Electric Clock for NanoMagnet Logic Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 72

they work only at temperatures near absolute zero. The same thing applies also
to semiconductor QCA, which use a complex semiconductor structure as the
basic cell. Molecular QCA instead use the charge stored in complex molecules
to represent logic values, they can work at room temperature and at very high
speed. Another possibility is instead to use rectangularly shaped magnets as
the basic cell, as shown in Fig. 1 . If magnet sizes are reduced to the nanoscale
level, around 50-100 nm, they are forced into the single domain condition and
their hysteresis cycle changes as shown in Fig. 1 . The consequence is that, at the
equilibrium, only two states are possible and therefore they can be used to repre-
sent logic values '0' and '1'. This particular QCA implementation is then called
NanoMagnet Logic (NML) [ 8 - 10 ]. The characteristic of NML logic is that devices
work at room temperature and they can be fabricated with currently available
technological process. As a consequence very advanced experimental activity is
going on with it [ 11 - 13 ]. Since NML circuits are based on magnets, they have
some specific advantages over other QCA implementations. They have no sta-
tic power consumption and potentially a very low dynamic power absorption.
They have an high heat and radiation resistance, making them ideal for hard
environment applications. Finally, due to their magnetic nature, they combine
logic and memory in the same device, opening up completely new possibilities
in the development of logic circuits. Their only disadvantage is that they work
at relatively low speed, around 50-100 MHz [ 14 ].

M/Mmax

M/Mmax = −1 M/Mmax = +1

+1

H

Logic 0 Logic 1

−1

Fig. 1. Reducing the size of a rectangular shaped magnet at nanoscale level
(50-100 nm) force it in the single domain condition. Their hysteresis cycle changes
and at the equilibrium only two states are possible, thus representing the logic values
'0' and '1'.

In NML technology, as it happens in general for QCA, circuits are built plac-
ing cells on the same plane. Information propagation and logic computation are
caused by magnetic interaction between neighbor cells. For example in Fig. 2 (A)
a NML wire is shown. Magnets are aligned horizontally and they align antifer-
romagnetically to reach the minimum energy state. Since every magnet is in the
opposite state of its neighbors, an inverter can be simply built making a wire
with an odd number of magnets (Fig. 2 (B)). The basic logic gate available on this
technology is the majority voter, shown in Fig. 2 (C). It is a three inputs gate
where the value of the central magnet is equal to the majority of the inputs.
While this gate coupled with the inverter allows to design any kind of logic