Tunable magnonic spectra in two-dimensional magnonic crystals with variable lattice symmetry

Tunable magnonic properties are demonstrated in two-dimensional magnonic crystals in the form of artificial ferromagnetic nanodot lattices with variable lattice symmetry. An all-optical time-domain excitation and detection of the collective precessional dynamics is performed in the strongly magnetostatically coupled Ni₈₀Fe₂₀ (Py) circular dot lattices arranged in different lattice symmetry such as square, rectangular, hexagonal, honeycomb, and octagonal symmetry. As the symmetry changes from square to octagonal through rectangular, hexagonal and honeycomb, a significant variation in the spin wave spectra is observed. The single uniform collective mode in the square lattice splits in two distinct modes in the rectangular lattice and in three distinct modes in the hexagonal and octagonal lattices. However, in the honeycomb lattice a broad band of modes are observed. Micromagnetic simulations qualitatively reproduce the experimentally observed modes, and the simulated mode profiles reveal collective modes with different spatial distributions with the variation in the lattice symmetry determined by the magnetostatic field profiles. For the hexagonal lattice, the most intense peak shows a six-fold anisotropy with the variation in the azimuthal angle of the external bias magnetic field. Analysis shows that this is due to the angular variation of the dynamical component of magnetization for this mode, which is directly influenced by the variation of the magnetostatic field on the elements in the hexagonal lattice. The observations are important for tunable and anisotropic propagation of spin waves in magnonic crystal based devices. Tunable magnonic spectra in two-dimensional magnonic crystals with variable lattice symmetry are studied using an all-optical time-resolved magneto-optical Kerr microscope. The magnonic mode profiles are interpreted in different lattices by micromagnetic simulations. A six-fold anisotropy is observed in the hexagonal lattice due to the variation of the magnetostatic stray field on the elements.