Non-uniform spin wave softening in 2D magnonic crystals as a tool for opening omnidirectional magnonic band gaps

TL;DR

This paper investigates how non-uniform spin wave softening in 2D magnonic crystals can be utilized to create omnidirectional magnonic band gaps, enabling highly tunable spin wave filters controlled by small magnetic field changes.

Contribution

It demonstrates the role of non-uniform mode softening in opening omnidirectional band gaps and highlights the importance of demagnetizing field enhancement for tunable magnonic devices.

Findings

Non-uniform mode softening causes omnidirectional band gaps.

Demagnetizing field enhancement is crucial for mode softening.

Magnonic gaps can be tuned with small external magnetic field changes.

Abstract

By means of the plane wave method we study spin wave dynamics in two-dimensional bi-component magnonic crystals based on a squeezed hexagonal lattice and consist of a permalloy thin film with cobalt inclusions. We explore the dependence of a spin wave frequency on the external magnetic field, especially in weak fields where the mode softening takes place. For considered structures, the mode softening proves to be highly non-uniform on both the mode number and the wave vector. We found this effect to be responsible for the omnidirectional band gap opening. Moreover, we show that the enhancement of the demagnetizing field caused by the squeezing of the structure is of crucial importance for the non-uniform mode softening. This allows us to employ this mechanism to design magnonic gaps with different sensitivity for the tiny change of the external field. The effects we have found should be useful in designing and optimization of spin wave filters highly tunable by a small external magnetic field.