The mechanism “splits” electronic rotations in a magnetic material – can create new energy-efficient memory devices

Researchers at Cornell have identified an approach to switching magnetization in thin layers of a ferromagnet by holding the desired material at right angles – a method that could eventually lead to the development of more energy-efficient magnetic memory devices.

The document of the research group “Inclined spin current generated by collinear antiferromagnet ruthenium dioxide” was published today (May 5, 2022) in the journal Natural electronics. The co-authors of the article are PhD student Arnab Boz and PhD students Nathaniel Schreiber and Rakshit Jane.

For decades, physicists have tried to change the orientation of the spins of electrons in magnetic materials by manipulating them with magnetic fields. But scientists, including Dan Ralph, a physics professor F.R. Newman of the College of Arts and Sciences and senior author of the paper, instead looked at the use of spin currents carried by electrons that exist when electrons have spins oriented predominantly in one direction.

When these spin currents interact with a thin magnetic layer, they transmit their angular momentum and generate enough torque to switch the magnetization by 180 degrees. (The process of switching this magnetic orientation is how information is written to magnetic memory devices.)

Ralph’s group focused on finding ways to control the direction of the spin in spin currents by creating them using antiferromagnetic materials. In antiferromagnets, the backs of every second electron are controlled in the opposite direction, so there is no pure magnetization.

“In essence, the antiferromagnetic order can reduce the symmetry of the samples enough for there to be non-traditional orientations of the spin current,” Ralph said. “The mechanism of antiferromagnets seems to also allow for fairly strong spin currents.”

The team experimented with ruthenium antiferromagnetic dioxide and measured how its spin currents tilt the magnetization in a thin layer of a nickel-iron magnet.[{” attribute=””>alloy called Permalloy, which is a soft ferromagnet. In order to map out the different components of the torque, they measured its effects at a variety of magnetic field angles.

“We didn’t know what we were seeing at first. It was completely different from what we saw before, and it took us a lot of time to figure out what it is,” Jain said. “Also, these materials are tricky to integrate into memory devices, and our hope is to find other materials that will show similar behavior which can be integrated easily.”

The researchers eventually identified a mechanism called “momentum-dependent spin splitting” that is unique to ruthenium oxide and other antiferromagnets in the same class.

“For a long time, people assumed that in antiferromagnets spin up and spin down electrons always behave the same. This class of materials is really something new,” Ralph said. “The spin up and spin down electronic states essentially have different dependencies. Once you start applying electric fields, that immediately gives you a way of making strong spin currents because the spin up and spin down electrons react differently. So you can accelerate one of them more than the other and get a strong spin current that way.”

This mechanism had been hypothesized but never before documented. When the crystal structure in the antiferromagnet is oriented appropriately within devices, the mechanism allows the spin current to be tilted at an angle that can enable more efficient magnetic switching than other spin-orbit interactions.

Now, Ralph’s team is hoping to find ways to make antiferromagnets in which they can control the domain structure – i.e., the regions where the electrons’ magnetic moments align in the same direction – and study each domain individually, which is challenging because the domains are normally mixed.

Eventually, the researchers’ approach could lead to advances in technologies that incorporate magnetic random-access memory.

“The hope would be to make very efficient, very dense and nonvolatile magnetic memory devices that would improve upon the existing silicon memory devices,” Ralph said. “That would allow a real change in the way that memory is done in computers because you’d have something with essentially infinite endurance, very dense, very fast, and the information stays even if the power is turned off. There’s no memory that does that these days.”

Reference: “Tilted spin current generated by the collinear antiferromagnet ruthenium dioxide” by Arnab Bose, Nathaniel J. Schreiber, Rakshit Jain, Ding-Fu Shao, Hari P. Nair, Jiaxin Sun, Xiyue S. Zhang, David A. Muller, Evgeny Y. Tsymbal, Darrell G. Schlom and Daniel C. Ralph, 5 May 2022, Nature Electronics.
DOI: 10.1038/s41928-022-00744-8

Co-authors include former postdoctoral researcher Ding-Fu Shao; Hari Nair, assistant research professor of materials science and engineering; doctoral students Jiaxin Sun and Xiyue Zhang; David Muller, the Samuel B. Eckert Professor of Engineering; Evgeny Tsymbal of the University of Nebraska; and Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry.

The research was supported by the U.S. Department of Energy, the Cornell Center for Materials Research (CCMR), with funding from the National Science Foundation’s Materials Research Science and Engineering Center program, the NSF-supported Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM), the Gordon and Betty Moore Foundation’s EPiQS Initiative, and the NSF’s Major Instrument Research program.

The devices were fabricated using the shared facilities of the Cornell NanoScale Science and Technology Facility and CCMR.