The interest in artificial magnetic media such as magnonic crystals increased substantially in recent years due to their potential applications in information processing at microwave frequencies. The main features of these crystals are the presence of band gaps in the spin-wave spectra, usually formed due to Bragg reflections of spin-waves on the artificially created periodic structures. Here, we study spin-wave propagation in longitudinally magnetized width-and thickness-modulated yttrium iron garnet waveguides by means of Brillouin light scattering and microwave spectroscopy techniques. It is found that the width modulated crystal does not manifest noticeable Bragg reflections, but still demonstrates a pronounced band gap in its transmission characteristic. The phenomenon can be explained by the destructive interference between different frequency-degenerated spin-wave modes excited by the crystal. Such a reflection-less crystal is promising for future design of multi-element magnonic devices.

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A theory has been developed for the waveguiding characteristics of dual-tunable multiferroic magnonic crystals (MCs). The crystals are constructed as periodically width-modulated microwave transmission slot-lines placed in between thin ferrite and ferroelectric films. Dispersion characteristics of the spin-electromagnetic waves (SEWs) in the investigated periodic waveguiding structure were derived using the method of approximate boundary conditions and the coupled-mode approach. The transmission-loss characteristics (TLCs) were calculated by the transfer-matrix method. The results show that the TLCs of the structures consist of pass-bands and stop-bands. The stop-bands are due to Bragg reflections in the structure. The magnetic and electric fields control the stop-band frequencies. The ferroelectric film polarization produced with the application of control voltage to the slot-line electrodes reduces its dielectric permittivity and provides up-shift of the stop-band frequencies. The most effective electric tuning is achieved in the area of the maximum hybridization of SEWs. As a result, the investigated multiferroic MCs combine the advantages of thin-film planar topology and dual tunability of magnonic band-gaps.

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