Magnon Dynamics in Parity-Time-Symmetric Dipolarly Coupled Waveguides and Magnonic Crystals

Abstract

We consider the magnonic properties of two dipolarly coupled magnetic stripes, both deposited on a normal conductive substrate with strong spin-orbit coupling. A charge current in the substrate acts on the adjacent magnets with spin-orbit torques, which result in magnonic damping or antidamping of the spin waves, and hence a gain-loss coupling of the two magnetic stripes. The whole setup is demonstrated to exhibit features typical for parity-time- ($PT$) symmetric systems. Phenomena are demonstrated that can be functionalized in magnonic devices, including reconfigurable magnonic diodes and logic devices. Alternative stripe designs and $PT$-symmetric, periodic, coupled magnonic textures are studied. Analytical and full numerical analysis identify the conditions for the appearance of exceptional points (EPs), where magnonic gain and loss are balanced and evidence nonreciprocal magnon propagation and enhanced magnon excitation around EPs. Furthermore, the dipolar coupling is shown to bring in a wave-vector-dependent $PT$-symmetric behavior. Proposing and simulating a $PT$-symmetric magnonic crystal, we show how EPs and hence associated phenomena can be steered to a particular wave vector in a gaped spectrum via material design. The phenomena offer additional tools for magnonic based communication and computational devices.