  We propose a new type of multi-bit and energy-efficient magnetic memory based
on current-driven, field-free, and highly controlled domain wall motion. A
meandering domain wall channel with precisely interspersed pinning regions
provides the multi-bit capability of a magnetic tunnel junction. The magnetic
free layer of the memory device has perpendicular magnetic anisotropy and
interfacial Dzyaloshinskii-Moriya interaction, so that spin-orbit torques
induce efficient domain wall motion. Using micromagnetic simulations, we find
two pinning mechanisms that lead to different cell designs: two-way switching
and four-way switching. The memory cell design choices and the physics behind
these pinning mechanisms are discussed in detail. Furthermore, we show that
switching reliability and speed may be significantly improved by replacing the
ferromagnetic free layer with a synthetic antiferromagnetic layer. Switching
behavior and material choices will be discussed for the two implementations.
