A new approach to controlling magnetism in a microchip could open the door to memory devices, computing, and sensors that consume dramatically less power than existing versions. The approach could also overcome some of the inherent physical limitations that have slowed progress in this area until now.
Researchers at MIT and Brookhaven National Laboratory have demonstrated that they can control the magnetic properties of a thin film material simply by applying a small voltage. Changes in magnetic orientation made in this way remain in their new state without the need for any continuous power, unlike the current standard memory chips, the team found.
The new discovery is being reported today in the journal Natural materials, in an article by Geoffrey Beach, professor of materials science and engineering and co-director of MIT's Materials Research Laboratory; graduate student Aik Jun Tan; and eight others at MIT and Brookhaven.
As silicon microchips approach the fundamental physical limits that may limit their ability to continue to increase their capacities while decreasing their energy consumption, researchers have been exploring a variety of new technologies that can circumvent these limits. One of the promising alternatives is an approach called spintronics, which makes use of an electron property called spin instead of its electric charge.
Because spintronic devices can retain their magnetic properties without the need for constant power, which requires silicon memory chips, they need much less power to operate. They also generate much less heat – another major limiting factor for today's devices.
But spintronic technology suffers from its own limitations. One of the largest missing ingredients has been a way of quickly and easily controlling the magnetic properties of a material by electrically applying a voltage. Many research groups around the world have pursued this challenge.
Previous attempts have depended on the accumulation of electrons at the interface between a metal magnet and an insulator, using a capacitor-like device structure. The electric charge can change the magnetic properties of the material, but only for a very small amount, making it impractical to use in real devices. There have also been attempts to use ions instead of electrons to alter magnetic properties. For example, oxygen ions have been used to oxidize a thin layer of magnetic material, causing extremely large changes in magnetic properties. However, the insertion and removal of oxygen ions causes the material to swell and shrink, causing mechanical damage that limits the process to only a few repetitions – making it essentially useless for computational devices.
The new finding demonstrates a way around this, using hydrogen ions instead of the much larger oxygen ions used in previous attempts. Because hydrogen ions can easily get in and out, the new system is much faster and offers other significant advantages, say researchers.
Because hydrogen ions are much smaller, they can enter and exit the crystalline structure of the spintronic device, changing its magnetic orientation each time without damaging the material. In fact, the team has now demonstrated that the process does not produce material degradation after more than 2,000 cycles. And, unlike oxygen ions, hydrogen can easily pass through the layers of metal, which allows the team to control the properties of the layers in a device that can not be otherwise controlled.
"When you pump hydrogen into the magnet, the magnetization rotates," says Tan. "You can actually switch the direction of magnetization by 90 degrees by applying a voltage – and it's totally reversible." Since the orientation of the magnet poles is what is used to store information, it means that it is possible to easily write and erase "bits" of data in spintronic devices using this effect.
Beach, whose laboratory discovered the original process to control magnetism through oxygen ions several years ago, says the initial discovery triggered extensive research into a new area nicknamed "magnetic ionic" and now this new finding "has put an end to all this field". "
"This is really a significant breakthrough," says Chris Leighton, a distinguished McKnight University professor in the Department of Chemical Engineering and Materials Science at the University of Minnesota, who was not involved in this work. "Currently, there is a great worldwide interest in the control of magnetic materials simply by the application of electrical voltages. It is not only interesting from a fundamental point of view, but it is also a potential change factor for applications where magnetic materials are used to store and process digital information. "
Leighton says, "Using hydrogen insertion to control magnetism is nothing new, but to be able to do this in a voltage-actuated way in a solid-state device with a good impact on magnetic properties – that's quite significant!" "This is something new, with the potential to open up additional new areas of research. … At the end of the day, controlling any kind of function of materials, literally reversing a switch is very exciting. Being able to do it fast enough, in enough cycles, would be a fantastic breakthrough for science and engineering in general. "
Essentially, Beach explains, he and his team are "trying to make a magnetic analogue of a transistor," which can be turned on and off repeatedly without degrading its physical properties.
Just add water
The discovery came in part through serendipity. While experimenting with layered magnetic materials for ways to change their magnetic behavior, Tan found that the results of his experiments varied greatly from one day to the next for reasons that were not apparent. Finally, examining all the conditions during the different tests, he realized that the main difference was the humidity of the air: the experiment worked better on wet days than on dry ones. The reason, he finally realized, was that the water molecules in the air were being divided into oxygen and hydrogen on the charged surface of the material, and as the oxygen escaped into the air, the hydrogen became ionized and penetrated the magnetic device. – and changing its magnetism.
The device the team produces consists of a thin multilayer sandwich, including a cobalt layer where magnetic changes occur, interspersed between layers of a metal such as palladium or platinum, and a layer of gadolinium oxide. layer of gold to connect to the electric voltage driving.
The magnetism is changed only with a brief tension application and then it is stopped. The inversion does not require power, only the short circuit of the device to connect its two sides electrically, while a conventional memory chip requires constant power to maintain its state. "As you are applying a pulse, energy consumption may decrease," Beach says.
The new devices, with low power consumption and high switching speed, may eventually be especially useful for devices such as mobile computing, Beach says, but the work is still in its early stages and will require further development.
"I can see lab-based prototypes within a few years or less," he says. Making a full working memory cell is "quite complex" and may take longer, he says.
The work was supported by the National Science Foundation through the Research Program in Materials Science and Engineering (MRSEC).