Magnetism loses under pressure

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Scientists have found that the magnetic force of magnetite – the most abundant magnetic mineral on Earth – drops dramatically when pressurized. Researchers at the Carnegie Institution Geophysical Laboratory, along with colleagues at the Advanced Photon Source of Argonne National Laboratory, found that when magnetite is subjected to pressures between 120,000 and 160,000 times atmospheric pressure, its strength magnetic halves. They discovered that the change is due to something called electron spin pairing.

Magnetism comes from unpaired electrons in magnetic materials. The strength of a magnet is the result of the spin of unpaired electrons and how the spins of different electrons are aligned with each other. This research showed that the fall in magnetism was due to a decrease in the number of unpaired electrons.

“Magnetite is found in small amounts in some bacteria, in the brains of some birds and insects, and even in humans,” commented Yang Ding, lead author of the study with the High Pressure Synergistic Consortium led by Carnegie. “Early navigators used it to find the magnetic North Pole and birds use it for navigation. And now it is used in nanotechnology. There is intense scientific interest in its properties. Understanding the behavior of magnetite is difficult because the strong interaction between its electrons complicates its electronic structure and magnetic properties.”

To study the mineral, the researchers developed and applied a new technique, called X-ray magnetic circular dichroism (XMCD) at the Advanced Photon Source, a high-energy synchrotron facility. The technique uses high-brilliance circularly polarized X-rays to probe the magnetic state of magnetite as a diamond anvil cell subjects a sample to several hundred thousand atmospheres. The researchers combined their experimental results with theoretical calculations by collaborators* to determine why the magnetic force changes. The study, which will be published in February in Physical Review Letters, reveals that the electron spin configuration in the mineral’s iron sites is behind the phenomenon.

This discovery not only shows the profound effects of pressure on magnetism, but also reveals, for the first time, that pressure induces a spin-pairing transition that leads to changes in electron mobility and structure.

“The discovery is significant,” Ding said. “It advances our understanding of the correlation of magnetism, electron transport and structural stability in materials with strong electronic interactions, such as magnetite.”

“It is not surprising to see that a new phenomenon was triggered by pressure in the oldest magnet. Pressure can directly alter electron-electron interactions by reducing the spacing between them,” Ho said. -kwang Mao, director of High Pressure. Synergetic Consortium and the High Pressure Collaborative Access Team. “In the future, the integration of high pressure with new synchrotron techniques will undoubtedly lead to new discoveries.”

*Collaborators are at the Kirensky Institute of Physics (Russia). The other authors of the article are Daniel Haskel, Sergei G. Ovchinnikov, Jonathan C. Lang, Yuan-Chieh Tseng and Yuri S. Orlov.

This work was supported by the US Department of Energy (Basic Energy Sciences and NNSA), the National Science Foundation, the WM Keck Foundation, and the Carnegie Institution.

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Material provided by Carnegie Institution. Note: Content may be edited for style and length.

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