Superconducting Technology Assessment - nitrd

SMT MRAM
Another direct selection scheme makes use of the recently observed interaction between a spin-polarized current
and the bit magnetization, through so-called spin-momentum transfer (SMT). If a spin-polarized current is incident
on a magnetic bit, the spin of the current-carrying spin-polarized electrons will experience a torque trying to align
the electrons with their new magnetic environment inside the bit. By conservation of angular momentum, the
spin-polarized current will also exert a torque on the bit magnetization. Above a critical current density, typically
10 7 to 10 8 A/cm 2 , the current can switch the magnetic polarization by spin-transfer. SMT switching is most effective
for bit diameters less than ~ 300 nm and gains importance as IC geometries continue to shrink.
Figure 2. Schematic of a proposed SMT MRAM cell structure showing the common current path for sense and program operations.
Successful development of this architecture would result in a high-density, low power MRAM.
The main advantages of SMT switching are lower switching currents, improved write selectivity, and bits highly stable
against thermal agitation and external field disturbances. However, SMT research is currently focused on
understanding and controlling the fundamentals of spin-transfer switching. Several significant challenges remain
before defining a product based on this approach. One such challenge is making a cell with a reasonable output
signal level, as observations of spin-transfer so far have been confined to GMR systems, which have lower
resistance, MR, and operating voltages compared to magnetic tunnel junctions. However, MTJ materials have very
recently been identified with MR > 100% at very low resistances, and MR > 200% with moderate resistances. Such
materials could potentially improve both the signal from SMT devices and lower the minimum currents needed for
switching. Ongoing R&D in this area is directed at further decreasing the minimum switching currents and
establishing materials with the necessary reproducibility and reliability for the low-resistance range. The outcome
of these R&D activities will determine which SMT architectures can be used and at what lithography node. If switching
currents are driven low enough for a minimum-size transistor to supply the switching current, this technology will
meet or exceed the density of the established semiconductor memories, while providing high speed and
non-volatility. Although SMT technology is less developed than MTJ MRAM is, it has the potential for higher density
and orders-of-magnitude reduction in power dissipation
We note here that for the specific case of MRAM integrated with RSFQ electronics, the low resistance of the
all-metal GMR devices (instead of the MTJ) may be beneficial, as described in the follow-on section Integrated
MRAM and RSFQ Electronics.
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DC
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Free Magnet
Tunnel Barrier
Fixed Magnet
Isolation Transistor