Bias-field-free spin Hall nano-oscillators with an out-of-plane precession mode

Nakano

Shirokura

et al. 2020

Self Cite

The large spin Hall effect in topological insulators (TIs) is very attractive for ultralow-power spintronic devices. However, evaluation of the spin Hall angle and spin-orbit torque (SOT) of TIs is usually performed on high-quality single-crystalline TI thin films grown on dedicated III-V semiconductor substrates. Here, we report on room-temperature ultralow power SOT magnetization switching of a ferrimagnetic layer by non-epitaxial BiSb TI thin films deposited on Si/SiO 2 substrates. We show that non-epitaxial BiSb thin films outperform heavy metals and other epitaxial TI thin films in terms of the effective spin Hall angle and switching current density by one to nearly two orders of magnitude. The critical SOT switching current density in BiSb is as low as 7 × 10 4 A/cm 2 at room temperature. The robustness of BiSb against crystal defects demonstrate its potential applications to SOT-based spintronic devices. Charge-to-spin conversion utilizing the strong spin-orbit coupling (SOC) in non-magnetic materials has become a very attractive concept with possible applications to various spintronic devices, such as spin-orbit torque (SOT) magnetoresistive random access memories (MRAM) 1 , racetrack memories 2 , and spin torque nano-oscillators 3,4. These SOT-based spintronic devices are superior to their spin-transfer torque (STT)-based counterparts in terms of driving current, speed, and long-term durability 5,6. In SOT-based devices, a perpendicular pure spin current density J s is generated by an in-plane charge current density J e in the non-magnetic layer through the spin Hall effect (SHE) 7-9 , whose charge-to-spin conversion efficiency is characterized by the spin Hall angle θ SH = (2e/ℏ) J s /J e. Thus, finding spin Hall materials with large θ SH and high electrical conductivity is crucial for SOT applications, and there have been huge efforts so far to achieve that goal. In the well-studied heavy metals (HMs) such as Pt 10-13 , Ta 14 , and W 15-17 , θ SH is of the order of ~ 0.1, and the typical critical switching current density J c in bilayers of heavy metals/ferromagnet with perpendicular magnetic anisotropy is typically of the order of 10 7 A/cm 2 for continuous direct currents (DC) and 10 8 A/cm 2 for nano-second (ns) pulse currents. The large switching current density requires large driving transistors, whose size limits the bit density of SOT-MRAM. Meanwhile, large θ SH (> 1) have been observed in topological insulators (TIs) 18,19 thanks to their strong SOC and Dirac-point-driven singularity of the Berry phase on their topologically protected surface states 20. Thus, significant reduction of the driving current density from 10 7-10 8 A/cm 2 to 10 5-10 6 A/cm 2 can be expected for SOT-based devices 21-25 , particularly in SOT-MRAM whose their large writing current density is the major obstacle for reducing the writing power consumption and increasing the bit density. However, evaluation of θ SH and SOT switching by TIs is usually performed on single-crystalline TI thin films deposited on dedicated III-V...

Section: Introductionmentioning

confidence: 99%

Ultralow power spin–orbit torque magnetization switching induced by a non-epitaxial topological insulator on Si substrates

Nakano

Shirokura

et al. 2020

Self Cite

“…The small switching current density and switching power also help suppress failure of the spin Hall layer due to electromigration and Joule heating 42 . Our results demonstrate the feasibility of BiSb for not only ultralow power SOT-MRAM but also other SOT-based spintronic devices, such as race-track memories 43 and spin Hall oscillators 44 , 45 .…”

Section: Sot Performance Benchmarking and Future Prospectivementioning

confidence: 68%

“…Our results demonstrate the feasibility of BiSb for not only ultralow power SOT-MRAM but also other SOT-based spintronic devices, such as race-track memories 32 and spin Hall oscillators. 33,34…”

Section: Sot Performance Benchmarking and Conclusionmentioning

confidence: 99%

Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers

Fan

Nakano

et al. 2022

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Spin orbit torque (SOT) magnetization switching of ferromagnets with large perpendicular magnetic anisotropy has a great potential for the next generation non-volatile magnetoresistive random-access memory (MRAM). It requires a high performance pure spin current source with a large spin Hall angle and high electrical conductivity, which can be fabricated by a mass production technique. In this work, we demonstrate ultrahigh efficient and robust SOT magnetization switching in fully sputtered BiSb topological insulator and perpendicularly magnetized Co/Pt multilayers. Despite fabricated by the magnetron sputtering instead of the laboratory molecular beam epitaxy, the topological insulator layer, BiSb, shows a large spin Hall angle of θSH = 10.7 and high electrical conductivity of σ = 1.5 × 105 Ω−1 m−1. Our results demonstrate the feasibility of BiSb topological insulator for implementation of ultralow power SOT-MRAM and other SOT-based spintronic devices.

“…Pure spin current induced by the spin Hall effect in materials with strong spin–orbit coupling is a promising magnetization manipulation method for various next-generation spintronic devices, such as spin–orbit torque magnetoresistive random-access memories (SOT-MRAM) 1 , 2 , race track memories 3 , and spin Hall oscillators 4 , 5 . There are two families of materials that are currently studied for efficient spin current sources: heavy metals (HMs) and topological insulators (TIs).…”

Section: Introductionmentioning

confidence: 99%

Efficient spin current source using a half-Heusler alloy topological semimetal with back end of line compatibility

Shirokura

Fan

et al. 2022

Self Cite

Topological materials, such as topological insulators (TIs), have great potential for ultralow power spintronic devices, thanks to their giant spin Hall effect. However, the giant spin Hall angle (θSH > 1) is limited to a few chalcogenide TIs with toxic elements and low melting points, making them challenging for device integration during the silicon Back-End-of-Line (BEOL) process. Here, we show that by using a half-Heusler alloy topological semi-metal (HHA-TSM), YPtBi, it is possible to achieve both a giant θSH up to 4.1 and a high thermal budget up to 600 °C. We demonstrate magnetization switching of a CoPt thin film using the giant spin Hall effect of YPtBi by current densities lower than those of heavy metals by one order of magnitude. Since HHA-TSM includes a group of three-element topological materials with great flexibility, our work opens the door to the third-generation spin Hall materials with both high θSH and high compatibility with the BEOL process that would be easily adopted by the industry.