Two dancers on one stage: one pulls the partner in one direction, the other in the opposite. In the crystal Cu2(OH)3Br, known as botallackite, copper atoms form chains where the spins (magnetic moments) of neighboring ions align either parallel, as in a ferromagnet, or antiparallel, as in an antiferromagnet. These two subsystems coexist and influence each other, but how exactly an external magnetic field rearranges their dance remained a puzzle.

To observe this dance, scientists used terahertz spectroscopy, a technique that detects spin oscillations at frequencies around a trillion times per second. They applied magnetic fields up to 31 tesla (a million times stronger than Earth's field) along different crystal axes and monitored changes in radiation absorption. When the field was directed perpendicular to the chains, at 5.4 tesla a sharp spin reorientation occurred, a so-called spin-flop transition. Low-frequency modes vanished, and new excitation branches appeared in their place.

With the field along the chains, the picture was different: no sharp reorientation, but at 16.3 tesla the ferromagnetic subsystem became fully polarized. The spectrum simplified to a single broad mode that stiffened with increasing field. This indicates that the coupling between the two subsystems weakened: the ferromagnetic chains froze, while the antiferromagnetic chains continued their dance, but now alone.

Previously, it was unclear how coexisting magnons and spinons exchange energy when external conditions change. Now it is clear: the field acts like a conductor who either makes the dancers abruptly change steps or smoothly sidelines one of them. Comparing spectra at different field orientations allowed the contributions of each subsystem to be separated and showed how their coupling weakens in strong fields.

This is pure knowledge: practical applications are not yet in sight. But it paves the way to understanding quantum phase transitions in low-dimensional magnets, where collective and fractional excitations intertwine like the steps of two dancers. And someday, perhaps, this dance will teach us to control new materials.