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August 15, 2011

Hybrid spintronics and straintronics: A magnetic technology for ultra low energy computing and signal processing

The magnetization of a 2-phase magnetostrictive/piezoelectric multiferroic single-domain shape-anisotropic nanomagnet can be switched with very small voltages that generate strain in the magnetostrictive layer. This can be the basis of ultralow power computing and signal processing. With appropriate material choice, the energy dissipated per switching event can be reduced to about 45 kT at room temperature for a switching delay of ∼100 ns and about 70 kT for a switching delay of ∼10 ns, if the energy barrier separating the two stable magnetization directions is about 32 kT. Such devices can be powered by harvesting energy exclusively from the environment without the need for a battery.

The proposed design runs on so little energy that batteries are not even necessary; it could run merely by tapping the ambient energy from the environment. Rather than the traditional charge-based electronic switches that encode the basic 0s and 1s of computer lingo, spintronics harnesses the natural spin – either up or down – of electrons to store bits of data. Spin one way and you get a 0; switch the spin the other way – typically by applying a magnetic field or by a spin-polarized current pulse – and you get a 1.

Arxiv - Hybrid spintronics and straintronics: A magnetic technology for ultra low energy computing and signal processing (16 pages)

The primary obstacle to continued downscaling of digital electronic devices in accordance with Moore’s law is the excessive energy dissipation that takes place in the device during switching of bits. Every charge-based device (e.g. MOSFET) has a fundamental shortcoming in this regard. Spin based devices, on the other hand, are switched by flipping spins without moving any charge in space and causing a current flow. Although some energy is still dissipated in flipping spins, it can be considerably less than the energy associated with current flow. This gives “spin” an advantage.

A composite structure consisting of a layer of piezoelectric material with intimate contact to a magnetostrictive nanomagnet (one that changes shape in response to strain). When a tiny voltage is applied across the structure, it generates strain in the piezoelectric layer, which is then transferred to the magnetostrictive layer. This strain rotates the direction of magnetism, achieving the flip. With the proper choice of materials, the energy dissipated can be as low as 0.4 attojoules, or about a billionth of a billionth of a joule.

If each operation needed that little energy and all of the other processes of a computer were maintained at that level then 2.5 exaflops would take 1 watt. 400 watts would be needed for a zettaflop. 400 kilowatts for a yottaflop system.



Recently, it has been shown that the minimum energy dissipated to switch a charge-based device like a transistor at a temperature T is about NkT ln(1/p), where N is the number of information carriers (electrons or holes) in the device and p is the bit error probability. On the other hand, the minimum energy dissipated to switch a single domain nanomagnet (which is a collection of M spins)
can be only about kT ln(1/p) since the exchange interaction between the spins makes all of them rotate together in unison like a giant classical spin. This gives the magnet an advantage over the transistor.

Unfortunately, the magnet’s advantage is lost if the method adopted to switch it is so inefficient that the energy dissipated in the switching circuit far exceeds the energy dissipated in the magnet. Regrettably, this is often the case.

Proposed high density Straintronic/Spintronic Hybrid

With a nanomagnet density of 10^10 cm−2 in a memory or logic chip, the dissipated power density would have been only 2 mW/cm2 to switch in 100 ns and 30 mW/cm2 to switch in 10 ns, if 10% of the magnets switch at any given time (10% activity level). Note that unlike transistors, magnets have no leakage and no standby power
dissipation, which is an important additional benefit. Such extremely low power and yet high density magnetic logic and memory systems, composed of multiferroic nanomagnets, can be powered by existing energy harvesting systems that harvest energy from the environment without the need for an external battery. These processors are uniquely suitable for implantable medical devices, e.g. those implanted in a patient’s brain that monitor brain signals to warn of impending epileptic seizures. They can run on energy harvested from the patient’s body motion. For such applications, 10-100 ns switching delay is adequate. Speed is not the primary concern, but energy dissipation is. These hybrid spintronic/straintronic processors can be also incorporated in “wrist-watch” computers powered by arm movement, buoy-mounted computers for tsunami monitoring (or naval applications) that harvest energy from sea waves, or structural health monitoring systems for bridges and buildings that are powered solely by mechanical vibrations due to wind or passing traffic.

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