In an ordinary crystal, atoms stand in neat rows like soldiers on a parade ground. Any deviation from this formation is usually considered harmful: disorder scatters waves and quickly dampens collective motions of particles. But in some materials, different atoms are deliberately mixed to create a so-called high-entropy structure — a kind of medley where each type of atom occupies a random site. Until now, it was believed that in such a jumble, magnetic waves (spin waves — oscillations of atomic magnetic moments, similar to a wave passing through a stadium crowd) should quickly fade away.

The authors tested this on the compound YBaCuFeO5. It contains two different magnetic atoms, copper and iron, with very different magnetic moments, so any local change in order immediately affects spin dynamics. Using resonant inelastic X-ray scattering (a method that directly measures the energy and momentum of spin waves), the scientists saw something unexpected: in the mixed phase, where disorder is maximal, spin waves not only do not disappear but become sharper and longer-lived. Moreover, they develop two well-separated branches, acoustic and optical, like sound vibrations in a lattice, but with a large energy gap between them.

This paradox changes our view of the role of disorder in magnetism. Previously, it was thought that coherent collective modes were a privilege of perfectly ordered systems with low entropy. Now it is clear: high-entropy magnets can preserve and even enhance coherence. This result is important for spintronics — a field where spin waves are used to transmit and process information without moving charges, and thus with lower losses.