Electrons are like cars with magnetic paint jobs. In a regular magnet, all cars are the same color; in an antiferromagnet, the colors balance out — no outward magnetism. An altermagnet gives each color its own lane. Apply a filter for one color, and the traffic becomes monochrome.

With computer modeling, scientists found a chemical transformation. They took a monolayer of iron selenide, a material known for superconductivity at –173 °C, and attached hydrogen atoms to one side. The symmetry broke, the magnetic order took on a wavelike pattern, and electrons of different spin separated thanks to their wave nature.

Two such layers were stacked into a tunnel junction. When the magnetic settings of the layers match, current flows; when opposite, the resistance jumps almost 20-fold. A switch with no stray fields — a building block for ultra-dense, fast memory. Unexpectedly, the same material that holds the superconductivity record now also operates as an altermagnet.