Controlling magnetism using a proton pump

14 Nov 2018 Belle Dumé

Magneto-ionic switching

Researchers at the Massachusetts Institute of Technology say they have discovered a new way to electrically control magnetism using a gate voltage that could be applied to a wide variety of magnetic materials, including oxides and metals. The “magneto-ionic” technique, which involves reversibly inserting and removing protons into the material structures, could help advance the field of spintronics (a technology that exploits the spin of the electron rather than its electrical charge) for the post CMOS-world.

Complementary metal-oxide semiconductor (CMOS) technologies are reaching the end of their roadmap and scientists are looking for alternatives to silicon microchips. Spintronics devices show promise in this context because they retain their magnetic state even when the power supply is switched off, something that it is not true for silicon memory chips. They also require much less power to operate and generate far less heat than their silicon counterparts.

One of the most important phenomena being studied in spintronics today is spin-orbit coupling, explains MIT Materials Research Laboratory co-director Geoffrey Beach, who led this research effort. “In many spintronics systems, emergent effects are generated at the interface between, for example, a metallic ferromagnet and a nonmagnetic heavy metal (like platinum or palladium),” he says. “Heavy metal/ferromagnetic interfaces have long been exploited to engineer magnetic thin films with perpendicular magnetic anisotropy, that is, films that spontaneously magnetize in a direction perpendicular to the film plane, which is required for most applications.”

Gating novel spin transport phenomena

“More recently, such interfaces have been exploited to generate novel spin transport phenomena, such as the spin Hall effect, and exchange interactions like the Dzyaloshinskii-Moriya interaction, which is responsible for stabilizing magnetic quasiparticles known as skyrmions. In devices containing such interfaces, it would be highly desirable to be able to gate such effects with a small bias voltage.”

The problem is that electric fields cannot penetrate very far in a metal so metal/metal interfaces are “immune” to electric fields. Beach and colleagues say they have now found a way to overcome this problem and gate such interfaces by injecting hydrogen ions (protons) into and out of a solid state heterostructure. “We can load and unload hydrogen into the heavy metal to reversibly turn spin-orbit effects on and off,” Beach tells Physics World.

“The process is remarkably simple,” he says. “The device structure we studied resembles a capacitor and consists of several thin layers including a layer of ferromagnetic cobalt sandwiched between layers of a metal such a Pd or Pt. To finish, we overlay thin film gadolinium oxide and then a noble metal layer on the other side of the device to connect to the driving voltage.”

Facile magnetic switching without any damage

“When we operate the device in ambient conditions, we find that a gate voltage applied to the top noble metal electrode efficiently splits water from humidity in the atmosphere to create oxygen and hydrogen. The protons then inject into the solid material, can shuttle through the gate oxide and move into and back out of the magnetic heterostructure. Since protons are very small, they can move quite fast and do not damage the crystalline structure of the device, allowing for facile switching of the magnetic orientation without any damage.”

Indeed, the researchers show that the process produces no degradation whatsoever even after 2000 such cycles. And unlike oxygen ions, hydrogen can easily pass through the metal layers, so allowing layers deep in a device to be controlled in a way that was not possible before without damaging it.

“When you pump hydrogen toward the magnet, the magnetization rotates,” explains team member Aik Jun Tan. “You can actually toggle the direction of the magnetization by 90 degrees by applying a voltage – and it’s fully reversible. And since the magnet’s orientation is used to store information, this means that we can easily write and erase data ‘bits’ in spintronics devices using this effect.”

A wide variety of materials could be controlled by a gate voltage

“In terms of fundamental science, we now have a tool that allows us to toggle interactions at interfaces so we can explore the relation between the nature of the interface and the nature of the phenomena induced at that interface,” adds Beach. “The proton pumping technique can be applied to a wide variety of materials and interfaces, which means that they could all be controlled by a gate voltage.”READ MORE

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So, what about potential applications? “One of the key missing ingredients in spintronics thus far was an efficient way to gate magnetic properties in a manner analogous to using gate voltage to control the properties of semiconductors in transistors,” says Beach. “The phenomenon we have discovered provides this missing functionality and might even allow for novel operating modes.”

The advantages of the technique are many. For one, the magnetic states induced by the applied voltage are non-volatile (that is, they retain their orientation even without power). They are also analogue (multiple states can be induced depending on bias voltage and dwell time), he explains. “These modalities will allow not only for conventional solid-state magnetic memories but also for new approaches to computing, such as neuromorphic architectures that promise tremendous breakthroughs in information processing.”

The MIT researchers, reporting their work in Nature Materials 10.1038/s41563-018-0211-5, are now planning to apply their gating mechanism to other spintronics phenomena and identify materials in which the proton transport is even faster than demonstrated in this work. “We expect that switching speeds approaching the nanosecond regime are possible through materials optimization,” says Beach. “We also hope to produce novel device structures such as artificial neurons that will be able to harness the capabilities we have discovered.

15/11/2018 FROM PHYSICSWORLD.COM