Keynote

Ultrafast optical excitation of a metallic ferromagnetic nanostructure induces a simultaneous coherent response of the lattice and spin system; in other words, it generates coherent phonons and magnons. The range of generated wave vectors, determined by the structural design and the spatial profile of optical excitation, is between 10⁴ and 10⁶ cm^-1. In this range, the characteristic frequencies for both types of excitations are ~10-100 GHz, and the phonon and magnon dispersions intersect. This allows us to enhance the role of the magnon-phonon interaction in the coherent response of the spin system to optical excitation. The key parameters, which are considered crucial for the contribution of the magnon-phonon interaction to coherent spin dynamics, are the coupling strength of the phonon and magnon eigenmodes of the nanostructure and their spectral overlap. The phases of the optically excited modes are usually not considered. The present work illustrates that this parameter can crucially determine the coherent spin dynamics and proposes a concept of hybrid coherent control of magnons in a phononic nanoresonator. The talk will be based on a series of experiments with ferromagnetic (Fe,Ga) nanostructures that host spectrally narrow phonon and magnon modes [1,2]. The coherent response of the lattice and spin system to ultrafast optical excitation is measured in the time domain at room temperature using a pump-probe scheme with asynchronous optical sampling. We will review the main mechanisms contributing to the coherent magnon response, how these contributions depend on the nanostructure design and experimental conditions, and how to control the contribution of coherent acoustic phonons to the coherent spin dynamics. In the central part of the talk, we will consider the interference effect, when the quasi-harmonic phonon driving of a magnon mode interferes with its response to ultrafast optical excitation [2]. By manipulating the phase relation of these two contributions by an external magnetic field and achieving constructive or destructive interference, we can either significantly increase the spectral amplitude of the magnon mode or completely suppress it. The demonstrated approach enhances our ability to manipulate coherent spin dynamics at the nanoscale through magnon-phonon interactions. [1] A. V. Scherbakov et al., Ultrafast magnetoacoustics in Galfenol nanostructures, Photoacoustics 34, 100565 (2023) [2] A. V. Scherbakov et al., Hybrid coherent control of magnons in a ferromagnetic phononic resonator excited by laser pulses, Phys. Rev. Research. 6, L012019 (2024).