A beam of circularly polarized light hits a MAPbI3 perovskite crystal. The electrons inside start behaving like tiny spinning tops, their axes aligning along the beam. This phenomenon is known as optical spin orientation.

At a temperature of about 4 Kelvin (minus 269 degrees Celsius), scientists applied a magnetic field and watched the spins lose their alignment. It turned out that the main culprit wasn't impurities or defects, but the atomic nuclei in the crystal itself. Their random magnetic fields, like trembling needles, disrupt the orderly array of electronic tops.

The researchers went further: they made the nuclear spins themselves align in one direction. To do this, they used the same polarized light, which transferred its angular momentum to the nuclei via the electrons. The result was an internal magnetic field — the Overhauser field. For electrons it reached 5 millitesla, and for holes as much as 30 millitesla. It's as if a permanent magnet appeared inside the crystal, but only while illuminated.

This trick opens the way to controlling spins without external magnets — using light alone. That means simpler and more compact spintronic devices, where information is encoded not by charge but by spin direction.