We propose a quantum theoretical model for magnon-phonon interaction in composite multiferroic systems (CMS) under external fields. The Hamiltonian of magnon-phonon interaction under external fields is expressed by considering the equivalent anisotropic field $H_K$ and coupling constant ${\sigma }_k$ that exhibit explicit functional dependence on external fields. Here, the coupling strength and frequency range of magnon-phonon interaction under different external fields are quantified by excitation spectrum. The results show that the external fields enable a giant enhancement of the strength of magnon-phonon interaction by about a factor of 334% in the case of CMS consisting of aluminum nitride and yttrium iron garnet. At the anticrossing regime (i.e., strong interaction regime) of the magnon-phonon interaction, we demonstrate the existence of two resonance peaks characterized (including S11, impedance and magnetic field) by the finite element analysis model. The simulation results showcase that the frequency interval has an obvious external field dependence, which agrees with our quantum theoretical model. Furthermore, we quantify the external field conditions of strong coupling, which demonstrated the tunability of the coupling strength by the external fields. Our work provides a universal method for revealing the evolution of magnon-phonon interaction under external fields, which also provides a fundamental theoretical tool for understanding and designing magnetoelastic coupling phenomena in multiferroic systems.

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• Magnetoacoustic effect
• Magnons
• Phonons
• Spin-phonon coupling
• Temperature
• Multiferroics
• Thin films