Band Structure Dependence on the External Perpendicular Magnetic Field and Zn Concentration of Photonic Crystals Made of Co1–xZnxFe2O4 Nanoparticles

Ferrofluids based on the Co
_1-x
ZnxFe
₂
O
₄
magnetic nanoparticles are considered as soft magnetic materials with interesting technological applications in nanotechnology when the ferrofluids are placed in an external magnetic field. The morphological characterization of Co
_1-x
ZnxFe
₂
O
₄
magnetic nanoparticles was obtained by means of high-resolution transmission electron microscopy studies. In addition, we observed self-assembles of Co
_1-x
ZnxFe
₂
O
₄
magnetic nanoparticle aggregates forming columns in a hexagonal 2-D photonic crystal (PC) when the ferrofluid was encapsulated in a cell glass under the action of a tunable magnetic field applied perpendicular to the film. The dependence of the ferrofluid effective refractive index with the applied magnetic field strength was determined by laser interferometry measurements using a Mach-Zehnder interferometer. To determine the optical response of 2-D magnetocontrollable PCs based on ferrofluids under the action of a dc external magnetic field, the Maxwell-Garnett theory, experimentally calculated effective refractive index, and plane-wave expansion method were used. The photonic band structure (PBS) of 2-D PCs for these ferrofluids was simulated, and we found that the optical response of PC depends on the ferrofluid effective refractive index, area ratio, permittivity of magnetic columns, and composition of Co
_1-x
ZnxFe
₂
O
₄
ferrofluids. The results showed that the PBS of ferrofluid with the less Zn concentration (x = 0.25) presented a complete bandgap in the IR range (λ ~ = 10.6 μm) at low perpendicular applied magnetic field. Finally, the procedure used in this paper may be considered as an alternative method to characterize the optical response of nanostructured ferrofluids in the presence of external magnetic fields. These results can be considered for technological applications of ferrofluids as magnetocontrollable PCs.