Band structure of magnonic crystals with defects: Brillouin spectroscopy and micromagnetic simulations

Abstract

Using Brillouin spectroscopy, the first observation has been made of the band structures of nanostructured defect magnonic crystals. The samples are otherwise one-dimensional periodic arrays of equal-width Ni 80 Fe 20 and cobalt nanostripes, where the defects are stripes of a different width. A dispersionless defect branch emerges within the bandgap with a frequency tunable by varying the defect stripe width, while the other branches observed are similar to those of a defect-free crystal. Micromagnetic and finite-element simulations performed unveil additional tiny bandgaps and the frequency-dependent localization of the defect mode in the vicinity of the defects.

Symmetry and localization properties of defect modes in magnonic superlattices

“…Thus, at ν = 10 for instance the mode inside the BG seems to have no dispersion, which is in agreement with recent BLS experiments and micromagnetic simulations. 29,30 Fig. 3(b) shows a zoom of Fig.…”

“…On the other side, unlike the defect modes, where the excitation is mainly located at the defect zone, the branches at the band-gap edges show an extended character, in such a way that these branches are excited in the entire unit supercell (see Ref. 29 for details).…”

“…Therefore, it is demonstrated that the symmetric modes have a limited range of field where they can be observed, since when these modes reach the band-gap edges they are extended along the crystal and therefore they can hardly be detected. 29 On the other side, once the antisymmetric mode is excited, it should be observable in a wider range of fields. The same behavior of the defect modes as a function of the field value is valid for different magnitudes of δ (not shown).…”

“…A theoretical analysis of shortwavelength perturbations in two-dimensional MCs with point defects was performed by Yang, et al, [42][43][44][45] where different configurations of the point defects were investigated. Defect-induced phenomena in one dimensional bi-component MCs with structural defects were more recently investigated by Brillouin light scattering (BLS) measurements and via numerical simulations, 29,30 where arrays of 250 nm wide Py stripes were fabricated in such a way that every tenth wire there is a defect wire having a larger width ranging from 300 to 500 nm. Since the recent experiments were performed in the Damon-Eshbach (DE) geometry at small wave vectors (around 10 µm −1 ), the dynamic dipolar contribution inevitably must be taken into account.…”

“…In (b) a zoom of the dispersion around of one boundary of the first Brillouin zone is shown.Figure (c) depicts the oscillation amplitude A f as a function of ν.quencies were extracted as the summation of the spatial FFT in the time domain within of each cell.IV. RESULTS AND DISCUSSIONTo study the dynamic properties of the system, standard values of Cobalt and Permalloy are employed 29. Namely, the magnetic properties of material A resemble Permalloy (Ni 80 Fe 20 ) which are: M A s = 730 kA/m and A A ex = 1.1 · 10 −11 J/m.…”