Researchers at the U.S. Naval Research Laboratory investigated a magnetic effect known as exchange bias when looking for unusual magnetism, and RuO₂ showed this effect when paired with iron.
Researchers in the United States have found the true source of a magnetic effect , a material called ruthenium dioxide . This can help resolve an active debate in the rapidly growing field of altermagnetism.
“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.” Newly predicted class of materials that could enable fasterThe team pointed out that 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, according to a press release.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 , 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, according to researchers.“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.” Researchers revealed that to investigate, they 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. It’s really what brought to light what was going on,” said Bennett. Neutrons are uniquely suited for studying magnetismThe team also revealed that 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. You can polarize them and use them as a direct probe of magnetism. It’s an incredibly powerful technique,” explained Bennett.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, as per the release.“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.”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. There are too many other contributions at the interface to use it by itself as definitive evidence,” said Bennett.
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