Magnonics

Magnonics aims at the exploitation of spin waves for information transport, storage, and processing. Practical realizations of disruptive magnonic technologies require versatile methods for efficient spin-wave excitation, manipulation, and detection. We pursue new approaches to these essential functions using low-loss YIG films, which we grow by pulsed laser deposition (PLD) [4]. We operate two PLD systems and use various optimized processes for the growth of YIG films with ultralow magnetic damping, substituted YIG films with perpendicular magnetic anisotropy, and suspended YIG microstructures. Using advanced microfabrication, we demonstrated nanometer-thick YIG-based magnonic crystals with large tunable bandgaps [6,7]. The magnonic crystals in these studies consist of a small number of air or CoFeB-filled grooves. We also integrated magnonic crystals into straight, curved, and Y-shaped YIG waveguides.   

As a new materials platform for low-loss nanoscale magnonics, we explore hybrid structures made of low-damping continuous YIG films and patterned ferromagnetic nanostructures. For instance, we have demonstrated short-wavelength spin-wave emission from an exchange-coupled YIG/CoFeB interface [3]. We also realized a magnonic analog of the Fabry-Pérot resonator by patterning ferromagnetic nanostripes on top of YIG films with a thin spacer [8]. In this configuration, dipolar coupling between the two magnetic layers produces an asymmetrical spin-wave dispersion within the bilayer region. Consequently, the two edges of the bilayer act as interfaces where the wavelength of spin waves converts upon reflection or transmission. Analogous to an optical Fabry-Pérot resonator, destructive interference of the spin waves entering and the spin waves circulating the bilayer region produces gaps at discrete frequencies in the transmission spectrum. Proper circulation of propagating spin waves within the resonator is facilitated by ultralow magnetic damping in the YIG film. Because the wavelength of spin waves within the bilayer converts down, the size of the resonator can be ultrasmall compared to the wavelength of incoming spin waves. In addition, magnetic switching in the Fabry-Pérot resonator actively controls the transmission signal. We currently work on the realization of magnonic neural networks using Fabry-Pérot nanoresonators.

As another example of a new hybrid magnonic structure, we demonstrated long-distance guiding of SWs along nanoscopic channels in continuous YIG films with patterned ferromagnetic stripes on top [12]. By taking advantage of ultralow damping in YIG and avoiding nanopatterning of this sensitive material, we measured spin-wave decay lengths of 20 micrometers in 160−400 nanometer wide waveguides at small magnetic bias fields. Our hybrid materials platform enables low-loss spin-wave transport through bends, providing a new strategy for the implementation of magnonic devices and integrated circuits without YIG nanopatterning.

Besides our work on YIG films, we also fabricate three-dimensional suspended YIG microstructures with low magnetic damping. We are particularly interested in bridges, which we operate as micromechanical resonators for studies on off-resonant magnon-phonon coupling, and suspended YIG microcavities for demonstrating strong coupling between optical whispering gallery modes and confined magnon states. We fabricate the suspended YIG structures by an advanced optical lithography process. Key steps in the process flow include lift-off of room-temperature grown YIG films and crystallization of the three-dimensional YIG structures using post-deposition annealing.

Leveraging the unique magnetic properties of ferromagnetic/ferroelectric bilayers, we have demonstrated short-wavelength spin-wave emission from oscillating magnetic domain walls [1] and globally excited magnetic anisotropy boundaries [2]. We have also shown that spin-wave transmission through a pinned magnetic domain wall can be switched on and off through active programming of the domain-wall spin structure. Most recently, we realized voltage control over the amplitude and phase of propagating spin waves in a multiferroic system [9] and we demonstrated zero-field routing of spin waves in a ferromagnetic/ferroelectric bilayer [10].