Strong coupling of quantized spin waves in ferromagnetic bilayers

TL;DR

This paper develops a strong-coupling theory for quantized spin waves in ferromagnetic bilayers, revealing non-integer quantization and spectrum splitting behavior that depends on interlayer exchange coupling, supported by micromagnetic simulations.

Contribution

It introduces a novel strong-coupling theoretical framework for PSSWs in ferromagnetic bilayers with interlayer exchange coupling, including analytical and simulation validation.

Findings

PSSWs remain quantized with non-integer quantum numbers.

Spectrum splitting is linear with IEC in weak coupling, saturates in strong coupling.

Analytical predictions agree well with micromagnetic simulations.

Abstract

We formulate a strong-coupling theory for perpendicular standing spin waves (PSSWs) in ferromagnetic bilayers with the interlayer exchange coupling (IEC). Employing the Hoffmann boundary condition and the energy-flow continuity across the interface, we show that the PSSWs are still quantized but with non-integral quantum numbers, in sharp contrast to that of a single layer. The magnon-magnon coupling is characterized by the spectrum splitting which is linear with the IEC in the weak-coupling region, but getting saturated in the strong-coupling limit. Analytical predictions are verified by full micromagnetic simulations with good agreement.