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

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.

12 pages, 3 figures, 1 table

References in corpus (8)

• Single-shot dynamics of spin-orbit torque and spin transfer torque switching in three-terminal magnetic tunnel junctions
• Maximizing Spin-Orbit Torque Generated by the Spin Hall Effect of Pt
• Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques
• Field-free switching of magnetic tunnel junctions driven by spin-orbit torques at sub-ns timescales
• Lack of simple correlation between switching current density and spin-orbit torque efficiency of perpendicularly magnetized spin-current generator/ferromagnet heterostructures
• Coherent control of nanomagnet dynamics via ultrafast spin torque pulses
• Spin Torque Oscillators with Thermal Noise: A Constant Energy Orbit Approach
• Charge-to-spin conversion efficiency in ferromagnetic nanowires by spin torque ferromagnetic resonance: Reconciling lineshape and linewidth analysis methods