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Innovative Method Paves Way to Ultra-Low-Power Microchips

A new method to manipulating magnetism in a microchip could pave the way for memory, computing, and sensing devices that consume extremely less power compared to present-day versions. The technique could also overcome a few of the intrinsic physical confines that have been slowing the advance in this domain until now.

Illustration shows how hydrogen ions (red dots), controlled by an electric voltage, migrate through an intermediate material to change the magnetic properties of an adjacent magnetic layer (shown in green). (Image: courtesy of the researchers, edited by MIT News)

Scientists at MIT and at Brookhaven National Laboratory have shown that they can regulate the magnetic properties of a thin-film material just by applying a small voltage. Deviations in magnetic orientation made in this way stay in their new state without the need for any constant power supply, unlike current regular memory chips, the team has discovered.

The new finding has been expressed in the recent issue of the journal Nature Materials, in a paper by Geoffrey Beach, a professor of materials science and engineering and co-director of the MIT Materials Research Laboratory; graduate student Aik Jun Tan; and eight others at MIT and Brookhaven.

Spin doctors

As silicon microchips approach the fundamental physical boundaries that could limit their ability to continue boosting their capabilities while reducing their power consumption, scientists have been examining a range of new technologies that might overcome these limits. One of the favorable alternatives is a method known as spintronics, which makes use of a property of electrons known as spin, in place of their electrical charge.

Since spintronic devices can preserve their magnetic properties without the necessity for constant power, which silicon memory chips require, they need a lot less power to function. They also produce a lot less heat — another key limiting factor for present-day devices.

However, spintronic technology suffers from its own restrictions. One of the biggest missing constituents has been a way to simply and quickly regulate the magnetic properties of a material electrically, by applying a voltage. Many research groups globally are pursuing that challenge.

Earlier attempts have depended upon electron accumulation at the interface between an insulator and a metallic magnet, using a device structure akin to a capacitor. The electrical charge can alter the magnetic properties of the material, but only by a very small quantity, making it unfeasible for use in actual devices. There have also been efforts made at using ions rather than electrons to alter the magnetic properties. For example, oxygen ions have been used to oxidize a thin layer of magnetic material, causing very large variations in magnetic properties. But, the insertion and removal of oxygen ions made the material to swell and shrink, resulting in mechanical damage that restricts the process to just a few repetitions, thereby making it basically unworkable for computational devices.

The new finding shows a way to sidestep that, by employing hydrogen ions instead of the much larger oxygen ions used in earlier attempts. Since the hydrogen ions can go in and out without difficulty, the new system is a lot faster and offers other major advantages, the scientists say.

As the hydrogen ions are a lot smaller, they can penetrate and exit from the crystalline structure of the spintronic device, altering its magnetic orientation each time, without degrading the material. Actually, the team has now shown that the process causes no degradation of the material after over 2,000 cycles. Plus, in contrast to oxygen ions, hydrogen can effortlessly pass through metal layers, which allows the team to manipulate the properties of layers deep in a device that couldn’t be regulated in any other way.

“When you pump hydrogen toward the magnet, the magnetization rotates,” Tan says. “You can actually toggle the direction of the magnetization by 90 degrees by applying a voltage — and it’s fully reversible.” As the orientation of the poles of the magnet is what is used to store information, this means it is feasible to simply write and erase data “bits” in spintronic devices using this effect.

Beach, whose lab found the original process for regulating magnetism through oxygen ions several years ago, says that preliminary finding gave a free rein to extensive research on a new area christened “magnetic ionics,” and now this latest finding has “turned on its end this whole field.”

“This is really a significant breakthrough,” says Chris Leighton, the Distinguished McKnight University Professor in the Department of Chemical Engineering and Materials Science at the University of Minnesota, who was not involved in this research. “There is currently a great deal of interest worldwide in controlling magnetic materials simply by applying electrical voltages. It’s not only interesting from the fundamental side, but it’s also a potential game-changer for applications, where magnetic materials are used to store and process digital information.”

Leighton says, “Using hydrogen insertion to control magnetism is not new, but being able to do that in a voltage-driven way, in a solid-state device, with good impact on the magnetic properties — that is pretty significant!” He adds, “this is something new, with the potential to open up additional new areas of research. … At the end of the day, controlling any type of materials function by literally flipping a switch is pretty exciting. Being able to do that quickly enough, over enough cycles, in a general way, would be a fantastic advance for science and engineering.”

Fundamentally, Beach explains, he and his team are “trying to make a magnetic analog of a transistor,” which can be switched on and off continually without corrupting its physical properties.

Just add water

The discovery came about by chance. While investigating with layered magnetic materials in search of ways of altering their magnetic behavior, Tan learned that the results of his experiments differed significantly from day to day for reasons that were not obvious. Ultimately, by probing all the conditions during the various tests, he understood that the main difference was the humidity in the air: The experiment was more effective on humid days compared to dry ones. The reason, he finally realized, was that water molecules from the air were being divided into hydrogen and oxygen on the charged surface of the material, and while the oxygen vanished into the air, the hydrogen became ionized and was entering into the magnetic device — and altering its magnetism.

The device created by the team comprises of a sandwich of several thin layers, including a layer of cobalt where the magnetic alterations occur, sandwiched between layers of a metal such as platinum or palladium, and containing an overlay of gadolinium oxide, and then a gold layer to connect to the driving electrical voltage.

The magnetism is switched on with merely a brief application of voltage and then stays put. No power is required to reverse it, just short-circuiting the device to connect its two sides electrically, while a conventional memory chip requires continuous power to keep its state. “Since you’re just applying a pulse, the power consumption can go way down,” Beach says.

The new devices, with their high switching speed and low power consumption, could ultimately be particularly useful for devices such mobile computing, Beach says, but the research is still at a primary stage and will necessitate further development.

“I can see lab-based prototypes within a few years or less,” he says. Creating a full working memory cell is “quite complex” and might take a while, he says.

The research was supported by the National Science Foundation through the Materials Research Science and Engineering Center (MRSEC) Program.

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