A Perspective on Electrical Generation of Spin Current for Magnetic Random Access Memories

TL;DR

This paper reviews charge-to-spin conversion mechanisms for MRAM, emphasizing the development of efficient channel materials and measurement techniques to enable high-performance, scalable spintronic memory devices.

Contribution

It highlights the importance of developing advanced channel materials with higher charge-to-spin conversion efficiency and discusses key metrics for optimizing MRAM device performance.

Findings

Current MRAM devices operate near maximum charge-to-spin efficiency.

Spin-orbit interactions can generate large spin currents in 3-terminal devices.

Identifies critical research needs for material measurement and optimization.

Abstract

Spin currents are used to write information in magnetic random access memory (MRAM) devices by switching the magnetization direction of one of the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) nanopillar. Different physical mechanisms of conversion of charge current to spin current can be used in 2-terminal and 3-terminal device geometries. In 2-terminal devices, charge-to-spin conversion occurs by spin filtering in the MTJ's ferromagnetic electrodes and present day MRAM devices operate near the theoretically expected maximum charge-to-spin conversion efficiency. In 3-terminal devices, spin-orbit interactions in a channel material can also be used to generate large spin currents. In this perspective article, we discuss charge-to-spin conversion processes that can satisfy the requirements of MRAM technology. We emphasize the need to develop channel materials with larger charge-to-spin conversion efficiency -- that can equal or exceed that produced by spin filtering -- and spin currents with a spin polarization component perpendicular to the channel interface. This would enable high-performance devices based on sub-20 nm diameter perpendicularly magnetized MTJ nanopillars without need of a symmetry breaking field. We also discuss MRAM characteristics essential for CMOS integration. Finally, we identify critical research needs for charge-to-spin conversion measurements and metrics that can be used to optimize device channel materials and interface properties prior to full MTJ nanopillar device fabrication and characterization.