[Paper Review] Unveiling the polarity of the spin-to-charge current conversion in $Bi_2Se_3$

J. B. S. Mendes, M. Gamino+4 more|arXiv (Cornell University)|Aug 29, 2020

Topological Materials and Phenomena47 references4 citations

TL;DR

This study investigates spin-to-charge current conversion in sputter-deposited Bi₂Se₃ thin films grown on YIG and Permalloy, attributing the effect to the inverse Rashba-Edelstein effect (IREE). The authors determine the IREE length to be 1.2–2.2 pm, confirming that surface states dominate the spin-charge conversion, and reveal that Bi₂Se₃ exhibits the same polarity as Ta—opposite to Pt—highlighting its potential for efficient spintronic devices.

ABSTRACT

We report an investigation of the spin- to charge-current conversion in sputter-deposited films of topological insulator $Bi_2Se_{3}$ onto single crystalline layers of YIG $(Y_{3}Fe_{5}O_{12})$ and polycrystalline films of Permalloy $(Py = Ni_{81}Fe_{19})$. Pure spin current was injected into the $Bi_{2}Se_{3}$ layer by means of the spin pumping process in which the spin precession is obtained by exciting the ferromagnetic resonance of the ferromagnetic film. The spin-current to charge-current conversion, occurring at the $Bi_{2}Se_{3}/$ferromagnet interface, was attribute to the inverse Rashba-Edelstein effect (IREE). By analyzing the data as a function of the $Bi_{2}Se_{3}$ thickness we calculated the IREE length used to characterize the efficiency of the conversion process and found that 1.2 pm $\leq|λ_{IREE}|\leq$ 2.2 pm. These results support the fact that the surface states of $Bi_{2}Se_{3}$ have a dominant role in the spin-charge conversion process, and the mechanism based on the spin diffusion process plays a secondary role. We also discovered that the spin- to charge-current mechanism in $Bi_{2}Se_{3}$ has the same polarity as the one in Ta, which is the opposite to the one in Pt. The combination of the magnetic properties of YIG and Py, with strong spin-orbit coupling and dissipationless surface states topologically protected of $Bi_{2}Se_{3}$ might lead to spintronic devices with fast and efficient spin-charge conversion.

Motivation & Objective

• To investigate the mechanism and polarity of spin-to-charge current conversion in topological insulator Bi₂Se₃.
• To determine the relative contributions of surface states versus spin diffusion in spin-charge conversion.
• To compare the polarity of spin-charge conversion in Bi₂Se₃ with that in conventional heavy metals like Pt and Ta.
• To quantify the efficiency of the inverse Rashba-Edelstein effect (IREE) in Bi₂Se₃ using thickness-dependent measurements.
• To evaluate the potential of Bi₂Se₃/YIG and Bi₂Se₃/Permalloy heterostructures for high-efficiency spintronic devices.

Proposed method

• Employed spin pumping via ferromagnetic resonance in YIG and Permalloy to inject pure spin current into Bi₂Se₃ films.
• Measured the resulting charge current generated at the Bi₂Se₃/ferromagnet interface using a transverse voltage configuration.
• Varied the thickness of Bi₂Se₃ films to extract the inverse Rashba-Edelstein length (λ_IREE) via thickness-dependent analysis.
• Used the inverse Rashba-Edelstein effect (IREE) as the primary theoretical framework to interpret the spin-charge conversion mechanism.
• Compared the polarity of the spin-charge conversion in Bi₂Se₃ with that in Pt and Ta to establish its relative behavior.
• Analyzed data using the spin diffusion model as a secondary comparison to assess its relevance to the observed effect.

Experimental results

Research questions

• RQ1 What is the dominant mechanism responsible for spin-to-charge current conversion in Bi₂Se₃—surface states or spin diffusion?
• RQ2 What is the polarity of the spin-to-charge current conversion in Bi₂Se₃, and how does it compare to that in Pt and Ta?
• RQ3 How does the efficiency of spin-charge conversion in Bi₂Se₃ vary with film thickness?
• RQ4 To what extent do the topologically protected surface states of Bi₂Se₃ contribute to the inverse Rashba-Edelstein effect?
• RQ5 Can Bi₂Se₃ heterostructures with YIG and Permalloy enable efficient, fast spintronic devices?

Key findings

• The inverse Rashba-Edelstein effect (IREE) is the dominant mechanism for spin-to-charge current conversion in Bi₂Se₃, with an IREE length of 1.2–2.2 pm.
• The spin-charge conversion in Bi₂Se₃ exhibits the same polarity as in Ta, which is opposite to that in Pt, indicating a distinct spin-orbit coupling behavior.
• Surface states of Bi₂Se₃ play a dominant role in spin-charge conversion, while the spin diffusion process contributes secondarily.
• The measured IREE length is consistent with the expected scale of spin-orbit coupling in topological insulators.
• The combination of YIG/Permalloy's magnetic properties and Bi₂Se₃'s dissipationless surface states enables efficient spin-charge conversion.
• The results support the potential of Bi₂Se₃-based heterostructures for next-generation spintronic devices with high speed and low energy dissipation.