Coupled oscillations in noncollinear microscale rectangular magnets

The eigenmode spectrums of dense arrays of single-layer rectangular elements periodically arranged in a two-dimensional (2D) magnonic crystal with a complex unit cell have been characterized using broadband ferromagnetic resonance (FMR) spectroscopy and magneto-optical Kerr effect measurements. The crystal's unit cell consists of noncollinear orientations of constituting elongated rectangular elements, which makes it significantly different from a normal 2D magnonic crystal with similar elements. Microscopy and numerical simulations of the ground state reveal that the elements are statically dipole coupled and that the ground state is of collective nature. Two distinct collective FMR modes were observed in the frequency range from 1 to 20 GHz, the frequency positions of which depend on the angle of the applied field with the selected crystal axis. These modes were explained as predominantly localized in elements with different easy-axis orientations. Coherence of oscillations in the elements of the same type is ensured by mediators in the form of the elements of the other types. Angular FMR measurements confirmed uniaxial configurational anisotropy for the array with a not-so-well-resolved frequency gap between the two main modes.