Spin-orbit-torque magnetization switching of a three terminal perpendicular magnetic tunnel junction

Abstract: We report on the current-induced magnetization switching of a three-terminal perpendicular magnetic tunnel junction by spin-orbit torque and the read-out using the tunnelling magnetoresistance (TMR) effect. The device is composed of a perpendicular Ta/FeCoB/MgO/FeCoB stack on top of a Ta current line. The magnetization of the bottom FeCoB layer can be switched reproducibly by the injection of current pulses with density $5\times10^{11}$ A/m$^2$ in the Ta layer in the presence of an in-plane bias magnetic field, leading to the full-scale change of the TMR signal. Our work demonstrates the proof of concept of a perpendicular spin-orbit torque magnetic memory cell.

• The paper introduces a novel SOT-MRAM design that decouples read/write paths, reducing power consumption and enhancing device endurance.
• It presents experimental evidence of a significant TMR change (up to 55%–90%) triggered by 50 ns current pulses.
• The study confirms that critical switching current scales with electrode area, aligning with theoretical predictions in optimized Ta/FeCoB/MgO/FeCoB stacks.

Overview of Spin-orbit Torque Magnetization Switching in Three-terminal Perpendicular Magnetic Tunnel Junctions

This paper investigates the magnetization switching of a three-terminal perpendicular magnetic tunnel junction (p-MTJ) utilizing spin-orbit torque (SOT). The study demonstrates a proof of concept for a novel non-volatile SOT-MRAM memory cell, highlighting the potential to overcome current inefficiencies in traditional STT-MRAM, notably related to power consumption, speed, and scalability.

Key Findings and Methodology

The authors describe a three-terminal device comprising a perpendicular Ta/FeCoB/MgO/FeCoB stack placed above a Ta current line. The configuration allows for the magnetization of the bottom FeCoB layer to be switched by injecting current pulses at a density of $5 \times 10^{11} \text{A/m}^2$ into the Ta layer, with an in-plane bias magnetic field. This switch generates a significant change in the tunneling magnetoresistance (TMR) signal, showcasing an effective mechanism for data storage and retrieval in MRAM devices.

The empirical analysis was facilitated by fabricating circular MTJ dots atop Ta electrodes using magnetron sputtering and various lithography techniques. Their dimensions ranged from 200 nm to 1 µm, with the critical current dependency observed to scale linearly with the cross-sectional area of the bottom electrode rather than its size alone. The researchers also observed that at high pulse amplitudes and biases, the switching current density showed minimal size dependency, corroborating theoretical predictions regarding domain wall motion at this scale.

Numerical Results

The experiments revealed a TMR signal change of up to 55%, with some samples achieving TMR values as high as 90%. This switching mechanism was activated by short current pulses of 50 ns, pointing to the potential for ultra-fast write operations. For various device sizes, the critical switching current densities were $j \approx 5 \times 10^{11} \text{A/m}^2$ and showed consistency across different dimensions within the tested range, demonstrating the robustness of the approach.

Implications and Future Directions

The research posits that the use of SOT for MRAM applications provides significant advantages over conventional STT-MRAM technologies, namely decoupled read/write paths. This decoupling is predicted to enhance device endurance by isolating the high-current paths required for write operations from the read circuitry. Additionally, the independence of TMR performance from writing constraints allows for optimized material configurations, addressing longstanding challenges in low RA and high TMR combinations.

Moving forward, the development of such SOT-MRAM structures could lead to adopting smaller and more efficient memory devices capable of operating at reduced power levels with enhanced speed. Further investigations into the interplay between material science advancements and spin-orbit interactions are necessary to refine the efficacy of SOT-MRAM. The adaptation of substrates and the variability of current pulses remain areas ripe for detailed exploration to maximize the potential of spintronic memory solutions.

In conclusion, this paper provides a significant step towards realizing non-volatile, energy-efficient, and high-speed memory technologies. The practical implications for computing architecture are profound, particularly in addressing ongoing challenges related to power dissipation in modern microprocessors. As the field advances, the exploration of alternative heavy metals and substrate materials could further revolutionize memory storage capabilities through enhanced spintronic mechanisms.