Magnon Kerr effect in a magnetic thin film strongly coupled to a microwave resonator

Cavity magnonics investigates hybrid systems where magnons interact coherently with photons, providing a platform to harness light-matter interaction in magnetic materials. Progress in this field hinges on achieving stronger and tunable nonlinear effects, which are essential for controlling magnon dynamics and frequency conversion. Here, we demonstrate the magnon Kerr effect in an anisotropic magnonic system comprising a 200~nm-thick yttrium iron garnet film strongly coupled to a three-dimensional microwave resonator. The strong shape anisotropy significantly enhances the magnon Kerr effect compared to a sphere of equivalent volume, while the cavity enables sensitive probing of magnetization dynamics. We demonstrate continuous tunability of the magnitude and sign of the Kerr shift by controlling the static orientation of the magnetization. Input-output modeling of the magnon-photon interaction provides a consistent description of our system and Kerr coefficients matching the experimental results. Our findings demonstrate a scalable approach to enhancing Kerr anharmonicity in hybrid magnon-photon systems while preserving strong coupling.

💡 Research Summary

In this work the authors demonstrate a pronounced magnon Kerr effect (MKE) in a hybrid magnon‑photon system that combines a 200 nm‑thick yttrium iron garnet (YIG) thin film with a three‑dimensional microwave resonator. By exploiting the strong shape anisotropy inherent to a planar magnetic film, they achieve Kerr coefficients that are two orders of magnitude larger than those reported for the smallest YIG spheres, while still preserving the strong‑coupling regime (g/2π ≈ 35.9 MHz, g > κ_tot, κ_m).

The experimental platform consists of a loop‑gap cavity resonator placed in a uniform electromagnet. The YIG film, grown by liquid‑phase epitaxy on both sides of a GGG substrate and subsequently polished on one side, is cut to 2.5 × 3.5 mm² and inserted into the cavity. By rotating the cavity the angle θ_H between the static magnetic field and the film normal can be varied continuously from in‑plane (IP, θ_H = 90°) to out‑of‑plane (OOP, θ_H = 0°). Vector network analyzer measurements of the transmission S₂₁ reveal the characteristic avoided crossing between the cavity mode and the Kittel magnon mode at low drive power (‑5 dBm). At higher drive powers (up to 25 dBm) the magnon‑polariton branches shift in frequency: for IP orientation the branches move to lower frequencies (higher magnetic fields), whereas for OOP orientation they shift in the opposite direction. This power‑dependent frequency shift is the hallmark of the magnon Kerr nonlinearity.

A mean‑field treatment of the Hamiltonian
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