Observation and control of collective spin-wave mode-hybridisation in chevron arrays and square, staircase and brickwork artificial spin ices

Abstract

Dipolar magnon-magnon coupling has long been predicted in nano-patterned artificial spin systems. However, observation of such phenomena and related collective spin-wave signatures have until recently proved elusive or limited to low-power edge-modes which are difficult to measure experimentally. Here we describe the requisite conditions for dipolar mode-hybridisation, how it may be controlled, why it was not observed earlier and how strong coupling may occur between nanomagnet bulk-modes. We experimentally investigate four nano-patterned artificial spin system geometries: 'chevron' arrays, 'square', 'staircase' and 'brickwork' artificial spin ices. We observe significant dynamic dipolar-coupling in all systems with relative coupling strengths and avoided-crossing gaps supported by micromagnetic-simulation results. We demonstrate reconfigurable mode-hybridisation regimes in each system via microstate control, and in doing so elucidate the underlying dynamics governing dynamic dipolar-coupling with implications across reconfigurable magnonics. We demonstrate that confinement of the bulk-modes via edge effects play a critical role in dipolar hybridised-modes, and treating nanoislands as a coherently precessing macro-spins or standing spin-waves are insufficient to capture experimentally-observed coupling phenomena. Finally, we present a parameter-space search detailing how coupling strength may be tuned via nanofabrication-dimensions and material properties.

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

“…Optical modes are often defined to be higher-frequency than acoustic modes when oscillators are coupled only at one end, as in the 1D-chain of atoms often used to describe acoustic and optical phonons or indeed single-layer artificial spin systems where adjacent islands couple in-plane across vertices 18,20 . Here, both ends of the oscillators are equally coupled due to the 3D multilayered architecture, which gives an out-of-phase optical mode at lower frequency than the in-phase acoustic.…”

“…Such edge-curvature modes are typically very weak and challenging to resolve in such nanopatterned arrays, with most magnon/spin-wave measurements failing to resolve them 4,17,18,20 and most studies concerning them using micromagnetic simulation or theory 12,14,45,52 . The strong edge-mode amplitude here is due to the higher degree of curvature induced by 3D inter-layer coupling, as discussed above.…”

“…In a conventional 2D magnonic crystal, relatively simple mode structures and small 10 − 10 2 MHz-scale shifts are typically observed between system states 17,18,46 . Here, we observe GHz-scale shifts, rich mode structures with up to 5 active modes and the three distinct reversal processes observed in MOKE data (at approximately 8, 26 and 40 mT).…”

“…Artificial spin systems such as artificial spin ice (ASI) 2,4,5,11 rely on relatively weak, longer-range dipolar coupling for information transfer between discrete islands rather than the strong, short-range exchange coupling used in SAFs. Impressive magnonic functionality has been demonstrated in ASI [12][13][14][15][16][17][18][19] including magnon mode hybridisation 18,20 and nonlinear multimagnon scattering 19 , but typically at smaller magnitudes rather than the many-GHz scale effects in SAFs 18,20 . To advance (d-g) 3D coupled nanoisland magnetisation states.…”

Adaptive Programmable Networks for In Materia Neuromorphic Computing

“…Longrange coupling provides high-dimensional, massively-parallel nonlinear processing which is sensitive to the local microstate 8,41 , a feature which must be engineered-in at-cost in other approaches. Nanomagnets switch at ns timescales and spin-waves (magnons) offer GHz computing 15,35,38,39,[42][43][44][45][46][47][48][49] .…”