Analysis of collective spin-wave modes at different points within the hysteresis loop of a one-dimensional magnonic crystal

The Brillouin light-scattering technique has been applied to study collective spin waves in a dense array of dipolarly coupled Ni80Fe20 stripes of alternating widths, during the magnetization reversal process. Both the saturated "ferromagnetic" state, where the magnetizations of wide and narrow stripes are parallel, and the "antiferromagnetic" state, characterized by an antiparallel alignment of the static magnetization in adjacent stripes, have been analyzed. The experimental data provide strong evidence of sustained collective excitations in the form of Bloch waves with permitted and forbidden magnonic energy bands. The measured frequencies as a function of the exchanged wave vector have been satisfactorily reproduced by numerical simulations which enabled us to calculate the spatial profiles of the Bloch waves, showing that some of the modes are preferentially localized in either the wide or the narrow stripes. We estimated the expected light-scattering cross section for each mode at different magnetic ground states, achieving a good agreement with the measured intensities. The alternating-width stripes system studied here represents a one-dimensional artificial magnonic crystal with a complex base and can be considered as a model system for reprogrammable dynamical response, where the band structure of collective spin waves can be tailored by changing the applied magnetic field.