Domaina

Meet Domaina, a piece of iron whose tiny magnetic regions have all decided to line up together.

In iron, the electrons of neighbouring atoms interact so that their magnetic moments prefer to point the same way. That alignment happens within small regions called domains, each strongly magnetised. In an ordinary iron nail the domains point in many directions and mostly cancel. Place it in a magnetic field and the domains aligned with the field grow at the expense of the others, so the net magnetisation M, the total magnetic moment per unit volume, becomes large. Pierre Weiss proposed domains in 1907.

Heat fights the alignment. Above iron's Curie temperature, about 770 °C, the order breaks down and iron loses its ferromagnetism. Cooling it again lets domains form once more.

Domains explain permanent magnets, transformer cores, and magnetic data storage, where tiny regions are magnetised one way or the other to record bits.

The sticker's arrows are a cartoon. Real domains are typically micrometres to millimetres across, and their walls move in small jumps, not smoothly.