Microwave Absorption Properties of Cobalt-doped BaFe12O19 Hexaferrites

Cobalt doping at the iron site of barium hexaferrites (BaFe12−xCoxO19, x = 0 − 2.0, Δx = 0.5) was done using a co-precipitation method followed by a heat treatment at 900 °C. Careful refinement of the X-ray diffraction patterns by using the Rietveld method revealed a transformation from a hexagonal to a mixed hexagonal and cubic spinel structure. The undoped sample was ∼95% barium M-type hexaferrites (BaFe12O19), with the other 5% being made up barium Y-type hexaferrites (Ba2Fe14O22: 3.50%) and spinel ferrites (Fe3O4: 1.62%). The replacement of Fe3+ by Co2+ led to a charge imbalance which promoted the growth of Y-type hexaferrites (Ba2CoxFe 14−xO22) and spinel ferrites (CoxFe3−xO4). The impurity phases grew linearly from 5.12% for x = 0 to 67.21% for x = 2.0, leading to a linear decrease in barium M-type hexaferrites (94.88% for x = 0 to 32.79% for x = 2.0). A structural study by using Raman scattering spectroscopy confirmed the growth of Y-type hexaferrites and spinel ferrites by detecting new modes at 319, 325, 519, 569, 640 and 675 cm−1. As to the electromagnetic properties, three of the four parameters were changed by doping in the frequency range from 2 to 18 GHz for samples with thickness of t = 1.50 mm. Both the real (μ′) and the imaginary (μ″) parts of the complex permeability and the real part (ε′) of the complex permittivity increased with increasing doping. This tuned the reflection loss (RL) from poor (RL = −5.0 dB at f = 12.5–13.0 GHz) for the undoped sample to fairly good for doped samples (RL = − 10.5 dB and −16.0 dB for x = 2.0 and 1.0, respectively, at f = 17.0 GHz). A study of the thickness (t) dependence of the RL in the range t = 2.75–1.50 mm pointed out that our samples also had some features similar to those of metamaterials, especially for thick samples at a frequency closeto18 GHz for the x = 0 sample.