Naval Research Laboratory Study Uses Neutron Scattering to Clarify Magnetic Behavior in Altermagnetic Material

Washington D.C.  –   Scientists at the U.S. Naval Research Laboratory (NRL) have identified the true source of a magnetic effect seen in the material ruthenium dioxide (RuO₂), helping resolve an active debate in the rapidly growing field of altermagnetism. The research was published by the American Chemical Society.

RuO₂ has drawn global attention as a possible “altermagnetic” material, a newly predicted class of materials that could enable faster, more energy-efficient computing technologies. The excitement has been fueled by theory and early experimental reports suggesting that RuO₂ might host an unusual magnetic state with major implications for spintronics and high-speed electronics.

“Altermagnets are a hot field of research right now,” said Steven Bennett, Ph.D., an NRL materials scientist and co-author of the study. “There’s been a rush to experimentally demonstrate what theorists predicted, because the impact on high-speed, energy-efficient computing could be significant.” 

One piece of evidence often cited in support of altermagnetism is magnetic behavior known as exchange bias. When thin films of RuO₂ are grown in intimate contact with a ferromagnet such as iron (Fe), researchers observe a shifted magnetic hysteresis loop, a signature commonly associated with hidden magnetic order such as antiferromagnetism.

But the NRL team suspected there might be hidden complexities.

“We’ve been studying exchange bias in other systems for years,” Bennett said. “When we looked at these results, we thought there could be other contributions at play.” 

Neutrons Reveal the Real Mechanism

To investigate, the team combined conventional magnetometry with advanced neutron scattering experiments conducted at Oak Ridge National Laboratory.

“I’d say the key thing that we did here was neutron scattering,” Bennett said. “It’s really what brought to light what was going on.” 

Neutrons are uniquely suited for studying magnetism. Because they carry a magnetic moment, they act as tiny probes of magnetic structure inside materials.

“Neutrons are little magnets,” Bennett explained. “You can polarize them and use them as a direct probe of magnetism. It’s an incredibly powerful technique.” 

The team used two complementary neutron techniques: polarized neutron reflectometry to examine magnetic behavior layer by layer, and neutron diffraction to probe bulk magnetic ordering.

“In both cases, we saw evidence that supported our observation that the exchange bias was not related to some intrinsic property of the ruthenium dioxide,” said Shelby Fields, Ph.D., an NRL materials scientist who conducted the neutron experiments. “Instead, it was due to the interfaces, something magnetometry alone can’t fully resolve.”
 
Further analysis showed that when iron was deposited onto RuO₂, oxygen migrated across the interface. This reaction formed a thin iron oxide layer containing magnetite (Fe₃O₄), a material known to contribute to exchange bias at low temperatures.

In other words, the magnetic effect came from a chemically formed interfacial layer, not from intrinsic altermagnetism within RuO₂.

Clarifying the Evidence

The findings do not rule out the possibility that RuO₂ could exhibit altermagnetism under specific conditions. However, the study makes clear that exchange bias alone cannot serve as proof.

“Exchange bias is not a smoking gun for antiferromagnetism in these materials,” Bennett said. “There are too many other contributions at the interface to use it by itself as definitive evidence.” 

That clarification is important as interest in altermagnets continues to grow.

What’s Next

The work is part of an ongoing NRL research program exploring altermagnetism and related phenomena. The team is now investigating how strain and compression may influence magnetic states in RuO₂ and other candidate materials.

“There’s still absolutely potential here,” Bennett said.  “We’re continuing to dig into what conditions might stabilize a magnetic phase.”

By carefully separating intrinsic material properties from interfacial chemistry, the researchers say they hope to provide a clearer framework for evaluating future altermagnetic candidates.

In emerging fields, progress often depends not just on finding new effects, but on understanding what truly causes them.

About the U.S. Naval Research Laboratory

NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL, located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.

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