Tongji researchers discovered a new type of bulk spin torque and its mediated magnetization switching in L10 FePt single layer. L10 FePt is one of the most important magnetic materials in holding great prospects for new generation spin storage and logic applications, and the demonstration of its spin torque effect goes beyond the present framework of spin-orbit and spin-transfer torques and potentially brings a paradigm-shift of spintronics development. These discoveries will push forward the frontier of spintronics and will have a transformative impact on magnetic storage and spin memory devices with simple architecture, ultrahigh density, and readily application.
Spintronics is an active research field whose aim is to exploit the spin degree of electrons to build new generation electronic devices. Recently, spin-orbit torque (SOT) emerges as the most efficient means for electrical operation of spintronic devices. However, SOT has faced several fundamental obstacles. Firstly, SOT has been mostly restricted in nonmagnetic metal/ferromagnet heterostructures and therefore it requires 3-terminal device architecture which is not favored in circuit design. Secondly, the interface nature of the SOT in heterostructures requires a thin ferromagnet which hinders thermal stability and nonvolatility.
Figure 1 (a) A schematic of spin torque effect in a L10 FePt single layer. (b) A TEM image of a 12-nm L10 FePt single layer.
Figure 2 (a) A schematic for measuring the SOT-driven magnetization switching. (b) RH versus pulsed DC current I. (c) Kerr images recorded for states 1–5, as indicated in (b). SOT effective fields HL (d) and HT (e), and switching efficiency ηSOT (f) as a function of the L10 FePt thickness t.
To remove these fundamental obstacles for SOT applications, the research group led by professor Xuepeng Qiu creatively proposed to explore SOT in the L10 FePt single layer, which simultaneously possesses strong magnetization and large spin-orbit coupling (Figure 1a). Epitaxial L10 FePt single-layer thin films were prepared by UHV sputtering tool, and then SOT effect was detected by both electrical Hall measurements and magneto optical Kerr effect (MOKE) imaging. Their experiment results reveal that intriguing SOT indeed exists in the L10 FePt single layer, and it can be utilized to electrically reverse the magnetization of the FePt (Figure 2a-c). More importantly, the SOT in L10 FePt shows bulk characteristic, i.e. SOT increases with the film thickness (Figure 2d-f). These results report bulk SOT and its driven magnetization switching in a single-layer film with strong perpendicular magnetic anisotropy for the first time, which shows great potential for removing above obstacles and brings new perspectives for SOT research and applications.
The emergence of spin-orbit-mediated torque in the L10 FePt single layer is intriguing, extending beyond the established theory framework of SOT. The researchers have devoted great efforts to elucidate the origin of the SOT in the L10 FePt single layer. After excluding several elements of symmetry breaking, such as antiphase boundary, current-induced temperature gradient, in-plane magnetization component, and surface Pt segregation, they finally attributed the SOT to the inherent structural gradient in the L10 FePt single layer, manifested as a composition gradient along the film normal direction. Then, the authors managed to reverse the structural gradient in an exquisitely designed L10 FePt single layer, and observed an opposite SOT effect, confirming the decisive role of the structural gradient in generating SOT in the L10 FePt single layer.
The authors further carried out a quantum transport calculation according to the experimental results. They theoretically built a ferromagnetic slab within the tight-binding approximation and computed its current-driven SOT using Kubo formula. The results show that both the two torque components are zero without structural gradient, and the torques increase almost linearly as a function of the structural gradient, thus indicating that a reasonable structural gradient can maintain a substantial torque throughout the volume of an otherwise centrosymmetric ferromagnet.
The results are published in Advanced Materials entitled “Bulk Spin Torque-Driven Perpendicular Magnetization Switching in L10 FePt Single Layer”. PhD student Meng Tang from Tongji University is the first author of the paper. Professor Xuepeng Qiu from Tongji University and professor Aurelien Manchon from King Abdullah University of Science and Technology (KAUST) and Aix-Marseille Université are the corresponding authors.
Paper link: https://doi.org/10.1002/adma.202002607