Boarino, L.

Celegato, F.

Chiolerio, Alessandro

Di Crescenzo, L.

Enrico, E.

Graziano, Mariagrazia

Lamberti, Andrea

Tiberto, P.

Vacca, Marco

Zamboni, Maurizio

English

PORTO Publications Open Repository TOrino

Electric clock for NanoMagnet Logic Circuits
M. Vaccaa, L. Di Crescenzoa, M. Grazianoa, M. Zambonia, A. Chioleriob, A. Lambertib, E. Enricoc,
F. Celegatoc, P. Tibertoc, L. Boarinoc
aDepartment of Electronics and Telecommunications, Politecnico di Torino, Italy
bIstituto Italiano di Tecnologia (IIT), Italy
cIstituto Nazionale per la Ricerca Metrologica (INRIM), Italy
Email: marco.vacca@polito.it, mariagrazia.graziano@polito.it, maurizio.zamboni@polito.it
Abstract—Among the Field-Coupled technologies NanoMagnet
Logic is one of the most promising. The low power consumption
coupled with the possibility to combine memory and logic in the
same device, makes this technology the ideal candidate for low
power and portable applications. However, the necessity to use
an external magnetic field to locally control the circuit represents
the weakest point of this technology. The high power losses in
the clock generation system wipes out most of the advantages of
this technology.
In this work we propose a clock system based on a piezoelectric
actuator that allows to electrically control NanoMagnet Logic
circuits. The low power consumption coupled with the fact that
electric fields are easier to generate at nanoscale makes this clock
system a competitive candidate as final clocking mechanism for
this technology. Moreover the solution here proposed is feasible
with current technology, and this represents a strong key point
of the proposed solution.
I. INTRODUCTION
In recent years the interest in Field-Coupled devices, like
NanoMagnet Logic, is rapidly increasing [1]. In this logic
single domain nanomagnets are used as base cell. Due to par-
ticular magnetic properties, like shape or magnetocrystalline
anisotropy, magnets can have only two stable states that are
used to represent the logic values ’0’ and ’1’ [2]. Circuits
are built placing many cells near each others, and informa-
tion propagates through magnetostatic coupling of neighbor
cells. A clock mechanism is required to correctly propagate
information through the circuit [3]. Magnets are forced in
an intermediate unstable state applying an external magnetic
field. When this magnetic field is removed magnets realign
themselves following the input element. Since the number of
magnets that can be cascaded is limited [4], a multiphase clock
system is necessary to build working circuits. Three clock
signals (Figure 1.A) with a phase difference of 120 degrees
are applied to small areas of the circuits called clock zones
composed by a limited number of elements. With this system
in every instant when magnets of a clock zone are in the
SWITCH state (Figure 1.B), magnets on their left are in the
HOLD state and act like an input and magnets on their right
are in the RESET state and have no influence. In this way
signals propagate in a specific direction [5].
Magnetic field is generated through a wire placed under
the magnets plane [6], however the required value of current
is quite high and the associated power consumption wipes
out the major advantage of this technology. Other solutions
uses multilayered magnets clocked using spin-torque coupling
CLOCK
PHASE 3
PHASE 2
CLOCK
PHASE 1
CLOCK
1 2 3
HOLD SWITCHRESET
RESET HOLDSWITCH
HOLD SWITCH RESET
(3)
(2)
(1)
FIELD
ELECTRIC
PZT NANOMAGNETS
ELECTRODES
V
A) B)
C) D)
Fig. 1. A) 3-phase clock signals. B) Signal propagation during the clock
period. C) Electric clock mechanism, when an electric field is applied the
strain induced by the piezoelectric layer rotates the magnetization vector of
90 degrees.
through a current that flows through the magnets [7] or
multiferroic structure where the magnets is composed by a
piezoelectric layer and a magnetostrictive magnetic layer that
can be clocked through an electric field [8]. Both solutions
show remarkably low power consumption compared to the
classic approach. We propose a solution similar to [8], with the
difference that magnets do not have a multilayered structure:
They are made by only one magnetic material and are de-
posited directly on a piezoelectric layer (Figure 1.C). When an
electric field is applied magnets are forced in the RESET state
by the strain induced by the piezoelectric layer (Figure 1.D).
While renouncing at some performance compared to [8], this
structure as the advantage that can be fabricated with available
technology.
II. RESULTS
The proposed circuit structure is shown in Figure 3. Single
logic gates on mechanically separated islands, obtained by
patterning with optical lithography a piezoelectric layer (PZT),
are used to build circuits. The basic logic gate is a 3-
magnets AND/OR gate obtained changing the shape of the
IN2
IN1
IN3
OUT1
OUT2
Fig. 2. Example of circuit layout using the proposed clock.
central (darker) element [9]. Since the horizontal coupling
in a chain of magnets is antiferromagnetic, every magnet
has the inverted value with respect to its neighbors. As a
consequence, if there is an odd number of magnets in a clock
zone the logic function is inverted and NAND/NOR gates are
obtained. NAND/NOR are universal gates that can be used to
build any circuit. Magnets can be made by IronTerbium with
sizes of 65x50x10 nm3 or Nichel with sizes of 55x50x10
nm
3
. Electrodes are buried under the PZT which can have
a thickness in the range of 70-600nm. Magnets cannot be
placed directly above the electrodes because in those areas
the electric field is near 0. Shielding blocks are used to
avoid errors in the vertical signal propagation [10]. Every
NAND/NOR is in an independent clock zone, and can have a
width of 3 or 5 magnets. The smaller the structure is the lower
energy is required to reset the magnets, as can be seen from
Figure 3, where the switch energy of a gate for different gate
sizes, composition and oxide thickness is shown. Circuits are
simulated using the free tool Magpar [11]. Simulation shows
that the energy consumption, considering all the source losses,
is 1000 times lower than the classic magnetic field approach.
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 0
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 0  50  100  150  200  250  300  350  400
En
er
gy
 (K
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PZT thickness (nm)
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Ni 3x3
Ni 5x3
Fig. 3. Energy comparison with different material and sizes.
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Electric clock for Nano-magnet Logic Circuits

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