Drop a stone into a pond, and ripples spread across the water. Magnets have waves too — only instead of water molecules, it's the directions of atomic magnetic arrows that oscillate. These waves are called magnons, and their behavior reveals a lot about the invisible forces between atoms.
In the new experiment, a beam of neutrons was sent through crystals of Na₂IrO₃ — a compound of sodium, iridium, and oxygen. Neutrons, acting like microscopic billiard balls, bounced off atoms and lost energy, exciting magnons. It turned out that these waves have a minimum energy — a gap of 1.7(1) meV. It's as if ripples on water couldn't be smaller than a certain size. This gap arises from the zigzag ordering of magnetic moments: atoms line up not in a straight row but in a serpentine pattern, and a wave has to overcome that pattern.
Physicists are interested in these materials because they may host so-called Kitaev physics — a special type of magnetic state akin to a quantum liquid of spins. In such a liquid, atomic magnetic arrows never freeze even at absolute zero; they keep fluctuating. Na₂IrO₃ and α-RuCl₃ are two candidates for this role, but their behavior differs due to the sign of interactions between neighboring atoms. The new data help refine the model: in Na₂IrO₃, exchange between nearest neighbors is antiferromagnetic, while in α-RuCl₃ it's ferromagnetic. It's like the difference between two springs: one pushes neighboring arrows in opposite directions, the other in the same direction.
Understanding such materials could lead to new quantum technologies where information is stored and processed using spins rather than charges. For now, this is fundamental research, but every precise experiment brings us closer to controlling magnetic states at the atomic level.
An energy of 1.7 meV is roughly a hundred million times smaller than the energy of a visible-light photon. To heat one gram of water by one degree, you'd need about 10²⁰ such magnons.