The Future of Computing could be Magnetic

Magnetic-based spintronic computers could match the raw computing power of quantum computers, without the need for electricity.

Researchers at Massachusetts Institute of Technology (MIT) have developed an innovative new circuit that allows for precise computing without the need for electricity. The novel design instead relies on magnetic waves — an advance that makes a significant step towards magnetic devices. Dubbed ‘spintronics’ these devices have the potential to be more powerful and efficient than electronics.

Classical computers which depend on the consumption of massive amounts of electricity generate a huge amount of wasted heat. In contrast to this, spintronic devices use very little electricity in comparison and thus, generate far less heat — practically none, in fact.

An MIT-invented circuit uses only a nanometer-wide “magnetic domain wall” to modulate the phase and magnitude of a spin-wave, which could enable practical magnetic-based computing — using little to no electricity. (MIT)

Spintronic devices use a particular property of electrons on a quantum-level called the spin-wave in magnetic materials in a lattice-like arrangement. The process involves utilising modulation of this spin-wave to produce a measurable output correlated to computing.

“People are beginning to look for computing beyond silicon. Wave computing is a promising alternative,” says Luqiao Liu, a professor in the Department of Electrical Engineering and Computer Science (EECS) and principal investigator of the Spintronic Material and Device Group in the Research Laboratory of Electronics.

The spin doctors

The significant breakthrough made by the MIT team is the development of a circuit architecture that does away with the need for bulky components used to inject electrical currents. This is an advantage as such components can cause signal noise, thus reducing performance.

The team negated the need for such components by developing a nanometer-wide magnetic domain wall in layered nanofilms of magnetic material to modulate a passing spin-wave, with no need for any extra components or electrical current. In turn, the spin-wave can be tuned to control the location and width of this domain as needed, giving precise control of two changing spin-wave states. These spin states corresponding to the 1s and 0s used in classical computing.

Spin states corresponding to the 1s and 0s used in classical computing. (MRAM)

Future applications of these spin-waves could see pairs fed into a circuit through dual-channels. Each member of this pair could be modulated for different properties — combining to generate measurable quantum interference. This is analogous to the use of photon-wave interference in quantum computing.

“By using this narrow domain wall, we can modulate the spin-wave and create these two separate states, without any real energy costs. We just rely on spin waves and intrinsic magnetic material,” Liu continues.

As such, the researchers suggest that such spintronics based devices relying on interference could, in theory, match quantum computers in terms of raw computing power, executing complex tasks that conventional computers struggle with.

Flipping out

Spin waves are ripples of energy with small wavelengths, the collective spins of many electrons are called magnons. Although these magnons are not true particles they can be measured in a similar way to electrons to be used in computing applications.

The team layered a pattern of cobalt/nickel nanofilms — each a few atoms thick — with certain desirable magnetic properties that can handle a high volume of spin waves. Then placing this wall in the middle of a magnetic material with a special lattice structure. This system was then integrated into a circuit.

On one side of the circuit, the researchers excited constant spin waves in the material. As this wave passes through the wall, its magnons immediately spin in the opposite direction, flipping from north in the first region to south in the second region —beyond the wall. This flip causes a dramatic shift in the wave’s phase — or its angle of orientation — and a slight decrease in its magnitude — or its power.

In their experiments, the researchers placed a separate antenna on the opposite side of the circuit, detecting and transmitting an output signal. R

Their results indicated that, at its output state, the phase of the input wave flipped 180 degrees. The wave’s magnitude — measured from highest to lowest peak — had also decreased by a significant amount.

Putting a spin on it

The team also discovered a mutual interaction between spin-waves and the magnetic domain wall. This interaction enabled them to efficiently switch between two states. Without the domain wall, the circuit would be uniformly magnetized. But, with the domain wall in place, the circuit has a split, modulated wave.

By controlling the spin-wave, the MIT researchers found that they were able to control the position of the domain wall. This process relies on a phenomenon involving spinning electrons essentially ‘jolting’ a magnetic material in order to flip its magnetic orientation — otherwise known as ‘spin-transfer torque.’

A group of spins — known as a magnon (WWU Munster)

The team boosted the power of injected spin waves to induce a specific spin in the magnons — pulling the domain wall toward the boosted wave source. Doing this results in the wall getting stuck under the antenna — effectively making it unable to modulate waves and ensuring uniform magnetization in this state.

Using a special magnetic microscope, the team were able to identify that this process causes a micrometre-sized shift in the wall. Just enough to position it anywhere along the material block.

Liu notes that the mechanism of magnon spin-transfer torque was proposed, but not demonstrated, a few years ago. “There was good reason to think this would happen,” the researcher adds. “But our experiments prove what will actually occur under these conditions.”

Liu describes the whole circuit is like a water pipe. The domain wall acting as a valve controls how the spin-wave flows through the material — just like water flows through a pipe.

“But you can also imagine making water pressure so high, it breaks the valve off and pushes it downstream,” Liu elaborates. “If we apply a strong enough spin-wave, we can move the position of domain wall — except it moves slightly upstream, not downstream.”

Such innovations could enable practical wave-based computing for specific tasks, such as the signal-processing technique, called ‘fast Fourier transform.’

As a further step in their work, the researchers hope to build a working wave circuit that can execute basic computations. In order to do this, they must optimize materials, reduce potential signal noise, and further study how fast they can switch between states by moving around the domain wall.

“That’s next on our to-do list,” concludes Liu.