By positioning the appropriate material at the appropriate angle, scientists from Cornell University were able to discover a way to switch the magnetization in a ferromagnet’s thin layers — an approach that could ultimately result in the creation of magnetic memory devices with high energy efficiency.
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The study has been published in the May 5th issue of Nature Electronics. The co-lead authors of the research paper include postdoctoral researcher Arnab Bose and doctoral students Rakshit Jain and Nathaniel Schreiber.
For decades, physicists have attempted to alter the alignment of electron spins in magnetic materials by influencing them with magnetic fields. But scientists including Dan Ralph, the F.R. Newman Professor of Physics in the College of Arts and Sciences and the study’s senior author, have instead looked to using spin currents conveyed by electrons, which exist when electrons have spins normally oriented in a single direction.
When these spin currents interact with a thin magnetic layer, they transfer their angular momentum and produce sufficient torque to change the magnetization 180 degrees. (The process of altering this magnetic alignment is how one writes data in magnetic memory devices.)
Ralph’s team has concentrated on discovering ways to regulate the direction of the spin in spin currents by producing them using antiferromagnetic materials. Every other electron spin in antiferromagnets is pointed in the opposite direction, therefore there is no net magnetization.
Essentially, the antiferromagnetic order can lower the symmetries of the samples enough to allow unconventional orientations of spin current to exist. The mechanism of antiferromagnets seems to give a way of actually getting fairly strong spin currents, too.
Dan Ralph, Senior Study Author and F.R. Newman Professor of Physics, College of Arts and Sciences, Cornell University
The researchers had been testing the antiferromagnet ruthenium dioxide and assessing the ways in which its spin currents switched the magnetization in a thin layer of a nickel-iron magnetic alloy known as Permalloy, which is a soft ferromagnet. To chart the various components of the torque, they computed its effects at a range 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. 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.
Rakshit Jain, Study Co-Lead Author and Doctoral Student, Cornell University
The scientists ultimately discovered a mechanism known as “momentum-dependent spin splitting” that is exclusive to ruthenium oxide and other antiferromagnets in the same category.
“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.
Dan Ralph, Senior Study Author and F.R. Newman Professor of Physics, College of Arts and Sciences, Cornell University
This mechanism had been theorized but has not been documented before. When the antiferromagnet’s crystal structure is aligned suitably within devices, the mechanism enables the spin current to be slanted at an angle that can allow more efficient magnetic switching compared to other spin-orbit interactions.
Currently, Ralph’s team is hoping to find a way to create antiferromagnets wherein they can regulate the domain structure — i.e., the regions where the magnetic moments of electrons orient in the same direction — and examine each domain separately, which is difficult because the domains are usually mixed.
In due course, the scientists’ approach could result in advances in technologies that integrate 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. 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.
Dan Ralph, Senior Study Author and F.R. Newman Professor of Physics, College of Arts and Sciences, Cornell University
The study’s 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; Evgeny Tsymbal of the University of Nebraska; David Muller, the Samuel B. Eckert Professor of Engineering; and Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry.
The study was supported by the U.S. Department of Energy, the Cornell Center for Materials Research (CCMR), with financial backing from the National Science Foundation’s Materials Research Science and Engineering Center program, Analysis and Discovery of Interface Materials (PARADIM), the Gordon and Betty Moore Foundation’s EPiQS Initiative, the NSF-supported Platform for the Accelerated Realization, and the NSF’s Major Instrument Research program.
The devices were made using the shared facilities of CCMR and the Cornell NanoScale Science and Technology Facility.
Journal Reference:
Bose, A., et al. (2022) Tilted Spin Current Generated by the Collinear Antiferromagnet Ruthenium Dioxide. Nature Electronics. doi.org/10.1038/s41928-022-00744-8.