Microwave Absorption Properties of Cobalt-doped BaFe12O19 Hexaferrites

Abstract

Cobalt doping at the iron site of barium hexaferrites (BaFe 12− x Co x O 19 , 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 (BaFe 12 O 19 ), with the other 5% being made up barium Y-type hexaferrites (Ba 2 Fe 14 O 22 : 3.50%) and spinel ferrites (Fe 3 O 4 : 1.62%). The replacement of Fe 3+ by Co 2+ led to a charge imbalance which promoted the growth of Y-type hexaferrites (Ba 2 Co x Fe 14 − x O 22 ) and spinel ferrites (Co x Fe 3− x O 4 ). 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.

Topics & Concepts