We use the phase-resolved imaging to directly study the nonlinear modification of the wavelength of spin waves propagating in 100-nm thick, in-plane magnetized YIG waveguides.We show that, by using moderate microwave powers, one can realize spin waves with large amplitudes corresponding to precession angles in excess of 10 degrees and nonlinear wavelength variation of up to 18% in this system. We also find that, at large precession angles, the propagation of spin waves is strongly affected by the onset of nonlinear damping, which results in a strong spatial dependence of the wavelength. This effect leads to a spatiallydependent controllability of the wavelength by the microwave power. Furthermore, it leads to the saturation of nonlinear spectral shift's effects several micrometers away from the excitation point. These findings are important for the development of nonlinear, integrated spin-wave signal-processing devices and can be used to optimize their characteristics.

We study, experimentally and by micromagnetic simulations, the propagation of spin waves in 100-nm thick YIG waveguides, where the width linearly decreases from 2 to 0.5 μm over a transition region with varying lengths between 2.5 and 10 μm. We show that this geometry results in a downconversion of the wavelength, enabling efficient generation of waves with wavelengths down to 350 nm. We also find that this geometry leads to a modification in the group velocity, allowing for almost-dispersionless propagation of spin-wave pulses. Moreover, we demonstrate that the influence of energy concentration outweighs that of damping in these YIG waveguides, resulting in an overall increase in the spin-wave intensity during propagation in the transition region. These findings can be utilized to improve the efficiency and functionality of magnonic devices that use spin waves as an information carrier.

Since magnonics takes advantage of not only the amplitude of spin waves but also their phase, tunable phase shifters are key elements for the implementation of magnonic circuits. Therefore, one of the major challenges in nano-magnonics is to find a physical mechanism to manipulate the spin-wave phase practically in simple and miniature devices. In this work, we experimentally demonstrate that intrinsic magnetic nonlinearities allow the implementation of efficient microscopic tunable phase shifters, where the phase is controlled by wave intensity. In the proposed devices, we achieve the tunability of the phase shift of more than 360° by a microwave power of few milliwatts over a propagation distance of about 10  μm. We show that the figure of merit of the demonstrated phase shifters is close to that of macroscopic devices based on alternative technologies. Our results also indicate that the ability to control the phase shift is primarily limited by nonlinear spin-wave damping and can be significantly improved by suppressing this effect. Our findings are important for the further development of integrated nano-magnonics for beyond-Moore computing.