A Novel Theoretical and Experimental Investigation of Magnon-Phonon Coupling in Multiferroic Thin Films for Spintronics Devices

Multiferroic materials exhibiting simultaneous ferroelectric and ferromagnetic order offer unique opportunities for electric-field control of magnetic properties and vice versa, with potential applications in low-power spintronic devices and magneto-electric sensors. The microscopic coupling mechanism between spin waves (magnons) and lattice vibrations (phonons) in multiferroic thin films remains incompletely understood at the level of predictive theory. This research develops a novel theoretical description of magnon-phonon coupling in bismuth ferrite thin films and validates it through systematic experimental studies on thin film specimens fabricated at the Solid State Physics Laboratory of the University of Lagos, South West Nigeria. The theoretical model employs a spin-lattice Hamiltonian incorporating Heisenberg exchange, Dzyaloshinskii-Moriya interaction, and anharmonic phonon-magnon coupling terms. Renormalization group methods are applied to derive effective coupling constants and temperature-dependent spectral functions. Predictions for magnon-polaron dispersion relations and anomalous thermal conductivity dips at magnon-phonon crossing points are derived analytically as original contributions. Bismuth ferrite thin films of 20 to 200 nm thickness were deposited by pulsed laser deposition on SrTiO3 substrates, and magnon-phonon coupling signatures were probed by inelastic light scattering (Raman and Brillouin spectroscopy) and ferromagnetic resonance linewidth measurements as functions of temperature, film thickness, and applied magnetic field. The theoretical coupling constant magnitude derived from experimental spectral fitting is consistent with density functional theory predictions within 15 percent, providing validation of the theoretical framework. Keywords: magnon-phonon coupling, multiferroic, thin film, spintronics, Brillouin spectroscopy