Theory of Spin-Wave Frequency Gaps in 3D Magnonic Crystals. Application to Manganites

TL;DR

This paper develops a theoretical framework for understanding spin wave frequency gaps in 3D magnonic crystals, applying it to manganites and showing good agreement with experimental neutron scattering data.

Contribution

The study introduces a model for spin wave spectra in 3D ferromagnetic composites and applies it to manganites, suggesting they can be viewed as natural magnonic crystals.

Findings

Frequency gaps depend on magnetic contrasts and structural parameters.

The theory aligns well with neutron scattering experiments on manganites.

Manganites may be considered as natural 3D magnonic crystals.

Abstract

his study is an investigation of spin wave spectrum in macrostructures composed of two ferromagnetic materials and showing a 3D periodicity: spherical ferromagnetic grains disposed in the nodes of a 3D crystal lattice are embedded in a matrix with different ferromagnetic properties. Frequency ranges forbidden to spin wave propagation are found in the calculated magnonic spectra. Both the position and the width of the gaps are found to depend on the magnetic (exchange and magnetization) contrasts in the composite material, as well as on its structural parameters (filling fraction, crystal lattice type). Having applied our theory to interpretation of the existence of a spin wave gap in doped manganites, recently revealed in neutron scattering experiments by S. Hennion {\em et al.}, we obtained a good (though approximate) quantitative agreement with the experimental results. A working hypothesis is proposed on this basis, supposing that certain manganites can be treated as natural 3D magnonic crystals.