Nonreciprocal collective magnetostatic wave modes in geometrically asymmetric bilayer structure with nonmagnetic spacer

TL;DR

This paper demonstrates a simple method to induce and control nonreciprocal spin wave modes in asymmetric ferromagnetic bilayer structures with a nonmagnetic spacer, advancing magnonic device design.

Contribution

It introduces a geometrical asymmetry approach to achieve frequency nonreciprocity in coupled ferromagnetic layers, supported by experimental and theoretical analysis.

Findings

Nonreciprocity reaches several percent at high wavenumbers.

Layer thickness influences the nonreciprocal shift of spin wave modes.

The method enables tailored spin wave dispersion for magnonic applications.

Abstract

Nonreciprocity, i.e. inequivalence in amplitudes and frequencies of spin waves propagating in opposite directions, is a key property underlying functionality in prospective magnonic devices. Here we demonstrate experimentally and theoretically a simple approach to induce frequency nonreciprocity in a magnetostatically coupled ferromagnetic bilayer structure with a nonmagnetic spacer by its geometrical asymmetry. Using Brillouin light scattering, we show the formation of two collective spin wave modes in Fe$_{81}$Ga$_{19}$/Cu/Fe$_{81}$Ga$_{19}$ structure with different thicknesses of ferromagnetic layers. Experimental reconstruction and theoretical modeling of the dispersions of acoustic and optical collective spin wave modes reveal that both possess nonreciprocity reaching several percent at the wavenumber of $22~\cdot~10^4$ rad cm$^{-1}$. The analysis demonstrates that the shift of the amplitudes of counter-propagating coupled modes towards either of the layers is responsible for the nonreciprocity because of the pronounced dependence of spin wave frequency on the layers thickness. The proposed approach enables the design of multilayered ferromagnetic structures with a given spin wave dispersion for magnonic logic gates.