Unveiling the Mechanism of Bulk Spin‐Orbit Torques within Chemically Disordered Fe_xPt_1‐x Single Layers

Abstract

The recent discovery of spin‐orbit torques (SOTs) within magnetic single‐layers has attracted attention. However, it remains elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered FexPt1‐x, the mechanism of the “unexpected” bulk SOTs is unveiled by studying their dependence on the introduction of a controlled vertical composition gradient and temperature. The bulk dampinglike SOT is found to arise from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient. The SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which is analog to the strain behaviors. These characteristics suggest that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non‐zero net torque is most likely a strain non‐uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This study points to the possibility of developing low‐power single‐layer SOT devices by strain engineering.

Current-induced perpendicular effective magnetic field in magnetic heterostructures

“…2(d)), excluding any relevance of the 𝐻 𝑧 eff to thermal effects. Note that the "hidden" vertical symmetry breaking that leads to bulk SOT of σy in ferromagnets 21,26 and ferrimagnets 27 cannot explain the 𝐻 𝑧 eff because the latter requires a lateral rather than vertical asymmetry. As shown in Fig.…”

Enhancement in Interfacial Dzyaloshinskii–Moriya Interaction in Pt/CoFe(B)/MgO Structures by Suppression of FePt Interface Phases with the Addition of Boron

“…As a result, the fcc (111) peaks of Pt, CoFe, and disordered FePt (a.k.a. A 1 structure), and the peaks of rock-salt MgO are clearly identified from the obtained spectra. – Notably, the actual chemical formula of the alloy formed at the interface between the CoFe­(B) and Pt layers is probably Fe­(Co)­Pt; however, the alloy is expressed as FePt for concise notation, since the CoFe used in this study is iron rich (10:90). With increasing boron concentration, each peak position and area (or intensity) change, and particularly remarkable changes occur in the peaks of FePt (111), MgO (002) and CoFe (111), as shown in Figure b–d.…”

Switching of Perpendicular Magnetization by Spin–Orbit Torque

“…Exerting a nonzero damping-like spin-orbit torque on a magnetic layer typically requires an external transverse spin current, while an in-plane charge current usually cannot generate any total spin-orbit torque or switching behaviors within centrosymmetric magnetic single layers (e.g., polycrystalline 3d FeCoB [149,150] and Co [151] ). Recently, experiments have established that a strong bulk-type net damping-like spin-orbit torque (Figure 15a) can be generated within single-crystalline GaMnAs with bulk inversion asymmetry [46] and some thick, strong spin-orbit coupling magnetic single layers, e.g., Co1-xPtx, [52,114,115] Fe1-xPtx, [53,120,121] Fe1-xTbx, [54] Co1-xTbx, [122,152] GdFeCo, [116,153] and L10-FeCrPt. [154] Switching of perpendicular magnetization by such bulk SOTs has been demonstrated.…”