Direct Observation of Magnon Modes In Kagome Artificial Spin Ice with Topological Defects

Abstract

Kagome artificial spin ice (KASI) is a network of Ising type nanobars on a kagome lattice [1,2]. Great progress has been made in understanding defects including disordered states in KASI via quasistatic imaging techniques [1,3]. However, the magnetodynamic study of disordered states has been confined to global magnetodynamic (e.g. broadband spin wave spectroscopy) and micromagnetic simulations [4]. Furthermore, the simulation study on KASI has shown the presence of novel magnetodynamic microstates that offer excellent insights into the KASI’s disordered regime [4]. From the fundamental physics side, dynamically controlled microstates may offer a way to create Dirac strings via microwave-assisted switching interior to the KASI lattice in a controlled manner and study the disordered regime systematically. From the magnonic application perspective, experimental studies of microstates in KASIs are key towards their usage as a new type of microwave filter [5] and reprogrammable magnonic crystal [6].We investigate spin dynamics of a KASI consisting of Ni81Fe19 nanomagnets arranged on an interconnected kagome lattice using broadband spin wave spectroscopy (Fig. a,b), magnetic force microscopy (MFM) (Fig. c), and micro-focus Brillouin light scattering (BLS) microscopy (Fig. e-f). Micro-focus BLS performed on magnetically disordered states exhibit a series of magnon resonances that depend on topological defect configurations that we image by magnetic force microscopy. Nanomagnets on a Dirac string and between a monopole-antimonopole pair show pronounced modifications in magnon frequencies. Our work is key for the creation and annihilation of Dirac strings via microwave-assisted switching and reprogrammable magnonics based on ASIs. **