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Magnets
Ferrite
Yoke
H
H
H
t
Copper
I
B)
A)
Wire
Copper Wires
(Bit and Source lines)
Magnetic Material
I
I
I
I
I
I
I
I
Piezoelectric Material
C)
D)
Fig. 5. NML clock systems. (A) Magnetic field clock. A magnetic field is generated
by a current flowing through a wire placed under the magnets plane. (B) Out-of-plane
NML clock. In this particular implementation of NML, the magnetization of each
dots points out-of-plane. In this case the clock is an oscillating magnetic field applied
perpendicularly to the plane. No clock zones are present because clock field is applied
uniformly to the entire circuit. (C) STT-current clock. Spin-Torque coupling due to a
current flowing through the magnets, which in this case are MTJ junctions. (D) Strain
based clock. Magnets are forced in the RESET state through a mechanical deformation.
called Out-of-Plane NML [ 27 ], where magnets are made by Cobalt-Platinum
structures and the magnetization lies perpendicularly to the plane. In this case
the magnetic field is perpendicular to the plane (Fig. 5 (B)). A different app-
roach uses instead STT-coupling with a current flowing through the magnets as
clock mechanism (Fig. 5 (C)). The basic element in this case is a Magneto-Tunnel
Junction (MTJ) and wires are used to contact them and to drive the current
[ 28 , 29 ]. This solution as the remarkable advantage that is based on the Magnetic
RAM memories structure, a technology that is already commercially available,
moreover the value of clock current required is lower than in the magnetic field
approach. Unfortunately all current based clock systems have a relatively high
power consumption, mainly due to the Joule losses in the clock wires. A differ-
ent solution was proposed in [ 30 , 31 ] and it is shown in Fig. 5 (D). Magnets are
multilayer structures made by a piezoelectric layer and a magnetic layer, and
they can be forced in the RESET state applying a proper electric field to them.
The use of an electric field allows an important reduction of the losses in the
clock generation system obtaining therefore true low power operations.
 
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