Magnetic materials can store oscillatory energy, like a tuning fork that keeps ringing after being struck. When such a material is placed in a microwave cavity, its oscillations (magnons) start interacting with light (photons). But to control this interaction, you need nonlinear effects — where the system's response depends on the strength of the drive, like a taut string that changes pitch when plucked hard.

The research team used a film of yttrium iron garnet just 200 nanometers thick — hundreds of times thinner than a human hair. Its shape is flat, not spherical, and that changes how magnetic oscillations behave. It turns out that in such a film, the Kerr effect (the dependence of oscillation frequency on amplitude) becomes noticeably stronger than in a sphere of the same volume. Moreover, this effect is tunable: by rotating an external magnet, the scientists changed not only the magnitude of the frequency shift but also its sign — from positive to negative.

Simulations of magnon-photon interactions matched the experimental data exactly, confirming that the enhanced Kerr effect does not destroy the strong coupling between oscillations. This means such a system can be used to control signal frequency — for example, in information processing devices where magnetic waves carry data.