A regular fridge magnet attracts iron because all its tiny magnetic arrows point in the same direction. But there are materials where the arrows arrange themselves more cunningly: neighboring rows point in opposite directions, so from the outside it looks like there's no magnetism at all. Such materials are called altermagnets. Inside them, researchers have found remarkable regions, domains, where the arrow order is the same, but the boundaries between regions are invisible to ordinary magnets.
The researchers studied the altermagnet MnTe using an infrared laser. It turned out that light reflected from different domains rotates differently, as if invisible patterns on glass suddenly became visible at a certain angle. The domains could be shifted with a magnetic field, but they quickly snapped back once the field was removed. Heating also didn't destroy them — the pattern persisted.
The ability to see and move domains with a simple optical method is an important step. Such materials could become the basis for a new type of memory where information is stored not in charge but in the orientation of magnetic patterns. And all this without bulky magnets and at room temperature.
Domains in altermagnets are so small that their width is only a few micrometers — thinner than a human hair.
In Greg Egan's novel 'Diaspora', materials with exotic magnetic properties are described, used to store consciousness. Altermagnets could be a step toward such science fiction.