Direct visualization of current induced spin accumulation in topological insulators

Abstract: Charge-to-spin conversion in various material systems is the key for the fundamental understanding of spin-orbitronis as well as the development of efficient means to manipulate the magnetization. We report the direct spatial imaging of current induced spin accumulation at the channel edges of Bi2Se3 and BiSbTeSe2 topological insulators by a scanning photovoltage microscope at room temperature. The spin polarization is along the out-of-plane direction with opposite signs for the two channel edges. The accumulated spin direction reverses sign upon changing the current direction and the detected spin signal shows a linear dependence on the magnitude of currents, which indicates that our observed phenomena are the current induced effects. The spin Hall angle of Bi2Se3 and BiSbTeSe2 is determined to be 0.0085 and 0.0616, respectively. We further image the current induced spin accumulation in a Pt heavy metal. Our results open up the possibility of optically detecting the current induced spin accumulations, and thus point towards a better understanding of the interaction between spins and circularly polarized light.

• The paper demonstrates direct current-induced spin visualization in topological insulators using a scanning photovoltage microscope at room temperature.
• It quantifies spin Hall angles (0.0085 in Bi2Se3 and 0.0616 in BiSbTeSe2) without ferromagnet layers, confirming effective charge-to-spin conversion.
• The study measures distinct spin lifetimes (~3.3 ps for Bi2Se3 and ~18.6 ps for BiSbTeSe2), advancing the understanding of spin dynamics in these materials.

Direct Visualization of Current Induced Spin Accumulation in Topological Insulators

This paper presents a significant advancement in the understanding of spintronic phenomena by employing a scanning photovoltage microscope to directly visualize current-induced spin accumulation in topological insulators (TIs) and heavy metals. Conducted at room temperature, this research investigates the role of charge-to-spin conversion, which is critical for both the contemporary understanding of spin-orbitronics and the development of mechanisms to manipulate spin and magnetization efficiently.

Key Findings

1. Spin Polarization in Topological Insulators: The study successfully visualizes the spin accumulation at the edges of Bi$_2$Se$_3$ and BiSbTeSe$_2$ TIs. The spin polarization is demonstrated to be out-of-plane with opposite signs on different edges. Importantly, the spin direction switches with a change in current direction and shows a linear relationship with current magnitude, confirming these phenomena are current-induced effects.
2. Quantification of Spin Hall Angle: The researchers determined the spin Hall angle of Bi$_2$Se$_3$ to be 0.0085 and 0.0616 for BiSbTeSe$_2$. Notably, these values are derived without a ferromagnet (FM) layer, circumventing possible confounding factors from FM/TI interfaces.
3. Spin Accumulation in Heavy Metals: The research extends the methodology to platinum (Pt), a heavy metal, to evaluate current-induced spin accumulation, noting observable phenomena similar to those in TIs. However, larger bias currents are required due to the high concentration of background electrons.
4. Spin Lifetime and Magnetic Field Dependence: Spin lifetime is measured using Hanle precession methods, revealing values such as ~3.3 picoseconds for Bi$_2$Se$_3$ and significantly higher ~18.6 picoseconds for BiSbTeSe$_2$. These experiments bolster the understanding of spin dynamics in TIs.

Implications and Future Directions

The implication of this work is twofold. Practically, the ability to optically detect spin accumulation in TIs and metals opens new avenues for designing spintronic devices free from FM layers, advertising enhanced interface transparency and reduced current shunting issues. This technology can aid in isolating intrinsic material properties like the spin Hall angle and spin lifetime, which are pivotal in optimizing spintronic devices.

Theoretically, the establishment of optical techniques for spin detection enriches the toolkit available for studying spin-charge conversion mechanisms. The study of spin-to-charge conversion without FM layers implies a clearer understanding of intrinsic TI properties, as opposed to systems where FM layers may obscure intrinsic effects.

Future research will likely focus on refining the optical detection methods and applying this technology to a broader range of materials. Expanding the scope to other TIs and heavy metals can help standardize measurements of spin-related parameters, further solidifying the understanding of spin-orbit interactions. Additionally, studies might explore lower-dimensional heterostructures or interfaces where the unique properties of TIs can be harnessed for targeted spintronic applications, potentially leading to breakthroughs in quantum computing technologies or high-efficiency data storage and processing solutions.