Magnetization alignment in spin-transfer-torque magnetic random-access memory

TL;DR

This study systematically analyzes how interlayer exchange coupling and asymmetry affect magnetic states in nanoscale p-STT-MRAM, providing insights for device stability and design.

Contribution

It offers a comprehensive micromagnetic phase diagram and publicly releases a large simulation dataset for various device configurations.

Findings

Asymmetry reduces the coupling strength needed for antiparallel SAF states.

Increasing SAF asymmetry raises SAF reversal barriers but lowers free-layer barriers.

Stray fields significantly influence energy barriers and magnetic stability.

Abstract

Reliable operation of perpendicular spin-transfer-torque magnetic random-access memory (p-STT-MRAM) requires control of magnetic alignment within the synthetic antiferromagnet (SAF) reference layer. At nanopillar dimensions, however, devices can exhibit magnetic states that are absent in extended thin films. We present a systematic micromagnetic study of 30 nm-diameter three-layer p-STT-MRAM nanopillars using experimentally motivated material parameters, and map equilibrium states as functions of bilinear and biquadratic interlayer exchange coupling. Phase diagrams show that introducing asymmetry between the SAF layers in saturation magnetization, anisotropy, and thickness reduces the coupling strength required to stabilize antiparallel SAF states and suppress competing configurations. Minimum-energy path calculations show that, for noncollinear antiparallel SAF states, increasing SAF asymmetry can raise SAF reversal barriers while lowering the free-layer barrier; this trade-off is absent for collinear antiparallel SAF states. Stray fields also significantly modify both SAF and free-layer energy barriers. To support the design of p-STT-MRAM devices with either collinear or noncollinear antiparallel SAF reference states, we publicly release the simulation dataset covering 4374 distinct device configurations.