Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures using Brillouin Light Scattering

Abstract

In recent years, artificial spin ice (ASI) systems have seen burgeoning interest due to their intriguing physics and potential applications in reprogrammable memory, logic and reconfigurable magnonics [1-2]. As the field progresses, direct in-depth comparisons of distinct artificial spin systems are crucial to advancing the field. While studies have investigated the effects of different lattice geometries, little comparison exists between systems comprising continuously connected nanostructures, where spin-waves (SWs) propagate via dipole-exchange interaction, and systems with nanobars disconnected at vertices where SW propagation occurs via stray dipolar-field. Here, we study the magnonic response of two kagome spin ices, a continuously connected system (c-ASI) and a disconnected system (d-ASI) with vertex gaps via Brillouin light scattering (BLS) (see Fig. 1(a) for schematic) and micromagnetic simulation [3]. Array of 25-nm-thick Ni80Fe20 (Py) kagome ASI were fabricated using electron-beam lithography and lift-off process. For both samples, the nanobars have length (l) and width (w) of 300 nm and 80 nm, respectively and d-ASI has a 50 nm vertex gap (Figs. 1(b,c)). Bias magnetic field (H) dependent SW spectra (Figs. 1(d,g)) measured by BLS reveal that both c-ASI and d-ASI exhibit frequency minimum, occurring at negative bias magnetic fields, when ramped down from a positive saturation field. A sharp jump in the mode frequencies was observed beyond the minima, linked to bar reversal via the switching field observed from the magneto-optical Kerr effect loops and the corresponding mixed magnetization state of nanobars aligned parallel and anti-parallel to the magnetic bias field. Furthermore, we elucidate striking differences in spatial mode-localization and mode quantization between dipole-exchange-mediated c-ASI and dipolar-coupled d-ASI (Figs. 1(e,f) and 1(h,i)). These observations are pertinent for the fundamental understanding of artificial spin systems and broader design and engineering of reconfigurable functional magnonic crystals.  Figure 1(a) Schematic of the BLS measurement geometry. (b,c) SEM images. (d-i) SW mode frequencies as a function of H together with the phase profiles for M1 and M3.