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

Abstract

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...

Magnetic memory driven by topological insulators

“…In order to be compatible with the industry-level manufacture, the topological insulator of BiSb is prepared by the magnetron sputtering method 29 , 46 , and the MTJ stack of Ru(5)/CoFeB(2.5)/MgO(2)/CoFeB(5)/Ta(8)/Ru(7) is in-situ deposited on top of the sputtered BiSb(10) without breaking the vacuum (thickness in nanometers), i.e., all sputtered BiSb-MTJ. The TMR ratio of the all sputtered BiSb-MTJ is around 90%, as shown in Fig.…”

Integration of the Rhombohedral BiSb(0001) Topological Insulator on a Cubic GaAs(001) Substrate

“…A low tilt can be observed (<2°) between the GaAs substrate and the BiSb layer, which corresponds to the blue family in figure 2b. The [110] direction of the GaAs corresponds to the [11][12][13][14][15][16][17][18][19][20] of the BiSb and the c axis is identical for both crystals.…”

Spin‐Orbit Torque in Van der Waals‐Layered Materials and Heterostructures

“…[167] In non-epitaxial Bi 2 Sb 3 /CoTb, the critical current density is as low as 7 × 10 4 A cm −2 (Figure 19m). [168] Recently, interfacial PMA magnetic layer was achieved by a thin insertion layer. Mahendra et al successfully prepared a PMA CoFeB layer by sputtering and demonstrated SOT switching in Bi x Se (1-x) /CoFeB heterostructure (Figure 20a,b).…”

“…Reproduced under the terms of the Creative Commons CC‐BY license. [ 168 ] Copyright 2020, The Authors. Published by Springer Nature.…”

“…[ 167 ] In non‐epitaxial Bi 2 Sb 3 /CoTb, the critical current density is as low as 7 × 10 4 A cm −2 (Figure 19m ). [ 168 ]…”