Our electronic devices are based on what happens when different materials are layered together: “The interface is the device,” as Nobel laureate Herbert Kroemer famously claimed over 40 years ago. Right now, our microchips and memory devices are based on the movement of electrons across and near interfaces, usually of silicon, but with limitations of conventional electronics become apparent, researchers are looking at new ways to store or process information. These “heterostructures” can also find applications in advanced batteries and fuel cells.
Now physicists at UC Davis have observed what’s going on at some of these interfaces as oxygen ions react with different metals, causing drastic changes in magnetic and electronic properties.
Ionic and electronic effects
Electronic devices operate, as the name implies, on the movement of electrons. But ions – here atoms with extra electrons and negative charges – can also move in and near interfaces and take part in magnetoelectric effects.
“Typically, the electronic structure is dominant, but now we’re realizing that the movement of ions is also important,” said Kai Liu, professor of physics at UC Davis and corresponding author on the paper published March 21 in Nature Communications. “If we can get a handle on ion movement, then we can get insight into how we alter the chemistry of these structures and their properties.”
But these effects are hard to measure, because the ions are buried beneath the interface.
Liu’s graduate students Dustin Gilbert (now a postdoctoral scientist at the National Institute of Standards and Technology, NIST), Justin Olamit and Randy Dumas studied thin films of gadolinium iron alloy, which is magnetic, placed over nickel-cobalt oxide, which is an antiferromagnet (with two opposite sets of magnetic moments). When these materials are put together, the antiferromagnet “pins” the moments in the magnetic material in place so they cannot move freely, allowing realization of desired magnetic configurations. This effect, called exchange bias, is widely used in devices such as hard disk drive read heads and magnetic random access memory.
But when the antiferromagnet oxide is layered with another magnetic metal with strong oxygen affinity, such as gadolinium iron, a chemical reaction occurs where the oxygen is pulled towards the gadolinium, drastically changing the pinning effect the original oxide has on the metal.
Liu’s group and collaborators were able to track down the origin of these unusual behaviors to physical processes occurring at the buried interface.
“We can study the oxygen migration across the interface through changes in the magnetic properties of the films,” Liu said.
New layer influences entire structure
The team used X-rays from the Advanced Light Source at the U.S. Department of Energy’s Lawrence Berkeley Laboratory to probe the interfacial magnetic signatures. They found elemental nickel and cobalt, showing that part of the nickel/cobalt oxide had been chemically reduced (lost oxygen). Using neutron scattering at the NIST Center for Neutron Research, they were able to show that these elements were at the interface between the two original materials. This new layer of nickel and cobalt, a result of the oxygen migration, can couple to both the magnetic gadolinium-iron and antiferromagnetic nickel/cobalt oxide and influence the behavior of the entire structure.
“This tells us what oxygen migration does in such a system, and it means that now we can design structures to take advantage of this effect,” Liu said. Devices based on this “magneto-ionics” principle could use much less energy, and therefore generate less heat, than conventional electronics, Liu said.
Other authors on the study were Brian Kirby, Alexander Grutter, Brian Maranville and Julie Borchers at the NIST Center for Neutron Research, Gaithersburg, MD, and Elke Arenholz at the Advanced Light Source, Lawrence Berkeley Laboratory. The work was supported by the National Science Foundation, NIST and the Department of Energy, Office of Basic Energy Sciences.