Spin wave behavior of a novel hopfion-like chiral state in Co/Pt nanodiscs

Abstract: This work discusses the rich phase diagram of non-trivial chiral spin textures in confined ferromagnetic/heavy-metal (FM/HM) bilayer nanomagnets of circular cross-section. These spin textures are realized as a minimum-energy ground state during an external bias field sweep for a range of nanomagnet's diameter (d). Our study, based on micromagnetic simulations, has revealed a novel Hopfion-like state which can be stabilized for a wide range of diameters and external magnetic fields. We explored the dynamical characteristics of this novel Hopfion-like state under a transient magnetic field applied along the plane's perpendicular direction. Simulation results have demonstrated the excitation of nonreciprocal spin wave (SW) modes for this novel chiral state, in contrast to other stabilized chiral states. These modes are characterized as breathing and quantized radial modes, which also exhibit hybridization with azimuthal modes. The resonant SW modes have been used to demonstrate the switching from a Hopfion-like state to a skyrmion within a few nanoseconds of SW excitation. Furthermore, we establish a correlation between the behavior of excited SW modes as a function of external magnetic field strength and underlying chiral spin texture states.