Magnetic properties tunable in high-entropy oxides

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. High-entropy materials pose an interesting research challenge. For one class of material, high-entropy oxides, there hasn’t been a lot of work on their magnetic properties. This piqued the interest of Alannah Hallas at the University of British Columbia in Canada. In this relatively new field, it’s important to start by clarifying what a high-entropy oxide is.

ALANNAH HALLAS: We sort of divide things along two separate axes. So one is the idea that a material is entropy stabilized, meaning that the entropy is playing some role in selecting the crystal structure. And a nuance here is that to have this stabilization you don’t actually have to be in this regime of having five or six metal cations, you can get entropy stabilization with as few as two cations. That’s sort of one important aspect of a high-entropy material and then the other is just, sort of, the magnitude of the configurational entropy which you can calculate. In the extreme limit of having very high configurational entropy, the entropy need not actually be doing anything all that important; it may be the case that the entropy is present but it’s not actually responsible for selecting the crystal structure. And so our sort of thinking is that perhaps the most interesting regime is one where you have both of these factors happening at the same time: both entropy stabilization and high entropy.

LAURA LEAY: With a significant input from two members of the research group –  Graham Johnstone and Mario González-Rivas – Alannah’s team began by choosing five elements that would be magnetic and combining them in oxide form. The result was a spinel structure.

ALANNAH HALLAS: The spinel structure chose us. We did not pre-select spinel.

LAURA LEAY: The team was interested in understanding how progressive substitution of the magnetic metal cations with non-magnetic gallium would affect the magnetic properties of the spinel. This effect, known as percolation, is well known for simple materials and the team wanted to understand whether the same would be true for the more complex high-entropy oxides. They found that gallium substitution led to precise control of the configurational entropy which may help to stabilize the spinel structure. X-ray absorption spectroscopy at the Canadian Light Source was immensely helpful in determining the behavior of individual elements as a result of gallium substitution; manganese, cobalt, and iron were all redistributed throughout the structure whereas nickel and chromium were unaffected. 

ALANNAH HALLAS: I think this is going to be one of the most powerful tools for studying high-entropy materials.

LAURA LEAY: Gallium substitution also led to the ability to tune the magnetic properties of the material in some unexpected ways which are explained by what the team has dubbed “entropy engineering.”

ALANNAH HALLAS: One behavior that we expect is that the magnetic ordering is suppressed in temperature but the unexpected behavior is that the size of the saturated moment and the magnitude of the coarsivity both vary completely nonlinearly with gallium concentration. This is exciting because it implied that there’s this high degree of tunability in the magnetic properties of these materials. What’s exciting about magnetism and tunability in these high-entropy materials is that we have a lot of chemical knobs to tune to optimize the property of the material.

LAURA LEAY: The ability to tune the properties of this high-entropy oxide means that it could have a wide range of uses for energy applications, data storage, and beyond. It also means that, in the future, some devices or electronic components that require rare chemical elements could end up using more common ones and so could lead to more sustainable technologies. This work was published in a recent issue of the Journal of the American Chemical Society.  My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.