Magnetic vortices just tens of nanometers across are like tiny tornadoes frozen in metal. Called skyrmions, they are remarkably stable: hard to destroy but easy to move. That's why engineers see them as ideal 'bits' for future memory — fast, energy-efficient, and resilient.

But until now, no one knew how such a vortex is structured within the material's depth. From the surface, you only see a flat picture; what's inside is a mystery. A team of physicists applied a technique called vector ptycho-tomography — akin to computed tomography, but using coherent radiation instead of X-rays, with algorithms assembling a 3D image from thousands of 2D snapshots. This allowed them to 'illuminate' a piece of film over 0.4 cubic micrometers in volume with a resolution of 8 nanometers — a hundred times thinner than a human hair.

The results exceeded expectations. The vortices are not simple cylinders but barrels with variable twist. Near the surface, the magnetic arrows align in one type (Néel), while deeper they adopt another (Bloch), and the twist direction can even reverse. This complex structure is described as a 'fractional hopfion'—a three-dimensional topological knot with a non-integer index.

This is the first time a 3D magnetic texture has been reconstructed without prior assumptions about its shape. The method opens the door to studying other materials where magnetic knots may be even more exotic, and thus to new principles of information storage and processing.