NanoMagnet Logic: An Architectural Level Overview - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 222

the amazing clock speed (1 THz) that can be theoretically reached [ 9 ]. However,
the experimental fabrication is extremely challenging with current technologi-
cal processes. NanoMagnet Logic uses single domain nanomagnets (Fig. 1 (A))
as basic cells [ 10 ]. Due to magnetic properties, like shape or magnetocrystalline
anisotropy, magnets can have only two stable states that are used to represent
logic values '0' and '1'. The main interest around this technology lies in its mag-
netic nature, that allows to potentially mix logic and memory in the same device.
For the same reason circuits built with this technology have an high resistance
to radiations and heat and a power consumption potentially lower than that of
state-of-the-art CMOS circuits [ 11 ].
Fig. 1. (A) Single domain nanomagnets are used as basic cell in NML technology.
Only two stable states are possible and are therefore used to represent digital values
'0' and '1'. A third intermediate state is possible but it is unstable and can therefore
be forced only by external means. (B) To switch magnets from one state to the other a
RESET mechanism is required. Magnets are driven in the RESET state by an external
mechanism, a magnetic field in the most classical implementation. When the magnetic
field is removed, magnets realign following the input element.
Information propagation depends on the magnetostatic interaction among
neighbor magnets. Nonetheless, the magnetic field generated by one magnet is not
strong enough to switch a neighbor element from one stable state to the other. As
a consequence a mechanism called clock must be used to help magnets switching,
forcing them in an intermediate unstable state (RESET in Fig. 1 (A)). An exter-
nal mean (a magnetic field in the most classical implementation [ 12 , 13 ]) must be
used to force magnets in the RESET state (Fig. 1 (B)). When the magnetic field
is removed magnets realign following the input element, thereby propagating the
information through the circuit.
Signals within the circuit propagate with a Domino-like effect. Magnets
switch one by one in a sort of chain reaction. Unfortunately the number of
magnets that switch correctly in a chain is limited by the intrinsic instability of
the RESET state. The RESET state is unstable, so external stimuli (like thermal
noise [ 14 ]) can generate unwanted switches, therefore leading to errors during
signals propagation. To solve this problem a multiphase clock system must be
used [ 15 - 17 ]. Circuits are divided in small areas called clock zones. Every clock
zone is made of a limited number (less than 5 in the ideal case [ 14 ]) of cascaded