Superconducting Technology Assessment - nitrd

SMT MRAM
SMT MRAM has only one read/write current path and can yield much higher densities than FS-TMR. A nanomagnetic
bit has recently been demonstrated. Cornell University, NIST, and Freescale have demonstrated that pulse
current densities of 10 7 -10 8 A/cm 2 can rotate the magnetic polarization at 40 K and switching times decrease from
5 ns to ~600 ps at high pulse amplitudes. SMT switching is most effective for single magnetic domains, ideal for
bits smaller than ~300 nm. Present SMT structures rely on GMR with low MR, but higher MR materials that will
reduce the switching current and time have been identified. A 2-Ω, 90-nm device was simulated to switch at 0.1
mA, equivalent to a voltage bias of 0.2 mV, and a simulated device switched in 50 ps.
SMT memory may be more compatible with RSFQ logic than FS devices, but it is even more embryonic and
considerable effort will be required to optimize the materials and cell architectures. It should be emphasized, however,
that the opportunity afforded by combining MRAM with superconductive circuits would remain unexplored unless
it is funded by government investment for this ultra-high performance computing niche.
MRAM Roadmap
The panel considered a three-step sequence to implement MRAM for cryogenic operation, concluding with monolithic
RSFQ-MRAM chips at 4 K.
Table 3.2-8. THREE-STEP PROCESS TO IMPLEMENT MRAM FOR CRYOGENIC OPERATION
Step
FS-TMR MRAM chips
at 40 to 70 K
Action
Reduce the resistance of the wiring and increase the signal-to-noise
ratio. As a result, one may be able to implement a very large
memory in the cryostat, alleviating some burden on the I/O link
between 4 K and ambient.
SC-MRAM at 4 K
Replace all MRAM wiring with superconductors and operate
at 4 K, eliminating the dominant ohmic loss in the wiring and
further improving signal-to-noise ratio.
RSFQ MRAM at 4 K
Replace all CMOS circuits in SC-MRAM with RSFQ by monolithically
integrating RSFQ circuitry with MRAM cells, producing dense,
low-latency, low-power RSFQ-MRAM chips that can be placed
adjacent to the RSFQ processors.
62