High Charge-to-Spin and Spin-to-Charge Conversion Enhanced by Quantum Confinement Effect in Sputtered Topological Insulator Thin Films

Abstract

The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and fast magnetic-based storage and computational devices. The SOT switching of magnetization can be used in the SOT-memory and computational devices whereas the spin-to-charge conversion can be utilized for reading of magnetization state in computational devices. Recent reports on topological insulators show high SOT but the industry compatible growth process is still lacking. Furthermore, SOT switching of perpendicular magnetization from topological insulators is demonstrated but still with large current density and large external field. We investigated the SOT from magnetron-sputtered bismuth selenide thin films in BixSe(1-x)/Co20Fe60B20 heterostructures by using dc planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency ( ) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08, using the dc planar Hall and ST-FMR methods, respectively. Moreover, switching of perpendicular CoFeB multilayers using SOT from the BixSe(1-x) has been observed at room temperature (RT) with low critical magnetization switching current density ( ) 4.3 × 105 A/cm2. Quantum transport simulations using realistic sp3 tight binding model suggests that the high SOT in sputtered BixSe(1-x) is due to a quantum confinement effect, whose charge-to-spin conversion efficiency enhances with reduced size and dimensionality. The demonstrated , ease of growth of the films on a silicon substrate, and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1-x) film provide an avenue for the use of bismuth selenide thin films as a spin-density generator in SOT-based memory and logic devices. In addition to charge-to-spin conversion, we also performed spin-to-charge conversion by sputtered bismuth selenide thin films. For the spin-to-charge conversion experiment, we prepared Sub/Si/SiO2/Bi43Se57/Co20Fe60B20 heterostructures with in-plane magnetization. High spin-to-charge conversion voltage signals have been observed at room temperature. The spin-pumping voltage decreases with an increase in the size of the grains. The figure-of-merit of spin-to-charge conversion inverse Edelstein effect length ( ) is estimated to be as large as 0.32 nm. The large is due to the spin-momentum locking and is further enhanced by quantum confinement in the nano sized grains of the sputtered bismuth selenide films. We also investigated the effect on spin-pumping voltage due to the insertion of layers MgO and Ag. The MgO insertion layer has almost completely suppressed the spin-pumping voltage whereas Ag insertion layer has enhanced the spin-pumping voltage as large as 40%. The suppression of spin-pumping voltage due to the insertion of insulating layer indicates that the thermal effects are negligible in the spin-pumping signal. The enhancement of spin-to-charge conversion voltage by insertion Ag layer is due to the Rashba-Edelstein effect. Moreover, the conducting ferromagnetic layer can influence both SOT and spin-to-charge conversion voltage. We investigated spin-to-charge conversion in sputtered Y3Fe5O12(YIG)/BS bi-layers at room temperature. The spin current is pumped to the BS layer by the precession of magnetization at ferromagnetic resonance in the YIG layer. is estimated to be as large as (0.11 ± 0.03) nm in YIG/BS (4 nm). Moreover, also shows a dependence on the bismuth selenide film thickness in YIG/BS structure, which is consistent with the spin-to-charge conversion in conducting ferromagnet and also in case of charge-to-spin conversion.