Ultrastrong Magnon-Magnon Coupling and Chiral Symmetry Breaking in a 3D Magnonic Metamaterial

Strongly-interacting nanomagnetic arrays are ideal systems for exploring the frontiers of magnonic control. They provide functional reconfigurable platforms and attractive technological solutions across storage, GHz communications and neuromorphic computing. Typically, these systems are primarily constrained by their range of accessible states and the strength of magnon coupling phenomena. Increasingly, magnetic nanostructures have explored the benefits of expanding into three dimensions. This has broadened the horizons of magnetic microstate spaces and functional behaviours, but precise control of 3D states and dynamics remains challenging. Here, we introduce a 3D magnonic metamaterial, compatible with widely-available fabrication and characterisation techniques. By combining independently-programmable artificial spin-systems strongly coupled in the z-plane, we construct a reconfigurable 3D metamaterial with an exceptionally high 16N microstate space and intense static and dynamic magnetic coupling. The system exhibits a broad range of emergent phenomena including ultrastrong magnon-magnon coupling with normalised coupling rates of $\frac{\Delta \omega}{\gamma} = 0.57$ and magnon-magnon cooperativity up to C = 126.4, GHz mode shifts in zero applied field and chirality-selective magneto-toroidal microstate programming and corresponding magnonic spectral control

Spin-wave caustics in an extended thin film excited by a nanoconstriction

International audienceThe ability to control the directionality of spin waves is important for magnonic logic and computing application. Here, we demonstrate the emission of caustic-like spin waves in an extended yttrium iron garnet (YIG) thin film of 200 nm thickness by a nano-constricted rf waveguide. Using spatial resolved micro-focused Brillouin light spectroscopy technique in both the backward volume and the Damon-Eshbach geometry, we reveal the spin-wave directional spin-wave propagation in the YIG film. We excite spin waves in the film by passing a rf current through a constricted waveguide patterned on the top of the extended YIG film by electron-beam lithography. When the direction of the group velocity is the same for waves of different wavevectors, caustic beams are formed which propagate in two directions perpendicular to the isofrequency curve. We find that these beams are symmetric in intensity for a rf magnetic field perpendicular to the biasing magnetic field. On the other hand, one beam is more intense than the other one for a rf magnetic field parallel to the biasing field. We further show that the propagation direction of caustic-like spin-wave beams is frequency dependent. Our findings have important implications for the development of switchable spin wave splitters, passive spin-wave frequency-division demultiplexers[1, 2] and magnonic interferometry