Realization of novel topological phases in magnonic band structures represents a new opportunity for the development of low-dissipation magnonics 1-5. The previous proposals require materials with either special crystal symmetries 2-3 or artificially modulated structures that demand advanced nanofabrication techniques 4-5, both of which bring in difficulties for experimental realization.
In this work, we theoretically study the magnonic band structure of antiparallelly aligned magnetic multilayers (Fig. 1), and reveal their field-tunable topological phases 1. We show that the long-range dipolar interaction between propagating magnons is chiral in nature, whose strength depends on the wave vector direction and therefore breaks time-reversal symmetry. Consequently, the dipolar interaction plays a role like spin-orbit coupling in electronic systems and generates bulk bands with non-zero Chern integers and ultra-localized magnonic surface states that carry chiral spin currents (Fig. 1). Through an external magnetic field, the topological phases can be switched, which therefore provides a tunable for transferring spin angular momenta in this synthetic antiparallelly aligned heterostructure. Besides analytical study, we also carry out micromagnetic simulations on the simplest multilayer system – YIG/Py bilayers using MuMax3 6. The magnonic band structure with a degeneration point only in one half of the reciprocal space and the unidirectional propagation of spin waves (Fig. 2) both validate our analytical results.
Our study provides an easy-to-implement system for realizing topologically protected magnonic surface states and low-dissipation spin transport in a tunable manner, which is expected to benefit various areas of modern spintronics and magnonics.