2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO) The International Conference Centre Birmingham 20-23 August 20112, Birmingham, United Kingdom
Hybrid spintronic/straintronics: A super energy efficient computing scheme based on interacting multiferroic nanomagnets Jayasimha Atulasimhai, and Supriyo Bandyopadhya/ IDepartment of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA, 2Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284USA Email:
[email protected],
[email protected]
Abstract
-
nanomagnets
We have theoretically shown that multiferroic
employing currents to switch magnets either by generating a
(consisting
magnetic field or spin transfer torque.
of
a
piezoelectric
and
a
L
magnetostrictive layer) could be used to perform computing while dissipating - few 100 kT/bit at clock rates of -IGHz [1,2,3]. They can act as memory elements [2], binary logic gates [3, 4] and associative memory for four state logic [5, 6]. The latter enables signal processing functions such as ultrafast image reconstruction and pattern recognition [7]. This talk will provide an overview of our research in modeling stress induced nanoscale magnetization dynamics, its application to ultra low
The central idea in our approach is to replace an ordinary magnet
with
a
2-phase
multiferroic
and
rotate
its
magnetization through a large angle with a tiny voltage of10 mV at room temperature [1, 2]. A schematic of a multiferroic nanomagnet is shown in Fig. 1 and consists of a shape-anisotropic structure (an elliptical cylinder) made up
and
of a piezoelectric layer elastically coupled with an overlying
information processing, and discuss preliminary experimental
magnetostrictive layer. Magnetization is rotated by applying
energy
hybrid
spintronic/straintronics
memory
work in fabrication and experimental demonstration of these
an electric field perpendicular to the heterointerface between
devices.
the piezoelectric and magnetostrictive layer. This field
Index Terms - straintronics, nanomagnetic logic (NML).
spintronics,
multiferroic,
Excessive energy dissipation in CMOS devices during switching is the primary threat to continued downscaling of devices
in
accordance
in
the
mechanically
I. INTRODUCTION
computing
strains the former layer and ultimately generates uniaxial stress
with
Moore's
magnetostrictive restrained
from
layer
if
the
expanding
or
layers
are
contracting
along the minor axis of the ellipse. This stress causes the magnetization rotation.
law.
Furthermore, this large power consumption imposes severe constrains on the lifespan of battery powered stand-alone high
density
Application
Specific
Integrated
Circuits
(ASICs) for mobile devices (cell phones, hand held and medically implanted processors).
Fig. 1: A 2-phase multiferroic magnet II. HYBRID STRAINTRONIC/SPINTRONIC DEVICES
In the quest for alternatives to traditional transistor based electronics,
nanomagnet-based
computing
and
signal
processing are emerging as and attractive alternative since: (i) nanomagnets are intrinsically more energy-efficient than transistors due to the correlated switching of spins, and (ii) unlike transistors, magnets suffer from no leakage and hence have
no
standby
nanomagnetic technology
logic
because
power (NML) the
dissipation. has
methods
not
displace
employed
However, transistor to
switch
magnets have been extremely inefficient We have devised a new nanomagnet switching scheme that we have termed hybrid spintronics and straintronics since it is predicated on coupling between magnetic and mechanical degrees of freedom. NML paradigms based on this approach turns out to be roughly four orders of magnitude more energy-efficient than modern day CMOS and possibly five orders more energy-efficient than other NML paradigms
This presentation will summarize our current research on devices
based
on
the
new
hybrid
spintronics
and
straintronics switching scheme described above. We will show rigorously using theoretical simulations that: This
work
is
supported
by
the
US
National
Science
Foundation under the SHF-Small grant CCF-1216614, NEB 2020
grant
ECCS-1l24714
and
by
the
Research Corporation (SRC) under NRI Task
Semiconductor 2203.001.
I The "all-spin-Iogic" paradigm [So Srinivasan, et aI., IEEE Trans. Magn., Vol. 47, 4026 (2011)] uses current induced spin transfer torque, but is equally energy-efficient as the scheme proposed here owing to very low resistance in the paths of the currents.
I. The magnetization state of an isolated single domain
nonmagnetic element shown in Fig
I can be switched
reliably [2] with a stress cycle, even in the presence of thermal noise [8] . Thus, it can function as a memory element which can be switched at
�
I GHz rate at room
temperature with >99.99% reliability while dissipating
�
100
kT [9] . 2. A logic bit can be propagated unidirectionally in a nanomagnetic wire using Bennett clocking as shown in Fig 2 (left), while dissipating clocking speed
�
I GHz
�
[3].
few 100 kT/bit/magnet at a
A NAND gate with fan-out as
shown in Fig 2 (right), with a throughput of 0.5 GHz and latency 4ns can be implemented [4] while dissipating
�
1000
kT/bit for the entire sub-circuit.
Fig. 4: Clockwise from top left: Scanning electron micrographs of nickel nanodots on Si substrate, magnetic force micrographs showing single domain states, scanning electron micrograph of arrays of nickel nanodots for logic chains, atomic force micrographs showing the absence of islanding in the nanomagnets. ACKNOWLEDGMENT The authors' students Noel D'Souza, Mohammed Salehi Fashami and Kuntal Roy have contributed to the work
Fig. 2: Bennett clocking scheme for propagating a bit (left All multiferroic NAND gate fan-out scheme (right).
discussed here. REFERENCES
Biaxial
3.
magnetocrystalline
anisotropy
in
crystalline
[I)
magnets can be exploited to implement four state logic circuits [5]. A scheme has been devised for propagating 4state logic bits [6] unidirectionally between stages, and a 4-
[2]
state NOR gate [5] has been designed as well, as shown in Fig
3.
Such a 4-state element can be used as associative
[3]
memory for ultra-fast image reconstruction [7]. [4] 111"1 IHI",
11"°1
�:� .. i .. , Ii"".
liilll
Ls:::.-,'.
1",ul
[5]
[6]
[7]
Fig 3: Four-state element with biaxial magnetocrystalline anisotropy (top). Four state logic propagation and logic gate (bottom). 4. Finally, we will present preliminary experimental work to fabricate
�
100
nm
diameter
elliptical
single-domain
nanomagnets using e-beam lithography as shown in Fig 4.
[8]
[9]
J. Atulasimha and S. Bandyopadhyay, "Bennett clocking of nanomagnetic logic using multiferroic single-domain nanomagnets", Appl. Phys. Lett.. 97, 173105,20I0 K. Roy, S. Bandyopadhyay and 1. Atulasimha, "Hybrid spintronics and straintronics: A magnetic technology for ultra low energy computing and signal processing",Appl. Phys. Lett., 99, 063108,20II. M. S. Fashami, K.Roy, J. Atulasimha and S. Bandyopadhyay, "Magnetization dynamics, Bennett clocking and associated energy dissipation in multiferroic logic",Nanotechnology, 22, 155201,2011. M. S. Fashami, J. Atulasimha and S. Bandyopadhyay, "Magnetization dynamics,throughput and energy dissipation in a universal multiferroic nanomagnetic logic gate with fan-in and fan-out",Nanotechnology, 23, 105201,2012. . N. D'Souza, J, Atulasimha and S. Bandyopadhyay' "Four-state nanomagnetic logic using multiferroics", J. Phys. D: App. Phys., 44 265001,2011. N. D'Souza, J. Atulasimha and S. Bandyopadhyay, "An energy efficient Bennett clocking scheme for 4-state multiferroic logic", IEEE Trans on Nanotechnology, 11,418,2012. N. D'Souza, J. Atulasimha and S. Bandyopadhyay, "An ultrafast energy-efficient image reconstructor implemented with nanomagnets possessing biaxial magnetocrystalline anisotropy", arXiv: II09.6932vl, in press, IEEE Trans on Nanotechnology. K. Roy, S. Bandyopadhyay and J. Atulasimha, "Error-resilient switching of a bistable switch without introducing asymmetry in its potential profile",arXiv: 1II1.5390v I. K. Roy, S. Bandyopadhyay and J. Atulasimha, "Energy dissipation and switching delay in stress-induced switching of multiferroic devices in the presence of therrnal fluctuations",arXiv: 11 11.6129v I,in-press J. ofApp. Phys.
We will also discuss preliminary work on fabrication of multiferroic nanomagnetic logic devices and studying their stress induced switching behavior with magnetic force microscopy.
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