Multipolar magnon modes of the skyrmion lattice probed by magneto-optic and magneto-elastic coupling

Magnetic skyrmion lattices in noncentrosymmetric chiral magnets form regular hexagonal arrangements with lattice constants on the order of tens of nanometers. These topological periodic spin textures act as natural magnonic crystals, eliminating the need for challenging nano-fabrication and enabling bottom-up engineering of magnon band structures in the exchange-dominated spinwave regime. Such systems host a rich manifold of topological magnon bands with multipolar character and non-reciprocal dispersions. While low-energy excitations at small wavevector have been extensively investigated using microwave and optical spectroscopies, and excitations at large wavevector have been accessed through neutron scattering, the intermediate momentum regime, where the spinwave wavelength matches the lattice periodicity, remains largely unexplored. This regime is essential for understanding magnon propagation, hybridization, and interference in periodic magnetic textures, and is highly relevant for future magnonic applications. In this thesis, we investigate the spinwave dynamics at intermediate wavevectors in the chiral magnet Cu$_2$OSeO$_3$, focusing on the system’s response to magneto-optic and magneto-elastic coupling within the skyrmion lattice phase. ... mehrTwo complementary experimental techniques are employed: micro-focused Brillouin light scattering (BLS) and ultrasonic pulse-echo spectroscopy. In the first study, we develop a general theoretical framework for analysing BLS spectra, which allows for the unambiguous identification of the magnon bands observed experimentally. Beyond the well-known dipolar counterclockwise, breathing, and clockwise modes, we identify two additional multipolar modes with quadrupole and sextupole character, respectively. By incorporating magneto-elastic coupling into our theoretical model, we further describe the collective hybrid excitations that emerge from the interplay between magnetic and elastic degrees of freedom. In particular, we analyse the hybridization between low-energy magnon modes and acoustic phonons induced by the noncentrosymmetric cubic crystal lattice. The comparison with the experimental data reveal an additional low-energy excitation with quadrupolar character. Furthermore, we study the opening of a phason gap governed by the strength of the magneto-elastic coupling. Together, these results advance the experimental and theoretical understanding of skyrmion lattice dynamics into previously inaccessible regimes and establish a foundation for future studies and applications of hybrid magnonic systems based on topological spin textures.