Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance (1708.04111v3)

Abstract: Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. However, the major challenge of current-induced switching in the nanopillars with both a large tunnel magnetoresistance ratio and a low junction resistance is still to be met. Here, we report spin transfer torque switching in nano-scale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 {\Omega}.{\mu}m2, which consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient resonant tunnelling transmission induced by the atom-thick W layers could contribute to the larger magnetoresistance ratio than conventional structures with Ta layers, in addition to the robustness of W layers against high temperature diffusion during annealing. The switching critical current density could be lower than 3.0 MA.cm-2 for devices with a 45 nm radius.

• The paper demonstrates that atom-thin tungsten layers significantly enhance spin-transfer torque switching in p-MTJs.
• It employs Cs-corrected TEM and EELS mapping to confirm improved crystallinity and thermal stability at temperatures up to 410°C.
• Findings reveal reduced critical current densities (~3.0 MA/cm²) and high TMR (up to 249%), advancing next-generation STT-MRAM technology.

Current-Induced Magnetization Switching in Atom-Thick Tungsten Engineered Perpendicular Magnetic Tunnel Junctions

This study presents a comprehensive examination of spin-transfer torque (STT) switching in perpendicular magnetic tunnel junctions (p-MTJs) using atom-thin tungsten (W) layers. These junctions exhibit a notable tunnel magnetoresistance (TMR) ratio of up to 249% and a low resistance-area (RA) product of 7.0 Ω·µm^2. The innovative utilization of W layers, in contrast to the traditionally used tantalum (Ta), has been demonstrated to enhance performance characteristics significantly by improving thermal stability and reducing diffusion-related degradation.

Structural and Magnetic Analysis

The paper investigates p-MTJ stacks constructed from variously layered materials including CoFeB, MgO, and critically, atom-thick W layers. A primary advantage of W over Ta is its superior resilience against high-temperature diffusion, which is often encountered during annealing processes used to enhance the crystallinity of MgO barriers. The W-layer enhanced structures exhibit a superior ferromagnetic coupling at a thickness of a single atomic layer, which is pivotal for achieving efficient STT switching.

Using crystallographic analysis methods such as Cs-corrected TEM and EELS mapping, the study verifies that the introduction of W layers helps maintain the structural integrity necessary for robust performance, even under higher thermal conditions. Moreover, the presence of W facilitates a body-centered cubic (bcc) crystal template for adjacent CoFeB layers, enhancing the overall structural and magnetic quality of the p-MTJs.

Spin-Transfer Torque and Thermal Stability

The STT behavior was meticulously analyzed through magnetic field and current sweep experiments. Notably, the p-MTJs with W spacer layers sustain high TMR even after thermal processing at elevated temperatures (up to 410°C). The study reported that critical current densities required for STT switching were considerably lower, at approximately 3.0 MA/cm^2 for devices with a 45 nm radius, which is competitive with p-MTJs using Ta layers.

Theoretical modeling via first-principles calculations corroborated the empirical findings, indicating that atom-thick W layers are instrumental in enhancing resonant tunneling transmission efficiency. This finding aligns with observed TMR values, providing a detailed understanding of the underpinning electronic interactions at the CoFeB/MgO interfaces.

Implications and Future Directions

The implications of this research are significant for the development of next-generation STT-MRAM technologies that require low-power and high-density memory solutions. The results suggest that W serves as a potent replacement for Ta in specific structural configurations, providing pathways for improved scaling below 20 nm, essential for modern electronic applications.

In conclusion, this paper not only elucidates the pivotal role of atom-thick W layers in optimizing p-MTJ performance but also lays the groundwork for further exploration into alternative materials and configurations that may offer even greater enhancements to magnetic storage technologies. Future research could explore the nuanced interactions at the interface level to further harness the potential of resonant tunneling for high-efficiency memory devices.