Spin torque switching with the giant spin Hall effect of tantalum

Abstract: We report a giant spin Hall effect (SHE) in {\beta}-Ta that generates spin currents intense enough to induce efficient spin-transfer-torque switching of ferromagnets, thereby providing a new approach for controlling magnetic devices that can be superior to existing technologies. We quantify this SHE by three independent methods and demonstrate spin-torque (ST) switching of both out-of-plane and in-plane magnetized layers. We implement a three-terminal device that utilizes current passing through a low impedance Ta-ferromagnet bilayer to effect switching of a nanomagnet, with a higher-impedance magnetic tunnel junction for read-out. The efficiency and reliability of this device, together with its simplicity of fabrication, suggest that this three-terminal SHE-ST design can eliminate the main obstacles currently impeding the development of magnetic memory and non-volatile spin logic technologies.

• The paper quantifies β-Ta's giant spin Hall angle (0.12–0.15), surpassing values reported for Pt and highlighting its efficiency in spin-torque switching.
• It employs innovative three-terminal CoFeB/Ta device geometries that separate low-impedance switching from high-impedance read-out, enhancing device reliability.
• The results challenge previous measurements and pave the way for optimized nanomagnetic devices in advanced memory and logic applications.

Spin Torque Switching with the Giant Spin Hall Effect of Tantalum

The study of spin-transfer-torque (STT) mechanisms has advanced significantly with the identification of a giant spin Hall effect (SHE) in β-Ta, enabling efficient spin-torque switching in magnetic devices. This paper presents the quantification of the SHE in tantalum through three distinct techniques, demonstrating its ability to produce a significant spin current capable of reversing the magnetization in adjacent ferromagnetic layers. The pivotal finding in this research is the spin Hall angle $\theta_{\text{SH}}$ for tantalum, determined to be 0.12 - 0.15, surpassing previously observed values in other materials like Pt, which was reported around 0.07 to 0.1.

The experiment utilizes thin film bilayers of CoFeB/Ta, implementing a three-terminal device geometry that greatly mitigates the challenges observed in traditional two-terminal magnetic tunnel junctions (MTJs). In these devices, a low-impedance Ta-ferromagnet bilayer is used for inducing magnetization switching, and a high-impedance magnetic tunnel junction is utilized for read-out. This design not only ensures efficiency and simplicity in fabrication but also significantly enhances device reliability. The study's results suggest that such three-terminal SHE-ST devices could effectively supplant existing magnetic memory technologies by overcoming current limitations related to reliability and signal levels.

The research challenges previous understandings of spin currents generated in non-magnetic materials through the SHE, previously explored primarily with Pt and Au. The high resistance of β-Ta, when sputtered onto amorphous surfaces, facilitates a large spin Hall angle contrary to some earlier experimental results, such as that of Morota et al., which reported a low spin Hall angle due to non-local spin valve measurement techniques. The authors effectively counter these discrepancies by applying a more accurate analysis of charge flow in resistive spin Hall materials.

By integrating the SHE induced ferromagnetic resonance (ST-FMR) technique, the researchers successfully isolate and measure the distinct contributions of the SHE from β-Ta. The synchronized spin current creates a spin-torque that is both robust and distinct in sign compared to Pt, offering unique advantages for anti-damping STT mechanisms. Critically, the SHE-ST approach induces efficient current-driven switching for both in-plane and out-of-plane magnetized samples, establishing a versatile platform for the development of spin logic technologies.

The research further explores the practical implications of this discovery by creating an MTJ-based three-terminal device for in-plane magnetized nanomagnets. Notably, the study addresses concerns around the increased magnetic damping from non-magnetic layers, showing that the β-Ta does not significantly enhance energy dissipation, which is a distinct advantage over Pt-based devices. This point aligns with prior work indicating differences in damping based on the engineering of material interfaces and scattering time ratios.

Although the study showcases pioneering experimental work demonstrating remarkable spin-transfer efficiencies, significant work lies ahead in optimizing these devices for practical applications. Potential improvements include further reductions in switching currents and enhancing MTJ sensor efficiency—transforming the landscape of low-power, high-density memory technologies. The pursuit of materials with even larger spin Hall angles than β-Ta might enhance this promise further.

Ultimately, this paper lays a robust framework for future advancements in nanomagnetic device architectures, signaling a critical stride towards more efficient and reliable spintronics applications. As researchers continue to explore the potential of tantalum and its associated nanoscale magnetic phenomena, advancements in SHE-ST device fabrication are poised to redefine the parameters of magnetic memory and logic applications.