A frozen orchestra where musicians swap seats when the conductor, magnetism, sets a new score. In a spinel crystal, iron and magnesium atoms sit in two types of seats: some surrounded by four neighbors, others by six. At room temperature, they barely stir, but cool the material to 5 K and a mass migration begins. Thermal motion is nearly absent, yet the order changes dramatically.

A neutron beam passed through the powder revealed the picture of this migration. Iron moves from four-neighbor seats to six-neighbor ones, while magnesium moves in the opposite direction. At 200 K, iron occupied 78% of the four-seat chairs, but at 5 K, it occupies all 100%. This is not random jostling: the atoms follow a new score set by magnetic ordering.

To understand who is conducting, scientists compared two experiments. In one, the material was cooled; in the other, it was compressed under pressure in a diamond anvil cell. Compression gave the opposite result: magnesium increasingly occupied the four-seat chairs. So the effect is not due to the lattice contracting from cold. The driving force is magnetic coupling between atoms.

Magnetic measurements confirmed a complex scenario. Upon cooling, magnetization changes character twice: at 135 K and 90 K, and at 50 K the material becomes a hard magnet with a coercivity of 0.35 T. Each transition is like a new bar in the score, prompting atoms to reshuffle.

The orchestra is frozen, but the music continues. Previously, it was thought that at cryogenic temperatures, ionic diffusion in oxides is completely frozen due to high energy barriers. Now it is clear that magnetic order can move atoms even in deep cold. Practical applications are not yet in sight, but this changes our understanding of material stability in space and in superconducting electronics.