Hybrid straintronics and spintronics: An ultra energy-efficient paradigm for logic and memory l 2 Supriyo Bandyopadhyay and Jayasimha Atulasimha
' 2 Department of Electrical and Computer Engineering, Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA phone: 1-804-827-6275, facsimile: 1-804-827-0006, e-mail:
[email protected] Excessive energy dissipation during switching of logic and memory bits is the primary impediment to continued downscaling of electronic devices predicted by Moore's law. Nanomagnetic logic and memory switches are innately more energy-efficient than electronic switches because of correlated switching of spins that does not happen when charges are "switched" by moving them into and out of a transistor's channel. Furthermore, magnets do not "leak" unlike transistors. This results in much lower energy dissipation in a nanomagnetic switch compared to an electronic switch. However, this advantage is usually squandered in nanomagnetic logic (NML) paradigms because of very inefficient magnet switching schemes that result in mammoth dissipation in the switching circuit. We have devised a new magnet 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 employing currents to switch magnets either by generating a magnetic field or spin transfer ! torque . 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 of - 10 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 of a piezoelectric layer elastically coupled with an overlying magnetostrictive layer. The two stable magnetization states are along the major axis of the ellipse and encode bits 0 and 1. A voltage applied across the structure generates strain in the piezoelectric that is ° transferred to the magnetostrictive layer and rotates its magnetization by - 90 . Because of coupling between the in-plane and out-of-plane dynamics, if the stress is withdrawn quickly after the magnetization ° ° rotates by _90 , then it continues to rotate further and completes a -180 rotation ("magnetization flip") . Extensive simulations of the switching dynamics in the presence of thermal noise based on the stochastic Landau-Lifshitz-Gilbert (LLG) equations show that the energy dissipated per switching event is less than 1 aJ at -1 GHz clock rate [3]. This makes it one of the most energy-efficient paradigms extant. This talk will describe Bennett clocking of logic chains [1], writing of bits in memory with straintronics with associated energy dissipation [2], design of universal logic gates that perform logic operations while dissipating - 2 aJ of energy at -1 GHz sinusoidal clock rate [4], 4-state logic [5] and image processing based on the latter scheme that can reconstruct a 512x512 pixel grayscale image in - 2 ns [6].
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1.
1. Atulasimha and S. Bandyopadhyay,Appl.
2.
K. Roy,S. Bandyopadhyay and 1. Atulasimha, Appl.
Phys, Lett. Vol. 97,173105,2010. Phys. Lett., Vol. 99, 063108, 2011.
3.
K. Roy,S. Bandyopadhyay,and 1., Atulasimha,arXiv:1111.6129vl
4.
M. S. Fashami, 1.Atulasimha,S.Bandyopadhyay,Nanotechnology, Vol. 23 105201,2012.
5.
N. D'Souza, 1. Atulasimha and S. Bandyopadhyay,J.
6.
N. D'Souza, 1. Atulasimha and S. Bandyopadhyay,arXiv:ll09.6932v1.
Phys.
D:
Appl. Phys"
Vol. 44, 265001,2011.
The "all-spin-Iogic" paradigm [So Srinivasan, et aI., IEEE Trans. Magn., Vol. 47, 4026 (2011)] ], which is a variant of
NML, uses spin polarized current for switching magnets, but is apparently equally energy-efficient as the scheme proposed here.
978-1-4673-1164-9/12/$31.00 ©2012 IEEE
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Fig. 2: Bennett clocking scheme for propagating a bit
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Fig. 3: 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.
978-1-4673-1164-9/12/$31.00 ©2012 IEEE
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