Torque by Interfacial Modification

Abstract

SOT deterministic switching of perpendicular magnetization requires an external in-plane magnetic field to break the mirror symmetry in the current-magnetization plane. This extra magnetic field is not favored for the scalability and integration of the devices. Various efforts have been made to break the mirror symmetry by extra design of the device structures, such as a wedge shape of ferromagnetic layer, [4] titled anisotropy, [5,6] additional magnetic layer with in-plane shape anisotropy, [7,8] exchange bias by adding antiferromagnetic layer, [9,10] and spin current gradient along the current direction. [11,12] The chiral symmetry breaking by a gradient of magnetic anisotropy or saturation magnetization can also induce field-free switching. [13,14] In order to increase the switching efficiency it is common to search for the spin source layer with a high spin Hall angle to increase the anti-dampinglike torque-induced longitudinal effective field. It still remains elusive for the increase of the effective out-of-plane (OOP) field in those structures since the breaking mirror symmetry is only a qualitative prerequisite. Recent works reported that the fieldfree switching can be realized by utilizing the intrinsic properties of the material such as the crystal symmetry in WTe 2 /FM Spin-orbit torque (SOT)-induced switching of perpendicular magnetization in the absence of magnetic field is crucial for the application of SOT-based spintronic devices. Recent works have demonstrated that the low-symmetry crystal structure in CuPt/CoPt can give rise to an out-of-plane (OOP) spin torque and lead to deterministic magnetization switching without an external field. However, it is essential to improve OOP effective field for the efficient switching. In this work, the impact of interface oxidation on the generation of OOP effective field in a CuPt/ferromagnet heterostructure is systematically studied. By introducing an oxidized CuPt surface, it is found that the field-free switching performance shows remarkable improvement. OOP effective field measurement indicates that the oxidation treatment can enhance the OOP effective field by more than two times. It is also demonstrated that this oxidation-induced OOP SOT efficiency enhancement is independent of the device shapes, magnetic materials, or magnetization easy axis. This work contributes to improve the performance of SOT devices and provides an effective fabrication guidance for future spintronic devices that utilize OOP SOT.

The central role of tilted anisotropy for field-free spin–orbit torque switching of perpendicular magnetization

“…Additionally, by using a two-step fabrication process, we identified that the oblique angle deposition of the bottom Pt layer, while compared to other layers, plays a more decisive role in realizing field-free currentinduced switching (see Supporting Information S5). Such two-step fabricated samples might also experience some level of interfacial oxidation, which can potentially enhance the overall DL-SOT efficiency 37,38 .…”

Engineering Symmetry Breaking Enables Efficient Bulk Spin‐Orbit Torque‐Driven Perpendicular Magnetization Switching

“…[1][2][3][4][5][6][7] The SOT-driven perpendicular magnetization switching is usually studied in nonmagnetic/ferromagnetic (NM/FM) bilayers. [8,9] For the purpose of making SOT switching a feasible technology, the SOT-based devices are required to show high thermal stability and low switching current density (J c ) simultaneously. [10,11] However, the conventional SOT generated from the NM layer, is an interface effect in nature due to the exponential attenuation of spin current and short spin coherence length, that is, J c is proportional with FM thickness t FM .…”

Reduction of Operating Current by Harnessing the Field‐ and Damping‐Like Torque Ratios in Nonmagnet–Ferromagnet Heterojunctions

“…The SOT consists of two mutually orthogonal vector components: damping-like torque (DLT, τ DLT ¼ m  ð m  ŝÞ) and field-like torque (FLT, τ FLT ¼ m  ŝ), where m and ŝ are unit vectors of magnetization and spin polarization. [9][10][11][12][13][14][15][16][17][18] This study explores the potential of SOT devices with perpendicular magnetic anisotropy (PMA) as a foundation for true random number generators (TRNGs) based on stochastic physics. The DLT aligns the magnetization along the y-direction.…”

“…The SOT consists of two mutually orthogonal vector components: damping‐like torque (DLT, ) and field‐like torque (FLT, ), where and are unit vectors of magnetization and spin polarization. [ 9–18 ]…”