The development of electrically controlled nanomagnets for spintronic applications, particularly non-volatile magnetic memories (MRAM), is attracting strong interest due to the limitations of CMOS-based memories such as SRAM and eDRAM. Spin–orbit torque (SOT) MRAMs are promising candidates for addressing SRAM specifications; however, current materials still suffer from limited efficiency and high resistivity, leading to unmet write-current requirements. Recently, studies have highlighted orbital phenomena as a potential route to enhance SOT efficiency, owing to their larger magnitudes and availability in a broader set of materials. However, orbital currents do not couple to magnetization in the absence of spin–orbit coupling, requiring an orbital-to-spin conversion layer, which motivates studies of conversion mechanisms and associated physics.
In this PhD work, we evaluate promising orbital/HM/FM material systems for SOTMRAM applications. We present a comprehensive study of Ru/HM/FeCoB and Ta/W/ FeCoB systems, where Ru and Ta act as orbital current sources, while Ta, W, and Pt serve as orbital-to-spin conversion layers. Ru is predicted to exhibit one of the largest orbital Hall angles among transition metals while maintaining low resistivity. Ta, a heavy metal with a large spin Hall effect, is predicted to exhibit an orbital Hall angle approximately one order of magnitude larger than its spin counterpart. When a heavy metal is used as a conversion layer, multiple spin-current contributions can coexist and add linearly to the total effective spin Hall conductivity, potentially enhancing the overall SOT efficiency.
We characterized key parameters relevant to SOT magnetic tunnel junctions (MTJ) devices, including saturation magnetization, effective anisotropy field, and resistivity, and we quantified damping-like (ξDL) and field-like (ξFL) SOT efficiencies as a function of orbital and conversion layer thickness, both in as-deposited and 300°C annealed samples. These metrics are benchmarked against reference HM/FeCoB systems to isolate the effect of the additional orbital layer. For Ru/Ta and Ru/W stacks, limited enhancement ξFL of ξDL is observed relative to reference systems. In contrast, Ru/Pt exhibits a twofold increase in ξDL compared to Pt alone. This difference is attributed to the stronger SOC in Pt, which enables more efficient orbital-to-spin conversion. The independence of ξDL on Ru thickness further suggests an interfacial origin of the orbital contribution in Ru/Pt. However, thermal annealing strongly degrades ξDL, limiting its applicability for SOT-MRAM. In Ta/W systems, we observe a strong enhancement of ξDL by a factor of 4.4 relative to Ta and 3.2 relative to W. A parallel-resistor model indicates that conventional SHE contributions cannot fully account for this increase, pointing to an additional orbital-related mechanism. Extending the study to 400 °C annealing shows that ξDL remains largely stable, indicating good thermal robustness while maintaining perpendicular magnetic anisotropy.
Leveraging these advantages, we further integrate the Ta/W system into SOT-MTJs and benchmark it against standard W-based MTJs. We investigate the pulse-length dependence of the critical switching current and provide a first demonstration of integrated orbital-to-spin conversion in SOT-MTJs. Ta/W devices exhibit switching currents comparable to W-based devices but have a lower switching current density and improved perpendicular magnetic anisotropy stability. Finally, we present a proof-of-concept for vertical non-local switching of SOT-MTJ using orbital torques, simplifying bottom-pinned SOT-MRAM fabrication. Overall, these results demonstrate that orbital physics can be exploited to enhance SOT-MTJ performance, simplify fabrication, and provide a promising route toward scalable bottom-pinned MRAM technologies.

Plus d'information :https://www.spintec.fr/phd-defense-exploration-of-orbital-to-spin-conversion-materials-and-integration-in-3-terminal-spin-orbit-torque-magnetic-tunnel-junctions/
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Pour suivre la soutenance ​​​en visioconférence : https://univ-grenoble-alpes-fr.zoom.us/j/98769867024?pwd=dXNnT3RMeThjYStybGVQSUN0TVdJdz09​

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