  Due to the ever increasing power and cooling requirements of large-scale
computing and data facilities, there is a worldwide search for low-power
alternatives to CMOS. One approach under consideration is superconducting
computing based on single-flux-quantum logic. Unfortunately, there is not yet a
low-power, high-density superconducting memory technology that is fully
compatible with superconducting logic. We are working toward developing
cryogenic memory based on Josephson junctions that contain two or more
ferromagnetic (F) layers. Such junctions have been demonstrated to be
programmable by changing the relative direction of the F layer magnetizations.
There are at least two different types of such junctions -- those that carry
the innate spin-singlet supercurrent associated with the conventional
superconducting electrodes, and those that convert spin-singlet to spin-triplet
supercurrent in the middle of the device. In this paper we compare the
performance and requirements of the two kinds of junctions. Whereas the
spin-singlet junctions need only two ferromagnetic layers to function, the
spin-triplet junctions require at least three. In the devices demonstrated to
date, the spin-singlet junctions have considerably larger critical current
densities than the spin-triplet devices. On the other hand, the spin-triplet
devices have less stringent constraints on the thicknesses of the F layers,
which might be beneficial in large-scale manufacturing.
