ToPoliNano: NanoMagnet Logic Circuits Design and Simulation - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 272

Fig. 1. Single domain nanomagnets are used as basic cells. At the equilibrium only two
stable states are possible.
to reach the minimum energy state. The alignment is antiferromagnetic (every
element is in the opposite state of its neighbors) if magnets are aligned horizon-
tally, while the alignment is ferromagnetic (every element is in the same state
of its neighbors) if magnets are aligned vertically [ 17 ]. However, the magnetic
field generated by a magnet is not sucient to cause a state alteration in its
neighbors. To switch magnets from one state to the other it is necessary to use
a mechanism called clock [ 18 ]. The behavior is depicted in Fig. 2 . Magnets are
forced in an unstable (RESET) state through an external mean, like a magnetic
field [ 19 , 20 ]. When the magnetic field is removed magnets realign with a domino-
like effect following the input element. With this mechanism signals propagate
correctly through the circuit. As well as a magnetic field, other systems can be
used to clock circuits, like STT-current coupling [ 21 ] or an electric field [ 22 ].
Fig. 2. Clocking mechanism for NML logic. Magnets are forced in an intermediate
unstable state through an external mean, like a magnetic field that in a particular
portion of time reaches a maximum appropriate value.
The RESET state is unstable. If too many magnets are cascaded some of
them along the chain will switch in the wrong state due to external influences,
like thermal noise [ 23 , 24 ]. To have a safe signal propagation no more than 5
magnets should be cascaded [ 23 ]. As a consequence a multiphase clock system
is required. Circuits are divided in small areas, called clock zones. Each zone is
made by a limited number of magnets. Every clock zone requires the application
of a different clock signal, like shown in Fig. 3 (A) where three clock signals with
a phase difference of 120 ⓦ are used. As depicted in Fig. 3 (B), when magnets of
a clock zone are in the SWITCH state (the magnetic field is slowly removed)