Keywords:SOT-MRAM, Topological insulator, Nickel oxide
Topological insulators (TIs) are quantum materials with insulating bulk and topological surface states. They are attractive spin current sources for spin-orbit-torque (SOT) MRAM application thanks to their very large spin Hall angle. Among TIs, BiSb attracts great interest as BiSb shows both high electrical conductivity and giant spin Hall angle in epitaxial BiSb(012) thin films (θSH ~ 52, σ ~ 2.5×105) . However, in SOT-MRAM using BiSb, it is important to suppress Sb diffusion toward the magnetic free layer (FM) that may destroy its magnetic properties and reducing the spin injection efficiency. In this work, to resolve such problems, we introduce a NiO interface layer at the BiSb/FM interface. We prepared sapphire substrate/BiSb(10 nm)/NiO (0.5,1.0,1.5 nm)/Co (1 nm)/Pt(1 nm) multilayers by sputtering deposition, and evaluated SOT by the second harmonic Hall measurements. Figure 1 shows the high field second harmonic Hall resistance of a 100 mm ´ 25 mm Hall bar device for sample with NiO thickness of tNiO = 1.5 nm. A very strong signal was observed. Figure 2 shows the extracted antidampling-like field from the second harmonic Hall data as a function of BiSb current density. From these data, we estimate the effect spin Hall angle for this sample is as large as 9.1. Figure 3 shows as a function of tNiO. is moderate for tNiO = 0 - 0.5 nm, but rapidly increases at tNiO = 1.0 - 1.5 nm, demonstrating that NiO can enhance the spin injection efficiency from BiSb to Co. Furthermore, we observe a rapid decrease of the effective anisotropy magnetic field of the Co layer at increasing tNiO, indicating the existence of perpendicular magnetic anisotropy at the NiO/Co interface. Our results show that BiSb/NiO is very attractive for a highly efficient and practical spin current source in SOT-MRAM.
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