While cheese and bread are fine on their own, when they are stacked together and chemically bonded using a little heat from a frying pan, they take on new, special properties, especially at the interface where the gooey cheese and crisp bread meet.
Advanced crystalline materials can yield similar results but on a microscopic scale. Bonding them together can create interfaces where truly remarkable physics take place.
Recently, an international team of researchers did just that, successfully layering two crystalline materials called perovskites and antiperovskites together, creating an interface with unique electrical properties and opening up a whole new class of quantum materials. Led by Chang-Beom Eom, a professor of materials science and engineering at the University of Wisconsin-Madison, the group details the process in a paper in the July 24, 2020, issue of Science Advances.
These layered materials, or heterostructures, open new directions in spintronics, an emerging technology that could lead to new types of super-fast, low-power electronic components like transistors, memory chips and storage devices.
Perovskite crystals, which are usually oxides, have a specific structure of positively and negatively charged ions. Materials scientists have researched perovskites intensely for the last decade because of their promising electrical, magnetic and optical properties. Because a large number of elements can be combined to create different types of perovskite crystals, they are very tunable, and therefore have potential applications in electronics, battery development, solar energy and catalysis.
In antiperovskite crystals, the placement of the positively and negatively charged ions is flipped, creating another class of materials with different properties from perovskites. Eom, one of the world’s leading experts on creating and layering thin films of oxide perovskites, believed that it might be possible bond the two to create an interface between anti-materials with a new set of electrical and magnetic properties.
Eom and his team theorized how these layers might form. Using calculations, they proposed a scenario in which an antiperovskite composed of manganese, gallium and nitrogen could bond to oxide perovskites. “But calculations are only one aspect of this research,” Eom says. “I’m an experimentalist. I want to see how it works in the real world.”
Using a type of thin-film epitaxy, a crystal deposition method, the team grew the antiperovskite crystal on top of an existing perovskite layer, creating the novel heterostucture. When they examined the crystal layers using scanning transmission electron microscopy, they discovered that the two crystal films had indeed bonded, but that the materials had developed a chemically unique manganese and nitrogen monolayer interface between the perovskite and antiperovskite that went beyond the theoretical prediction.
Though the experiment was just a first step into the world of these new quantum materials, analysis of the heterostructure revealed exciting new properties in the interface, including a useful state called a frustrated spin structure. “This shows the possibility of creating very unconventional spintronic devices because the spin structure is so unique,” says Eom. “It can create what was thought to be impossible behavior.”
Eom plans to continue exploring these types of interfaces and hopes other researchers will also begin to look into this new class of heterostructures. “The very exciting part is the possibility of generating interesting physics and behavior at the interface,” says Eom. “I think this is fundamental science and opens up the possibility of a whole new class of perovskite/antiperoskite interfaces in infinite combinations. I’m excited about the exploration of the unknown.”
Eom is the Raymond R. Holton Chair Professor and Theodore H. Geballe Professor in materials science and engineering and physics at UW-Madison.
Camilo X. Quintela of UW-Madison is the first author. Tianxiang Nan, Neil Campbell and Mark S. Rzchowski of UW-Madison also contributed to the paper. Additional authors include Kyung Song of the Korea Institute of Materials Science, Ding-Fu Shao, Tula R. Paudel and Evgeny Y. Tsymbal of the University of Nebraska, Lincoln, Lin Xie of Nanjing University, Xiaoqing Pan of the University of California-Irvine, Thomas Tybell of the Norwegian University of Science and Technology in Trondheim, and Si-Young Choi of the Pohang University of Science and Technology.
The team at UW-Madison acknowledges support by the NSF under DMREF (grant no. DMR-1629270) the Army Research Office (Grant number W911NF-17-1-0462) and the U.S. Department of Energy (DOE) (award number DE-FG02-06ER46327).
Author: Jason Daley