A beam of charged particles, like a fine pen, draws an invisible pattern on a metal film. Where it touches, the material changes its internal atomic order: from a close-packed arrangement, like a pyramid of oranges in a crate, the atoms rearrange into a looser one, like cubes stacked in a box. Along with the rearrangement comes magnetism — where the film previously wouldn't attract iron, it now becomes a magnet itself.

The direction of the magnetic forces depends on how the beam moved. By varying its path, you can obtain eight different orientations of magnetic domains — regions with uniform magnetization. This is akin to how the same pencil, stroked across paper in different directions, leaves marks that reflect light differently.

The reason for this selectivity is residual stress in the crystal lattice after the rearrangement. Atoms can't settle into ideal positions, and this 'under-compression' forces the magnetic moments to align in a specific direction. Understanding this mechanism paves the way for creating magnetic nanostructures with a pre-designed magnetization pattern, like engraving on metal, but at a scale a thousand times thinner than a human hair.