Strong magnon-phonon coupling in a ferromagnetic nanograting

Abstract

The aim of this work is the development of a method to operate elementary spin excitations (magnons) in a ferromagnetic metal by means of elementary vibrational excitations of the lattice (phonons). The utilized magnon-phonon coupling was confirmed more than 50 years ago, but only recently the experimental approaches of ultrafast acoustics have made available the excitation and time-resolved detection of coherent phonons in the sub-THz frequency range. The most intriguing regime of the magnon-phonon coupling is the strong coupling regime, which guarantees a conversion of phonons to magnons and vice versa at unity fidelity. This regime is extremely difficult to achieve in ferromagnetic metals due to a typically weak magnon-phonon coupling and fast relaxation processes. In this work, we demonstrate a way to overcome these limitations and to achieve the regime of strong magnon-phonon coupling. We have utilized a lateral nanoscale pattering (nanograting) of a Galfenol nanolayer, in order to introduce two additional localized high-Q phonon modes (~ 10 GHz). By means of an in-plane external magnetic field, we control the frequency detuning between the magnon and phonons modes, while their spatial overlap is determined by the lateral pattern. We observe two bright manifestations of the magnon-phonon coupling, i.e. resonant phonon driving and avoided crossing. The latter one clearly indicates the regime of strong magnon-phonon coupling with formation of a hybridized state known as magnon polaron. Theoretically, the magnon-phonon coupling is considered in the frame of coupled oscillators, whose coupling strength is determined by the spatial overlap of the interacting modes. The presented experimental and theoretical results may aid in the development of energy-efficient transducers between magnonic and phononic systems.