Back in the 1980s, researchers discovered that a bismuthate oxide material was a rare type of superconductor that could operate at higher temperatures. Now, a team of engineers and physicists at the University of Wisconsin-Madison has found the material, “Ba(Pb,Bi)O3,” is unique in another way: It exhibits extremely high spin orbit torque, a property useful in the emerging field of spintronics.
The combination makes this and similar materials potentially important in developing the next generation of fast, efficient memory and computing devices.
The finding was an encouraging surprise to Chang Beom-Eom, a professor of materials science and engineering, and Mark Rzchowski, a professor of physics, both at UW-Madison. “We’re looking to expand the range of materials that can be used in spintronic applications,” says Rzchowski. “We had known from previous work these oxides have a lot of interesting properties, and so were investigating the spintronic characteristics. We weren’t anticipating such a large effect. The origins of this are not theoretically understood, but we can speculate about some interesting physical mechanisms.”
The paper was published Dec. 5, 2023, in the journal Nature Electronics.
In conventional electronics, positive and negative electric charges are used to flip millions or billions of tiny transistors on semiconductor chips or in memory devices. But in spintronics, magnetic fields, and interactions with other electrons, manipulate a fundamental property of electrons called the spin state, which records information. This is much faster, more energy-efficient and more powerful than current semiconductors and will advance the development of quantum computing and low-power devices.
However, unlike conventional electronics, which have relied on the cheap and abundant semiconductor silicon for half a century, spintronics has yet to find its ideal materials. That’s why researchers are searching for new or unconventional materials that might have spintronic properties.
Eom and Rzchowski decided to investigate Ba(Pb,Bi)O3 because of its rich electrical and structural properties. The team grew thin films of the material using a process called epitaxy, then layered them with films of other materials to create two types of “heterostructures,” or stacked devices.
The team tested these heterostructures to measure their spin properties—in particular, their charge-to-spin conversion efficiency. What the researchers found is that Ba(Pb,Bi)O3 has a charge-to-spin conversion efficiency as large as or greater than that measured in any other material. In fact, it was 70 times greater than their calculations first predicted.
So what do they think is going on? The researchers believe that some subtle modifications of the bonding structure not included in their original theoretical calculations could have very large effects on the spin orbit interactions.
The team is excited to look deeper into this material and better understand the possible interplay between the spin-orbit phenomenon and superconductivity. Once the researchers understand what’s happening at the atomic level, Eom says it will be possible to create or tune similar bismuthate-based heterostructured materials.
“We are exploring the possible mechanisms,” he says. “Once we understand that, we can design new spintronic materials based on oxides using the new mechanism. That’s why I think further exploration could be quite important.”
Chang Beom-Eom is Raymond R. Holton Chair Professor in Engineering and the Theodore H. Geballe Professor.
Other UW-Madison authors include Anthony L. Edgeton and Neil G. Campbell. Other authors include Isaac A. Harris and Xiaoxi Huang of the University of California, Berkeley; Yahong Chai and Tianxiang Nan of Tsinghua University, Beijing; Marcel Mazur and Daniel Ralph of Cornell University; Gautum Gurung and Evgeny Tsymbal of the University of Nebraska; Sandhy Susarla of Arizona State University, and Ramamoorthy Ramesh of Rice University.
The UW-Madison authors acknowledge support from a Vannevar Bush Faculty Fellowship (ONR N00014-20-1-2844); the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF9065 and DOE Basic Energy Science award number DE-FG02-06ER46327.
Featured image caption: Chang Beom-Eom, a professor of materials science and engineering, and Mark Rzchowski, a professor of physics, in the lab. Photo: Joel Hallberg.