A tiny magnet made of layers of sulfur, chromium, and bromine sits on a microscope stage. Inside it, like unexpected guests, sit erbium atoms — a rare-earth metal. These atoms can glow when hit with a laser, and their light tells us what's happening around them.

Usually, magnetic properties are studied from the outside: with a beam of particles or a light beam reflecting off the surface. But then the signal is averaged over a large chunk of material, like judging the mood of a crowd by its general hum, without hearing individual voices. Here, the erbium atoms act as microphones placed right inside the crowd.

When the sample was cooled to three kelvins (minus 270 degrees Celsius), the erbium atoms' glow changed dramatically. Upon warming back up, the changes repeated but with a delay, as if the magnetic 'memory' didn't want to let go. The brightness minimum occurred at 132 kelvins (minus 141 degrees)—exactly the temperature at which magnetic order in CrSBr was thought to disappear.

But the most interesting part: the beacon atoms kept signaling order even at temperatures above 132 kelvins. This means that at the nanoscale, in regions just tens of atoms across, order persists longer than the crystal average. It's like in a crowd that has dispersed, there are still small groups marching in formation.

Adding a weak magnetic field, just 0.3 tesla, like a fridge magnet — shifted the temperature at which order disappeared up by eight degrees. The field gave the atoms a gentle nudge toward cooperative behavior.

Such built-in glowing sensors could help create magnetic sensors that see not the average picture but details at the atomic scale. And perhaps control magnetic states in future devices — thin, fast, and heat-resistant.