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Scaling TMR reader down to sub 20 nm for magnetic storage technology beyond 4 Tb/in2 is challenging, due to its complex film stack and increasing noise at smaller size. Recently, a spin-orbit torque (SOT) reader based on the inverse spin Hall effect was proposed as an alternative candidate 1. The SOT reader consists of a SOT layer and a ferromagnetic layer (FM). A spin-polarized current is injected from the FM layer to the SOT layer, which generates an output voltage via the inverse spin Hall effect. However, the output voltage and signal-to-noise ratio (SNR) of SOT reader using traditional heavy metals are insufficient due to the relatively small spin Hall angle (SHA) and low sheet resistance 2. Hence, a SOT material with large SHA is essential to generate a large output and sufficient SNR. The topological insulator (TI) BiSb emerges as potential candidate for SOT reader thanks to its giant SHA 3 and larger sheet resistance.
In this work, we demonstrate a proof-of-concept for a BiSb-based SOT reader with large output voltage and SNR. Figure 1 shows the schematic structure of our device. The SOT reader is a 20 μm × 20 μm pillar of CoFe (5 nm)/MgO (2 nm)/BiSb (10 nm) on top of a 20 nm-thick Ta/Pt bottom current electrode deposited on an oxidized silicon substrate. Then, a top current electrode was deposited on top of the pillar. A perpendicular bias current was applied to the pillar, and the output voltage of the inverse spin Hall effect was read out during sweeping an in-plane magnetic field. Figure 2(a) shows a representative data at Japp = 6.25 kA/cm2 for the inverse spin Hall resistance RISHE = VISHE/Iapp of the reader. Figure 2(b) shows the characteristics of the reader. The output is as large as 15 mV, which is 3 orders of magnitude larger than that of Pt-based devices 2. We project a giant inverse spin Hall angle of 24 for this device, which demonstrates the potential of BiSb for SOT reader application.
References
1 P. A. V. Heijden, Q. Le, K. S. Ho, US patent US9947347B1 (2016).
2 V. T. Pham, I. Groen, S. Manipatruni, Nat. Elec. 3, 309 (2020).
3 N. H. D. Khang, Y. Ueda, and P. N. Hai, Nat. Mater. 17, 808 (2018).
Schematic device structure of our BiSb-based SOT reader.
(a) Inverse spin Hall resistance at Japp = 6.25 kA/cm2. (b) Output voltage at various applied current density.
