  Serial connection of multiple memory cells using perpendicular magnetic
tunnel junctions (pMTJ) is proposed as a way to increase magnetic random access
memory (MRAM) storage density. Multi-bit storage element is designed using
pMTJs fabricated on a single wafer stack, with a serial connections realized
using top-to-bottom vias. Tunneling magnetoreistance effect above 130%, current
induced magnetization switching in zero external magnetic field and stability
diagram analysis of single, two-bit and three-bit cells are presented together
with thermal stability. The proposed design is easy to manufacture and can lead
to increase capacity of future MRAM devices.
