  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.
