Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques (1709.02159v1)

Abstract: Topological insulators (TIs) with spin momentum locked topological surface states (TSS) are expected to exhibit a giant spin-orbit torque (SOT) in the TI/ferromagnet systems. To date, the TI SOT driven magnetization switching is solely reported in a Cr doped TI at 1.9 K. Here, we directly show giant SOT driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge to spin conversion efficiency of ~1-1.75 in the thin TI films, where the TSS is dominant. In addition, we find the current density required for the magnetization switching is extremely low, ~6x10^5 A cm-2, which is one to two orders of magnitude smaller than that with heavy metals. Our demonstration of room temperature magnetization switching of a conventional 3d ferromagnet using Bi2Se3 may lead to potential innovations in TI based spintronic applications.

• The paper demonstrates efficient room-temperature magnetization switching in Bi₂Se₃/NiFe heterostructures enabled by high spin-orbit torque efficiency (~1–1.75) and a low switching current density (~6×10⁵ A/cm²).
• The experiments use MBE-grown films and techniques like MOKE microscopy and ST-FMR to map charge-to-spin conversion and verify the dominance of topological surface states.
• The study paves the way for scalable, low-power spintronic devices by utilizing room temperature performance of topological insulators to overcome previous limitations.

Room Temperature Magnetization Switching in Topological Insulator-Ferromagnet Heterostructures by Spin-Orbit Torques

This paper investigates the phenomenon of magnetization switching at room temperature using topological insulator (TI)/ferromagnet heterostructures, specifically focusing on Bi$_2$Se$_3$/NiFe systems. The research provides robust evidence for significant spin-orbit torque (SOT) efficiency resulting from the topological surface states (TSS) in these systems. This development is noteworthy as it surpasses the previously reported low-temperature (1.9 K) magnetization switching in a chromine-doped TI system, expanding potential applications at room temperature for TI-based spintronic devices.

The primary achievement of this paper is the demonstration of room temperature magnetization switching driven by SOT in Bi$_2$Se$_3$/NiFe heterostructures with high charge-to-spin conversion efficiency. A remarkable aspect of the work is the significantly reduced current density required for magnetization switching—approximately $6 \times 10^5$ A cm$^{-2}$—which is one to two orders of magnitude lower than that typically necessary in systems with heavy metals. This highlights the potential for low-power spintronic applications.

The paper elucidates that the large SOT efficiency (~1 to 1.75) attained in thin Bi$_2$Se$_3$ films (in the range of 5-10 quintuple layers) is primarily due to the dominance of TSS over other states like bulk states (BS) and the two-dimensional electron gas (2DEG). The observed SOT efficiency contrasts with the variability (0.01-3.5) previously documented in similar Bi$_2$Se$_3$/ferromagnet configurations, which underscores the importance of minimizing the contribution of BS and 2DEG for optimal performance. Furthermore, the research demonstrates the magnetization switching using a magneto-optic Kerr effect (MOKE) microscope, validating the capability of TIs for robust room temperature applications.

High-quality Bi$_2$Se$_3$ films were fabricated using molecular beam epitaxy (MBE), with the thickness of the films carefully controlled to optimize the dominance of TSS. Through various measurements such as atomic-force microscopy (AFM), four-probe, Hall, and spin torque ferromagnetic resonance (ST-FMR), the paper constructs a detailed mapping of charge-to-spin conversion efficiency, stressing the critical role of TSS. Analytical models were employed to further dissect the individual contributions of TSS, BS, and 2DEG, effectively corroborating the experimental results.

The implications of this research are far-reaching for the field of spintronics. The realization of efficient, room temperature magnetization switching in TI/FM heterostructures without the need for an external magnetic field assists in overcoming significant barriers related to scalability and integration into existing semiconductor technologies. It marks an important step towards developing energy-efficient, spin-based memory and logic devices.

As we project into the future of AI and quantum computing, the innovations captured in this paper showcase the potential of exploiting quantum material properties for advanced electronic applications. In the quest for more compact and efficient devices, the methodologies and findings presented in this work could inspire further exploration of other TIs and ferromagnet combinations, potentially leading to new paradigms in the design of next-generation computational and data storage technologies.