Pressure Turns a Metal into a Magnet express
A crystal of osmium dioxide, OsO₂, is like a stubborn teenager: under ordinary conditions it behaves like a typical metal, but press hard enough and its character changes. Physicists grew this material as high-quality single crystals and found that at ambient pressure, OsO₂ is a good conductor with clear Fermi-liquid behavior, where electrons scatter strongly off each other, like a crowd in a narrow corridor.
Magnetic measurements showed that the crystals are isotropically paramagnetic: their magnetic moments are randomly oriented, like compass needles scattered on a table. But theoretical calculations predict that hidden inside the material is altermagnetism — a special state where electron spins are aligned antiparallel, yet due to the crystal symmetry, the energy of electrons with different spins splits, like twins taking different paths. Photoemission spectroscopy experiments have not yet seen this splitting in the bulk bands, but theory suggests that pressure is the key.
Calculations with hybrid functionals show that pressure enhances the Coulomb repulsion between electrons (the U parameter), and the material progresses from a paramagnetic metal to an altermagnetic metal, and then to an altermagnetic insulator. It's like tuning a guitar: turning the peg changes the pitch, and here, squeezing the crystal switches its magnetic state.
So far, the altermagnetic splitting has not been observed experimentally, but the calculations provide a clear roadmap. Further studies at high pressure may reveal this effect, paving the way to controlling magnetic properties of materials simply by squeezing them.
The pressure of 44 GPa at which OsO₂ becomes an insulator is comparable to the pressure in Earth's mantle at a depth of about 1200 kilometers.
In Arthur C. Clarke's novel "Childhood's End," aliens control material properties through unknown fields — here, scientists do it with plain pressure.